U.S. patent application number 11/889482 was filed with the patent office on 2008-06-12 for methods from removing heavy metals from water using chemical precipitation and field separation methods.
Invention is credited to Steven L. Cort.
Application Number | 20080135491 11/889482 |
Document ID | / |
Family ID | 39496724 |
Filed Date | 2008-06-12 |
United States Patent
Application |
20080135491 |
Kind Code |
A1 |
Cort; Steven L. |
June 12, 2008 |
Methods from removing heavy metals from water using chemical
precipitation and field separation methods
Abstract
A two-step chemical precipitation process involving hydroxide
precipitation and sulfide precipitation combined with "field
separation" technology such as magnetic separation, dissolved air
flotation, vortex separation, or expanded plastics flotation,
effectively removes chelated and non-chelated heavy metal
precipitates and other fine particles from water. In the
first-step, the non-chelated heavy metals are precipitated as
hydroxides and removed from the water by a conventional
liquid/solids separator such as an inclined plate clarifier to
remove a large percentage of the dissolved heavy metals. The
cleaned water is then treated in a second precipitation step to
remove the residual heavy metals to meet discharge limits. In the
second precipitation step, any metal precipitant more effective
than hydroxide for metal precipitation can be used. The invention
improves metal removal, lowers cost because fewer chemicals are
used, produces less sludge, and reduces the discharge of toxic
metals and metal precipitants to the environment. Magnetic
separation is preferred for the separation of particles
precipitated in the second stage. Similar methods can be employed
for separation of other particulates from water. Particulates can
also be removed by causing them to adhere to particles of expanded
plastic, forming a floc lighter than water, so that the floc can be
removed by flotation.
Inventors: |
Cort; Steven L.; (Cary,
NC) |
Correspondence
Address: |
Michael de Angeli
60 Intrepid Lane
Jamestown
RI
02835
US
|
Family ID: |
39496724 |
Appl. No.: |
11/889482 |
Filed: |
August 14, 2007 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
11135644 |
May 24, 2005 |
7255793 |
|
|
11889482 |
|
|
|
|
10152024 |
May 22, 2002 |
6896815 |
|
|
11135644 |
|
|
|
|
60294022 |
May 30, 2001 |
|
|
|
60330973 |
Nov 5, 2001 |
|
|
|
60352265 |
Jan 30, 2002 |
|
|
|
Current U.S.
Class: |
210/695 ;
210/223 |
Current CPC
Class: |
B03C 1/03 20130101; C02F
1/66 20130101; Y02P 10/20 20151101; Y10S 210/914 20130101; C02F
2103/06 20130101; C02F 1/54 20130101; Y10S 210/912 20130101; C02F
2101/20 20130101; C22B 3/22 20130101; C02F 2103/001 20130101; B03C
1/30 20130101; C02F 2101/22 20130101; C22B 3/44 20130101; B03C
1/015 20130101; C02F 1/488 20130101; B03C 1/286 20130101; C02F 1/38
20130101; Y10S 210/913 20130101; C02F 9/00 20130101; Y02P 10/234
20151101; C02F 1/56 20130101; C02F 1/5236 20130101; C02F 1/24
20130101 |
Class at
Publication: |
210/695 ;
210/223 |
International
Class: |
C02F 1/48 20060101
C02F001/48; B03C 1/30 20060101 B03C001/30 |
Claims
1-30. (canceled)
31. A method of treating water, comprising the steps of: a.
directing the water to be treated into a flocculation zone; b.
mixing magnetic seed and a flocculant with the water and causing
particulate matter in the water to attach to the magnetic seed to
form a magnetic floc that can be attracted by a magnetized surface;
c. mixing the magnetic floc and water in the flocculation zone and
maintaining at least some of the magnetic floc in suspension in the
flocculation zone; d. driving a first rotary magnetic separator in
the flocculation zone such that at least a portion of the first
rotary magnetic separator is submerged in the water in the
flocculation zone; e. collecting magnetic floc from the water in
the flocculation zone on the first rotary magnetic separator; f.
removing the magnetic floc from the first rotary magnetic
separator; and g. separating the magnetic seed from the magnetic
floc removed from the first rotary magnetic separator and recycling
the separated magnetic seed.
32. The method of claim 31 wherein the first rotary magnetic
separator is positioned adjacent an upper surface of the water in
the flocculation zone.
33. The method of claim 31 wherein said step of recycling the
magnetic seed includes the steps of returning the magnetic seed to
the flocculation zone and mixing the returned magnetic seed with
the water in the flocculation zone.
34. The method of claim 31 including the further steps of directing
the magnetic floc removed from the first rotary magnetic separator
to a shear tank, shearing the magnetic floc, and detaching the
magnetic seed from the magnetic floc to form sludge.
35. The method of claim 34 including the further steps of driving a
second rotary magnetic separator, collecting the detached magnetic
seed on the second rotary magnetic separator, removing the magnetic
seed from the second rotary magnetic separator, and recycling the
magnetic seed.
36. The method of claim 35 wherein the second rotary magnetic
separator is disposed adjacent a sludge collection surface and the
method includes the step of collecting the sludge on the sludge
collection surface.
37. The method of claim 35 including the further steps of scraping
the magnetic seed from the second rotary magnetic separator and
directing the magnetic seed scraped from the second rotary magnetic
separator to the flocculation zone.
38. The method of claim 35 including the further step of driving
the first and second rotary magnetic separators with a common drive
source.
39. The method of claim 31 including the further steps of driving a
second rotary magnetic separator and collecting the magnetic seed
separated from the magnetic floc on the second rotary magnetic
separator.
40. The method of claim 39 including the further step of shearing
the magnetic floc to separate the magnetic seed from the magnetic
floc prior to collecting the magnetic seed on the second rotary
magnetic separator.
41. The method of claim 40 including the further step of shearing
the magnetic floc in a shear chamber that is separated from the
flocculation zone.
42. The method of claim 39 including the further steps of disposing
at least a portion of the second rotary magnetic separator over the
flocculation zone, and removing the collected magnetic seed from
the second rotary magnetic separator such that as the magnetic seed
is removed from the second rotary magnetic separator, the magnetic
seed falls into the flocculation zone.
43. The method of claim 31 wherein the magnetic seed is magnetite
and the method includes the step of mixing magnetite with the
water.
44. The method of claim 31 wherein the flocculation zone is
disposed in a single tank and wherein substantially all of the
particulate matter removed from the water is removed while the
water is confined within the single tank.
45. The method of claim 35 wherein the flocculation zone is
disposed in a single tank and the method includes the steps of
directing the water to be treated into the single tank, removing
substantial particulate matter from the water while the water is
confined within the single tank, and directing a clean and treated
water effluent from the single tank.
46. The method of claim 45 wherein a third rotary magnetic
separator is positioned adjacent an outlet of said tank and the
method includes the further steps of directing the water towards
the outlet, driving the third rotary magnetic separator and
collecting magnetic floc from the water on the third rotary
magnetic separator prior to the water being discharged from the
outlet.
47. The method of claim 46 including driving the first, second and
third rotary magnetic separators with a common drive source.
48. A water treatment system for removing particulate matter from
water, comprising: a. a single tank having an inlet through which
water to be treated enters the tank and an outlet through which
treated water is discharged from the tank; b. one or more injection
sites for injecting magnetic seed and a flocculant into the water;
c. a mixer disposed in the single tank for mixing the magnetic seed
and flocculant with the water to form magnetic floc, and for
maintaining at least a portion of the magnetic floc in an upper
portion of the water contained within the tank; d. a first rotary
magnetic separator disposed in an upper portion of the single tank
such that when water to be treated is contained within the tank,
the first rotary magnetic separator is at least partially submerged
in the water; e. a drive for driving the first rotary magnetic
separator such that as the first rotary magnetic separator rotates
through the water in the tank, magnetic floc from the water is
collected on the first rotary magnetic separator; and f. whereby
particulate matter in the water is removed therefrom while the
water is confined within the single tank.
49. The system of claim 48 including a removing device for removing
magnetic floc from the first rotary magnetic separator, and a
shearing device for shearing the removed magnetic floc to yield
magnetic seed and sludge.
50. The system of claim 49 wherein the shearing device includes a
shear chamber, and wherein the system is configured to direct
magnetic floc removed from the first magnetic separator to the
shear chamber.
51. The system of claim 49 including a second rotary magnetic
separator for collecting magnetic seed after the magnetic seed has
been separated from the magnetic floc, and wherein the collected
magnetic seed can be recycled to the single tank.
52. The system of claim 51 further including a sludge collection
surface disposed adjacent the second rotary magnetic separator for
receiving the sludge.
53. The system of claim 51 wherein the second rotary magnetic
separator is disposed at least partially over the single tank and
disposed at least partially above the upper surface of the
water.
54. The system of claim 48 further including: a. a shear chamber
for shearing magnetic floc and yielding magnetic seed and sludge;
b. a first device for removing collected magnetic floc from the
first rotary magnetic separator; C. a transfer device for
transferring removed magnetic floc to the shear chamber; d. a
second rotary magnetic separator for collecting magnetic seed
produced by the shearing chamber; and e. a second removing device
for removing the magnetic seed from the second rotary magnetic
separator.
55. The system of claim 54 wherein the second removal device is
configured to direct the magnetic seed into the single tank.
56. The system of claim 54 including a sludge collection surface
disposed adjacent the second rotary magnetic separator for
collecting sludge.
57. A method of employing magnetic seeding and a magnetic separator
to treat and clarify water in a single tank, comprising the steps
of: a. directing water having particulate matter therein into the
single tank; b. injecting magnetic seed and a flocculant into the
water; c. forming magnetic floc in the tank by binding particulate
matter to the magnetic seed, such that the magnetic floc can be
attracted by a magnetic surface; d. agitating the magnetic floc in
the tank to maintain a substantial portion of the magnetic floc in
suspension and in an upper portion of the water within the tank; e.
driving a first rotary magnetic separator positioned in an upper
portion of the tank, and moving at least a portion of the first
rotary magnetic separator through an upper portion of the water in
the tank; f. removing particulate matter from the water in the tank
by magnetically collecting the magnetic floc on the first rotary
magnetic separator as the first rotary magnetic separator rotates
through the water; g. discharging clarified water from the tank;
and h. wherein substantially all of the particulate matter removed
from the water is removed while the water is confined within the
single tank.
58. The method of claim 57 including the further steps of
separating the magnetic seed from the magnetic floc and recycling a
portion of the separated magnetic seed to the single tank.
59. The method of claim 58 wherein said step of separating the
magnetic seed from the magnetic floc includes the steps of shearing
the magnetic floc and forming a mixture of sludge and magnetic
seed, directing the mixture to a second rotary magnetic separator,
collecting the magnetic seed on the second rotary magnetic
separator, scraping the magnetic seed from the second rotary
magnetic separator, transferring the magnetic seed to the single
tank, and collecting the sludge on a sludge collecting surface.
60. The method of claim 59 comprising the step of driving the first
and second rotary magnetic separators with a common drive source.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation-in-part of and claims
priority from Ser. No. 10/152,024, filed May 22, 2002, issuing May
24, 2005, as U.S. Pat. No. 6,896,815. Ser. No. 10/152,024 in turn
claimed priority from three provisional patent applications, Ser.
Nos. 60/294,022 filed May 30, 2001, 60/330,973, filed Nov. 5, 2001,
and 60/352,265, filed Jan. 30, 2002.
BACKGROUND OF THE INVENTION
[0002] The three provisional patent applications referred to above
were combined into one patent application Ser. No. 10/152,024,
because they all dealt with two-step chemical precipitation and
"field separation" technologies to remove fine metal precipitates
from water, which was the focus of parent application Ser. No.
10/152,024. This remains an important aspect of the invention of
this continuation-in-part application; however, this application is
also directed to other aspects of the invention, as will appear
more fully below. Thus, in the following disclosure, two-step
precipitation processes for removing heavy metals from water are
discussed first, with the other aspects of the invention being
discussed later.
[0003] The removal of heavy metals from water is an important
aspect of water treatment. There are many technologies for
accomplishing this; however, one of the most cost effective means
is chemical precipitation. "Chemical precipitation", as used herein
and generally in the art, refers to reacting dissolved metals with
an additive chemical of some sort so that the metals to be removed
are rendered insoluble, so that they can then be separated from the
water. Raising the pH to a neutral or an alkaline level will
precipitate most heavy metals as metal hydroxides. However,
hydroxide precipitation is usually not effective enough to meet
strict new discharge limits. Metal hydroxides are not insoluble
enough to meet these limits and metal ions that are chelated will
not precipitate at all. Therefore, more advanced treatments such as
reaction with organic or inorganic sulfides must be used. These
chemistries will produce metal sulfides that have lower solubility
than hydroxides and will break chelate bonds to allow the metals to
precipitate.
[0004] The Department of Army Engineering and Design Manual No.
1110-1-4012 on page 2-2 (Precipitation/Coagulation/Flocculation),
shows the difference between the solubility of metal hydroxides and
metal sulfides. Under ideal conditions, the optimum metal hydroxide
solubility ranges from 10.sup.2 to 10.sup.-2 mg/L. Under ideal
conditions, the optimum metal sulfide solubility ranges from
10.sup.-2 to 10.sup.-2 mg/L.
[0005] If all the metals (chelated and non-chelated) are
precipitated with sulfide chemicals in a one-step precipitation,
the removal is complete, but the cost of treatment is high, often
prohibitively high for waste streams containing high concentrations
of heavy metals. If most of the metals are first removed as metal
hydroxides in a first-step precipitation, and the remaining metals
are polished out in a second-step precipitation, the removal of
metals is improved and the cost of treatment is much lower. This
patent application shows that it is beneficial to use selected
"field separation" methods that have not been used or contemplated
before in combination with this two-step precipitation process.
[0006] The concept of removing heavy metals using sulfides and
ferrous compounds was described by Anderson in U.S. Pat. No.
3,740,331. However, Anderson fails to suggest refinements and
additions provided by the present invention that make this basic
technique cost-effective in today's processing environment.
Specifically, Anderson does not suggest that removing metals can be
performed more efficiently if the heavy metals are removed in a
two-step precipitation process. The teachings of the Anderson
patent are simply that using ferrous compounds with sulfides will
result in better metal removal. No suggestion is made to use "field
separation" methods effective in removing fine and fragile metal
precipitates.
[0007] The fundamental disadvantage of doing a sulfide
precipitation according to Anderson is that it produces very fine
colloidal particles that are hard to remove. The present inventor
attempted to remove these particles with a sand filter or with a
one micron sized back-washable filter and was unsuccessful.
[0008] Fender, in U.S. Pat. No. 4,422,943, describes the use of
iron pyrite as a source of sulfide to precipitate heavy metals as
metal sulfides. He also describes the benefits of using a two-step
precipitation process. In claim 2, Fender describes the step of
separating precipitated sulfides by filtration (specifically, sand
filtration), but does not contemplate using the "field separation"
methods described in this application. However, to accomplish
filtration, he uses a polymer to increase the particle size so the
sand filter can remove the metal sulfides. It is known in the art
that using an organic polymer to increase the size of the metal
sulfide precipitates will cause fouling problems with sand filters.
The "field separation" methods covered by this patent application
are not subject to fouling, as are filters. Furthermore, sand
filters have a limitation on the size of particles that can be
removed; even a well designed multi-media sand filter can remove
particles only down to about 20 microns in size. Metal sulfide
precipitation will produce colloidal-sized particles of less than
one micron in size, which will pass through a sand filter.
[0009] With the exception of microfiltration which can remove
sub-micron sized particles, the present inventor has found no
filtration equipment capable of consistently and economically
removing fine metal sulfide particles. More specifically, the
present inventor has experimented with a back washable filter
manufactured by Asahi. It had a plastic-mesh filtering element with
a one micron opening size. This was significantly smaller than the
metal sulfide precipitates, judged to be at least 30 microns
because they were visible to the naked eye. However, even low
operating pressures (about 10 psi), were sufficient to deform the
shape of the metal sulfide precipitates and force these >30
micron sized particles through one micron sized openings.
[0010] The only commonality between this patent application and the
Fender patent is they both recognize the economic importance of
using a two-stage precipitation process, which is known art. In
summary, this application deals with other forms of soluble and
insoluble sulfide treatment rather than iron pyrite and "field
separation" equipment rather than filters, which is an improvement
on the Fender patent. Further, the Fender patent only deals with
iron pyrite as a source of sulfide to precipitate heavy metals; the
present application deals with other sulfides that are known to
produce small metal sulfide particles that are difficult to
filter.
[0011] The art recognizes a difference between filtration equipment
and "field separation" equipment, as discussed in the Chemical
Engineering document dated February 1997, Volume 104, Issue 2, Page
66. Filtration equipment includes: straining, cake filtration, deep
bed filtration, and membrane filtration and always involves a
physical barrier that prevents the passage of particles over a
specific size. "Field separation" techniques include gravitational
settling, centrifugal settling, hydrocyclone separation, dissolved
air flotation, expanded plastics flotation, and magnetic
separation. The difference is that filtration involves a physical
barrier to capture particles while "field separation" involves
force fields, provided by inter-molecular, gravitational,
centrifugal, and/or magnetic forces to separate particles from
water.
[0012] U.S. Pat. No. 6,099,738 to Wechsler deals with a method and
system for removing solutes from a fluid using magnetically
conditioned coagulation. This method includes the steps of
magnetically conditioning the fluid by applying a magnetic field to
enhance the precipitation of solute particles for coagulation;
adding a coagulant to the fluid before, during, and after
application of the conditioning magnetic field to coagulate the
increased available solute particles to form colloids; and
collecting the colloids for removal from the fluid. Wechsler
neither contemplates combining magnetic seeding and polymer
addition with a two-step metal precipitation process as a means for
efficiently removing heavy metals from wastewater, nor combining
magnetic separation principles with gravity settling in one
treatment vessel, as described herein.
[0013] According to the present invention, any magnetic separation
method can be used; however, in the preferred embodiment the
magnetic separator used to capture the magnetic particles is
mounted in the treatment tank rather than as a separate collection
device, which is novel. This approach has three advantages: (1) one
less piece of equipment is needed, (2) the system can be cleaned
without interrupting the water flow, and (3) permanent magnets can
be used rather than electromagnets.
[0014] Magnetic seeding is used in some embodiments of the present
invention to remove precipitated pollutants and other non-magnetic
particles from water. Magnetic seeding is known per se for such
purposes. Specifically, the Department of Energy published studies
(C. Tsouris, et. al., Electrocoagulation for magnetic seeding of
colloidal particles, Physiochem Eng. Aspects (accepted paper)
December 1999; C. Tsouris, et. al., Flocculation of paramagnetic
particles in a magnetic field, Journal of Colloid and Interface
Science, 171, 319-330; T-Y Ying et. al., High-gradient magnetically
seeded filtration, Chemical Engineering Science 55(2000) 1101-1113)
addressing the effectiveness of magnetic seeding to remove
colloidal sized particles. The DOE investigators studied
magnetically seeded solid/liquid separation combining magnetic
seeding under turbulent-shear flow conditions with high field
gradient magnetic filtration. They concluded that magnetic seeding
was effective in removing fine particles. They used seed particle
concentration, solution pH, and ionic strength parameters that
determine the zeta-potential of particles to significantly affect
the particle removal performance. They did not use organic polymers
to bind the magnetic seed materials to the low-magnetic particles
to enhance removal, and did not apply magnetic seeding and
filtration principles to the second step of a two-step metal
precipitation process using sulfide precipitants.
[0015] Nilsson patent U.S. Pat. No. 3,980,562 shows an apparatus
for magnetic separation, including a device for removal of
accumulated particles from magnetic disks used to collect the
particles. More specifically, Nilsson teaches that suspended
particles and high molecular weight substances can be removed from
water by adding a ferromagnetic particulate to the water and using
a magnetic field to separate the combined particles. Nilsson also
suggests addition of chemical "flocking agents", giving as example
lime, alum, iron chloride, polyelectrolytes and water glass. Col. 1
lines 10-28. These materials are properly referred to as
coagulants, in that they affect the charge of the particles, as
compared to the polymer flocculants used in practice of the present
invention, which attach particles with long chain polymers as
discussed below. Nilsson shows collecting the magnetized particles
on opposed walls of disks enclosing permanent magnets, and then
scraping them off onto a conveyor belt for disposal. A sector of
the disks may be provided without magnets, to facilitate the
scraping. Col. 5, lines 18-20. In the Nilsson design, the sector
provided without magnets is located at the top of the collector
magnet disks because this is the location of the conveyor belts
removing the scraped magnetite. However, in the present invention,
the sector that is free of magnets is located at the bottom of the
collector magnet disk so that after the magnetite is scraped off
the disk, it settles by gravity to the bottom of the settling
chamber (74).
[0016] In researching the present work, the present inventor found
that a strong enough bond between the magnetic seed material and
the non-magnetic metal sulfide precipitates to enable reliable
separation could not be achieved unless a flocculating polymer was
also used. The polymer binds the magnetic seed material together
with the fine metal sulfide particles so they can be removed by a
low field strength magnetic separator or by gravity settling.
[0017] Another novel approach of this present patent is the removal
of fine precipitates formed, for example, in the second step of
this two-step precipitation process, by the use of expanded
plastics to enhance flotation. The present inventor successfully
attached fie metal precipitates to expanded polystyrene (EPS) with
a flocculating polymer. The EPS, having the precipitates attached
thereto by the flocculating polymer, floats, allowing the metal
precipitates to be removed from the waste stream. The same "EPS
flotation" technique of the invention can be used to remove
particulates from other water streams, e.g., remove dirt from
stormwater runoff.
[0018] The concept of enhanced flotation using highly buoyant EPS
is similar to the principle used in dissolved air flow (DAF)
equipment. DAF uses micro-bubbles to float fine particles out of
water, while (in at least one embodiment) the present invention
uses an expanded plastic like EPS; this eliminates the energy cost
involved with compression of air to form the micro-bubbles.
[0019] To date, two-step precipitations have been rarely used
because they require additional equipment and space. This level of
treatment was not necessary because existing regulatory limits
could be achieved with a one-step hydroxide precipitation. However,
with tighter regulations, a two-step precipitation process is now
justified but the traditional clarification approach is often
infeasible because of the high residence times required which
involve substantial cost and space requirements. According to the
present invention, the extensive tankage and time requirements
of-conventional settling techniques are replaced with more
sophisticated separation techniques which result in a faster, more
space-efficient, and less expensive process.
[0020] The present invention describes better ways to do a two-step
precipitation that is less costly and requires less equipment than
a traditional clarifier yet is able to handle the metal
precipitates gently, so as to prevent their breakup.
BRIEF SUMMARY OF THE INVENTION
[0021] It is the object of this present invention to provide a
cost- and chemically-effective process for treating wastewater and
all waters requiring the removal of metal precipitates or other
fine particles.
[0022] A fundamental aspect of the present invention is the use of
"field separation" methods, especially magnetic separation or ESP
flotation; for example, these separation techniques can be employed
to remove particulates created by a two-step chemical precipitation
process for heavy metal removal, to remove particulates created by
a one-step method for removing other fine pollutant particles, or
simply to remove suspended solids from water.
[0023] As applied to the removal of heavy metals from water,
particularly to the removal of cadmium, chromium, copper, lead,
mercury, nickel, and zinc, the present invention pertains to
combining methods for precipitating heavy metals in an efficient
two-step chemical precipitation process (preferably hydroxide and
sulfide precipitation steps) with improved methods for removing the
fine metal precipitates produced in each precipitation stage.
[0024] More specifically, the process disclosed by the present
application provides an effective way to remove fine metal sulfide
precipitates and metal hydroxide precipitates, which in turn makes
it possible to effectively use a two-step precipitation method. The
two-step precipitation method reduces chemical costs, reduces the
amount of sludge produced, allows metals to be recycled, and
reduces the amount of metals discharged to the environment.
[0025] The first step of the invention as implemented for this
purpose is to precipitate non-chelated metals as metal hydroxides.
This requires the pH to be raised as necessary to reach an optimum
precipitation point for the metals in question. This typically is
in the 6-10 pH range. However, as the pH of wastewater must
generally be adjusted to be in the 6-9 pH range before it can be
discharged, precipitating the heavy metals as hydroxides according
to this aspect of the present invention does not significantly
increase treatment cost. When this first step is completed, most
(85% to 95% depending on the level of chelating agents present in
the wastewater) of the heavy metals will precipitate as metal
hydroxides. Any alkaline material can be used to raise the pH in
the first step of the process. Alkaline materials that are lower in
cost and form less sludge are preferred. Suitable alkaline
materials are lime, limestone, caustic soda, soda ash, or magnesium
hydroxide.
[0026] The conventional approach for metal removal is the one-step
precipitation process described above. As typically implemented,
this process normally requires a pH control tank, a floc tank, a
clarifier, and a final filter. The main disadvantage of a one-step
precipitation process is that it either cannot meet the discharge
limits if hydroxide precipitation is practiced, cannot effectively
remove metal hydroxides that precipitate at different pH's, or it
is very costly if sulfide treatment is practiced when metal
concentrations are high.
[0027] This present invention solves these problems and provides a
better and more cost effective method for removing dissolved metals
from water. The invention is an improvement over a one-step
precipitation process because it reduces chemical usage, produces
less sludge, and gets better metal removal. Chemical usage is
reduced because all the metals are not precipitated with sulfide
chemicals. Sludge quantities are reduced because smaller amounts of
chelate-breaking chemicals, such as ferrous compounds, are needed
in a two-step precipitation. Since most of the metals are recovered
in the hydroxide form, they can easily be recovered by
electrowinning and hydrometallurgical processing techniques. The
amounts of metals released into the environment are less because
sulfide chemicals can produce lower metal concentrations when a
first stage hydroxide precipitation process lowers the starting
concentration of the metals.
[0028] Several embodiments of the invention involving two-step
precipitation techniques for removing heavy metals are described.
In all embodiments, the first step of the metal removal process is
hydroxide precipitation and removal with a clarifier or other
suitable "field separation" device. The second step of the process
is preferably sulfide precipitation followed by a second "field
separation" method capable of removing fine particles in the range
0.1 to 100 microns. The "field separation" process chosen must be
capable of removing small fragile metal precipitates. According to
a further aspect of the invention, similar processes can also be
employed for separating particulates other than metal precipitates
per se from water, e.g., separating silt from storm water.
[0029] Until now, there have been few applications for two-step
precipitation processes; the applicable regulations are considered
to be liberal by many, so that one-step precipitation is usually
sufficient. In the few known applications with two-step
precipitation, clarification using gravity settling was practiced;
more specifically, processes using hydroxide precipitation and
clarification followed by sulfide precipitation and clarification
have been employed. According to one aspect of the present
invention, hydroxide clarification and/or sulfide precipitation is
followed by other field separation methods, including magnetic
separation or expanded polystyrene separation.
[0030] Such sophisticated separation techniques are required
because the metal precipitates are fragile and will break or deform
when aggressively filtered. For example, the present inventor
attempted to use so-called "dead end" filtration using a
back-washable filter manufactured by Asahi. It was not successful
because the pressure across the filtration element was too great
(greater than 10 psi), causing the fragile metal precipitates to
deform and break through the filter cloth.
[0031] The present inventor also attempted to remove fine sulfide
precipitates in a sand filter. The particles were too small and
exceeded the limit of the sand filter to remove particles smaller
than 10 microns.
[0032] Clarifiers are not well suited for the light solids loading
found in polishing applications because they are dependent upon the
type and frequency of collisions between the particles. Clarifiers
are also large in size and cannot fit into many existing
facilities.
[0033] The following "field separation" processes have been tested
and found, with some modification according to the invention, as
discussed below, to be suitable when used in combination with a
two-step precipitation process.
Magnetic Separation:
[0034] The process and apparatus of this present invention
accomplish the efficient removal of fine particles from water by
using gravitational and magnetic forces in one treatment vessel. A
magnetic seed material is necessary when the fine particles to be
removed do not possess magnetic properties, and a flocculating
agent is necessary to bind the magnetic seed material to the
non-magnetic particles.
[0035] The magnetic separation techniques used according to this
aspect of this invention, as illustrated in FIG. 1, is optimized
for the removal of precipitated heavy metals from water, but the
apparatus and process of this invention will remove a wide variety
of suspended solids from water.
[0036] A required step in the chemical precipitation of heavy
metals from water is to precipitate the metals as either hydroxides
or sulfides, by pH adjustment or the addition of a sulfide
precipitant, respectively. These metal precipitates are small and
fragile and require gentle liquid/solid separation methods.
Furthermore, such metal precipitates generally do not exhibit
magnetic properties. Therefore, in this embodiment of the
invention, a magnetic seed material, preferably magnetite
(Fe.sub.3O.sub.4), is added to the non-magnetic metal precipitates.
A flocculation agent, preferably an anionic polymer, is added to
ensure that the magnetite is attached to the metal precipitates.
This attachment process provides a magnetic anchor for the heavy
metal precipitates and allows those particles that are not normally
magnetic to be removed by a magnetic separator.
[0037] The addition of the anionic polymer, preferably a
polyacrylamide based polymer A3040L sold by Stockhausen, causes the
mixture of metal precipitates and magnetite particles to
flocculate. Because the magnetite is heavy, the majority of the
flocculate quickly settles to the bottom of the treatment vessel,
becoming a sludge that can be removed and dewatered. Gentle
agitation of the solution promotes flocculation by keeping the
heavy magnetite particles in suspension at the bottom of the
treatment vessel to improve the flocculation and the absorption
process of dissolved heavy metals. This gentle agitation can be
caused by mechanical mixing or by a naturally induced vortex
action. However, caused by the upward flow of water, some
particles, particularly those of smaller size, will be carried to
the top of the treatment vessel. According to another aspect of the
invention, a magnetic separator (detailed in connection with FIGS.
5-10 below) captures these rising fine magnetic particles before
they are discharged. By comparison, in the absence of the magnetic
separator, given sufficient time, all of the flocculated fine
particles would settle out by gravity, but depending on the nature
of the particles, this could take a long time and would necessitate
greatly increasing the size of the treatment vessel. As the flow
through the treatment vessel increases, there are even greater
upward forces on these fine particles preventing them from
settling. Since the magnetic separator can capture the magnetic
particles at high velocities, the fine particles can be allowed to
rise in the treatment vessel. This allows the treatment vessel to
be smaller which results in higher water velocities. Thus, the
combination of gravity settling at the bottom of the treatment
vessel and magnetic separation at the top of the treatment vessel
reduces the size of the equipment and allows the inventive process
to remove fine particles, whether they have a tendency to sink or
to float.
[0038] A bench scale system was constructed with a five-gallon
tank, a variable speed mixer, and a permanent magnet. A mixture of
metal sulfides, magnetite, and polymer were added to the tank and
the variable speed mixer set at a moderate speed. This flocculated
the mixture and the density of the flocculated particles caused
most of them to settle rapidly to the bottom of the tank. The speed
of the mixer was slowed until only a relatively small percentage of
the particles were suspended. Water was injected into the tank at a
rate of 2.5 gallons per minute and excess water was discharged from
the top of the tank. A permanent magnet was placed at the discharge
point and collected the suspended particles, leaving the discharge
water almost completely free of suspended particles. When the
magnet was removed, the quantity of particles discharged was
unacceptably high. This test demonstrated that a permanent magnet
could be employed to remove a high percentage of magnetic particles
from a moving stream of water. It showed that placing the magnet in
the same treatment tank where gravity settling occurs reduces
capital cost and allows the system to operate continuously. The
magnet is so effective at removing particles comprising various
undesirable components flocculated with magnetite particles that a
high wastewater throughput is possible through a small sized
system. For example, the Surface Over Flowrate (SOR) measured in
gallons per minute per square foot of surface area for a
traditional clarifier is usually between 0.25 and 1. The SOR for
the gravity settling zone (74) of the inventive process is 8-10 and
the SOR for the final magnetic collector (76) is ten times this
level or about 80-100.0036
[0039] Another pilot-scale system was tested to better evaluate the
benefit of high throughput capacity and the ability to capture the
magnetic particles with a collection of permanent magnets. The
pilot scale system had a capacity of 15 gallons and at the
discharge point bar magnets were placed in a trough. The bar
magnets were constructed of a ceramic material and were laid flat
in the trough with the water containing the magnetic particles
flowing through the trough and over the bar magnets. Tests showed
that the system could operate at a flow rate of at least 10 gallons
per minute with no visible discharge of magnetic particles. At 15
gallons per minute, the residence time for the system would be one
minute. This compares very favorably with other ballast aided
clarification systems; for example, the Actiflo system manufactured
by US Filter requires a residence time of between 10 and 15
minutes.
[0040] In the preferred embodiment, detailed in FIGS. 5-10,
discussed below, the magnetic separator is cleaned continuously. In
one embodiment, the magnetic separator consists of several round
disks mounted on a revolving shaft. A stationary scraper blade
removes the heavy deposits of magnetic material from the revolving
magnets. The removed sludge quickly settles to the bottom of the
treatment vessel and is not re-entrained into the water flow. This
is because the collected particles retain a magnetic charge
imparted from the permanent magnets, causing the particles to clump
together. Therefore they quickly settle and are withdrawn and
circulated for reuse or recovery.
[0041] The flocculating polymer forms a bond between the magnetite
and the metal precipitate sufficient to withstand the forces of
gentle flow, magnetic separation, and gravity settling. However,
under high-shear mixing, the bond between the magnetite and metal
precipitate is broken, allowing the magnetite to be reused. The
liberated magnetite is either separated from the metal precipitate
by gravity or by magnetic forces. The cleaned magnetite is reused
and the remaining metal precipitates are removed from the system
and dewatered with appropriate dewatering equipment.
[0042] The precipitated metals or other fine particles can also be
separated from the magnetite chemically. Magnetite is chemically
stable and does not measurably dissolve with pH adjustment. This is
not the case with some metal precipitates. For example, metal
hydroxides can be easily dissolved by pH adjustment. Therefore, a
mixture of magnetite and metal hydroxide precipitates can be easily
separated by pH adjustment.
[0043] Once the metal hydroxide particles are dissolved, the
magnetite can then be easily separated and returned to the
wastewater treatment system for reuse. The dissolved metals then
can be re-precipitated and filtered out of the wastewater for
disposal or recovery.
[0044] The recovered magnetite can be reused many times and testing
has confirmed that there is no practical limit to the number of
times it can be reused. However, some small quantity of the
magnetite is lost in the process and must be replenished as
necessary.
[0045] The magnetite provides several advantages. It provides solid
particles to enhance chemical precipitation and coagulation. It
adsorbs dissolved metals. It is heavy and provides good settling
action so that most of the magnetic particles settle out of the
flow before reaching the magnetic separator. This reduces the
solids loading on the magnetic separator.
[0046] As noted, the use of magnetic seed material to remove
non-magnetic material is not new. However, several improvements in
known magnetic separation practices are made according to this
invention, including, but not limited to: (1) the combination of
magnetic seeding practices with a two-step chemical precipitation
of heavy metals to make it possible to cost effectively remove the
heavy metals down to very low levels, (2) combining gravity
settling and magnetic separation in one treatment vessel, (3)
recovering the magnetite by using forces to break the bond between
the magnetite and the fine non-magnetic particles, and (4) certain
specifics of the design of the components used and the process
steps employed, as discussed in detail below.
[0047] The second precipitation step of the present invention
preferably uses organic or inorganic sulfide chemicals. However,
any metal precipitants (i.e., organic sulfides, inorganic sulfides,
sodium borohydride, ferrous sulfate, ferrous chloride, etc.) that
are more effective than hydroxide precipitants are suitable for the
second precipitation step and are within the scope of the present
invention. It is sometimes necessary to add a ferrous salt as a
co-precipitant to break metal-chelate bonds to improve metal
removal levels by co-precipitation effects. Any ferrous product
will work but either ferrous sulfate or ferrous chloride is
preferred; these are economical water treatment chemicals that add
no extra toxicity to the water.
[0048] Magnetite is a naturally occurring magnetic material and is
preferred in the practice of this present invention. However, any
material showing good magnetic susceptibility can be used;
ferrosilicon is one material that is suggested. The literature
shows that magnetic materials can be formed either chemically from
ferrous and ferric materials or electrically from iron electrodes.
These sources of magnetic seed material are also acceptable and
within the scope of the present invention. The inventor's
experiments show that permanent magnets with a field strength of
between 0.2 and 3.0 tesla effectively remove these magnetite/metal
sulfide bonded particles. The preferred magnets are high strength
"NdFeB" rare earth magnets, e.g., magnets comprised of neodymium,
iron, and boron.
[0049] As far as known to the inventor, there is no prior art
teaching magnetic seeding in combination with a hydroxide/sulfide
two-step precipitation process, nor practicing magnetic separation
and gravity settling in one treatment vessel.
Dissolved Air Flotation (DAF):
[0050] As a further alternative to, or in addition to, magnetic
separation, enhanced flotation using micro bubbles can be employed
in this present invention. These devices are effective in gently
removing fine metal precipitates. Compressed air is dissolved in
the wastewater; when the pressure is released, the air comes out of
solution in the form of fine bubbles. These bubbles attach to the
fine metal precipitates causing them to float. These particles are
then skimmed off the surface of the water and disposed of.
[0051] There is no known prior art teaching the combination of DAF
units with a hydroxide/sulfide two-step precipitation process as
described in this patent application.
Vortex Separation:
[0052] Another alternative to magnetic separation in the practice
of this present invention is the use of vortex separation. Vortex
separators amount to centrifuges disposed in a tank, to perform
separation of large particles using tangential flow and gravity
forces. This process is enhanced according to this invention by
using magnetite and an anionic flocculant to increase the weight
and size of the precipitates to make them settle more rapidly. The
clear water rises to the top of the vortex separator and the solids
fall to the bottom.
[0053] This present invention is the first known proposal to use
vortex separators to remove metal precipitates following a
hydroxide precipitation step and the first use of magnetite as a
ballast to promote rapid settling in a vortex separator.
[0054] Vortex separation is the preferred embodiment of using field
separation technologies based on gravity because it does not cause
breakup of the fragile metal precipitate floc. However, this
present invention also includes other field separation technologies
(i.e., hydrocycloning and centrifugation) using centrifugal
forces.
Expanded Plastics Flotation:
[0055] In one embodiment, the invention accomplishes the efficient
removal of fine contaminant particles from water by using enhanced
flotation in combination with a two-step precipitation process for
heavy metal removal or as a separate treatment method in a one step
process to remove fine contaminant particles from water. A
flocculating polymer is used to attach a buoyant material, in this
case an expanded plastic, to the fine contaminant particles. In one
embodiment of this invention, an anionic polymer is first mixed
with the fine contaminant particles to be removed, and a second
cationic polymer mixed with the buoyant material, so as to ensure
attraction of the buoyant material to the fine contaminant
particles to be removed. The combined buoyant material and fine
contaminant particles then float to the surface of the water, where
they can readily be removed. After one or more uses, the fine
contaminant particles can be separated from the buoyant material,
enabling reuse thereof.
[0056] A pilot-scale system was constructed with a 15-gallon mix
tank and a variable speed mixer. A mixture of water containing 20
ppm metal sulfide particulates and an anionic flocculant flowed
into the tank, which contained expanded polystyrene (EPS) granules
of 0.025 inches mean size, which had been treated with a cationic
polymer. The variable speed mixer was set at a moderate speed, so
that the EPS was able to contact the metal sulfides and be bound
thereto by the flocculating polymer; more violent mixing would be
expected to prevent effective binding. The flocculant polymers
attached the metal sulfides to the EPS and the flocculated mixture
floated to the top of the tank, forming a floating mat. The use of
a mixer can likely be avoided in a commercial realization of the
invention, and the efficient contacting of the metal sulfides with
the EPS be accomplished by natural mixing in-line. This was
demonstrated in another pilot-scale test, where metal sulfide
precipitates were first flocculated with an anionic polymer. Then
they were mixed inline with EPS that had been treated with a
cationic polymer. The treated water flowed by gravity into the tank
at a rate of 10 gallons per minute and excess water, free of metal
precipitates, was discharged from the bottom of the tank. The EPS
formed a floating mat on the surface of the water. This removed any
particles that had not attached inline to the EPS. This test
demonstrated that the EPS could be attached inline to the metal
sulfides by flocculating polymers and that the floating mat formed
by the combined particles floated to form a secondary collector in
a separator tank. The water that percolated through the floating
mat of EPS and metal sulfides was clear and free of suspended
particles. It was thus demonstrated that the metal precipitates
that did not attach to the EPS in-line attached to the EPS in the
floating mat. Agitation of the ESP was also helpful in flocculating
the fine contaminant particles with the ESP.
[0057] As a final test to prove that the metal precipitates were
being removed by molecular forces provided by the cationic polymer
and not by in-depth filtration, the present inventor gently
agitated the bed to see if the metal precipitates released from the
EPS. The particles did not release from the EPS, showing that
in-depth filtering was not a factor. The particles were being
removed by molecular forces between the anionic and cationic
polymers.
[0058] While not proven, experience leads us to theorize that first
the negative-charged anionic polymer attracts the
positively-charged metal precipitates and forms a floc that now has
a negative charge. The positively-charged cationic polymer attaches
to the EPS giving the EPS a positive charge. When the
negatively-charged metal precipitate comes into contact with the
positively-charged EPS, the opposite charges attract, causing the
metal precipitates to bind to the EPS.
[0059] The flocculating polymer forms a bond between the EPS and
the metal precipitate sufficient to withstand the forces of
flotation and those encountered during removal of the flocculated
materials from the water. When it is necessary to separate the
metal precipitates from the EPS, high-shear mixing can be performed
to break the particle bonds. After the bonds are broken, the
liberated EPS can be separated from the metal precipitate by
flotation, cleaned, and reused. Alternatively, the separation and
cleaning process can include any process that can effectively
separate the fine contaminant particles from the buoyant seed
material, e.g., mechanical separation, pH or chemical treatment,
heat, biological treatment, or ultrasonic treatment. The remaining
fine contaminant particles are removed by gravity from the system
and dewatered with appropriate dewatering equipment. The recovered
EPS can be reused many times; testing has confirmed that there is
no practical limit to the number of times it can be reused.
However, some small quantity of the EPS is lost in the process and
must be replenished as necessary.
[0060] The addition of buoyant material according to the invention
to remove particulates from a water stream is effective when the
particles are small and lightweight and are amenable to flotation.
The invention is suitable for large-scale applications with high
flow rates because few moving parts are involved, the storage
vessels required need only be large enough to contain the water for
a short period of time, and flow is by gravity. The process can
also be performed inline and tanks are not necessary. This makes
the process especially attractive for large-scale operations that
have space and capital cost limitations.
[0061] There are a number of buoyant seed materials that can be
used. Any material that exhibits strong positive buoyancy, will not
become water-logged over time, and can be attached to fine
contaminant particles by flocculating polymers can be used. The
preferred embodiment of the invention is to use an expanded
closed-cell plastic material such as EPS. This material is
available as a waste product, has strong positive buoyancy and is
chemically inert in most circumstances. Suitable waste material
comes in all sizes and can be ground up into small granules without
severely affecting its closed cell structure and its buoyancy. EPS
waste material also comes in a variety of densities. Different
types of materials were tested with no apparent difference in
performance. However, it is preferred that the-materials used have
high buoyancy. The EPS granules are also strong and can withstand
repeated cleanings. If EPS is not chemically compatible with the
wastewater, other expanded plastics such as expanded polyethylene
and expanded polypropylene can be substituted.
[0062] Similarly, any of a wide range of well-known flocculating
polymers can be used. Those used in the tests reported herein,
which appear to be fully suitable, are available from Stockhausen
under product numbers K111L and A3040L.
BRIEF DESCRIPTION OF THE DRAWINGS
[0063] The present invention will be better understood if reference
is made to the accompanying drawings, in which:
[0064] FIG. 1 shows a schematic diagram of a system for practicing
the method of the present invention, using magnetic separation;
[0065] FIG. 2 shows a schematic diagram of a system for practicing
the method of the present invention, using dissolved air
flotation;
[0066] FIG. 3 shows a schematic diagram of a system for practicing
the method of the present invention, using vortex separation;
[0067] FIG. 4 shows a schematic diagram of a system for practicing
the method of the present invention, using expanded plastics
flotation;
[0068] FIG. 5 shows a schematic diagram in plan view of an improved
magnetic separation unit provided according to a preferred
embodiment of the invention;
[0069] FIG. 6 shows an elevation of the unit of FIG. 5;
[0070] FIGS. 7-9 are cross-sections taken along lines 7-7, 8-8, and
9-9, respectively, of FIG. 5; and
[0071] FIG. 10 shows an exploded perspective view of a preferred
assembly for removing the combined flocculated magnetite and
contaminant particulates from the water stream.
DETAILED DESCRIPTION OF THE INVENTION
[0072] The magnetic separation and the expanded plastics flotation
embodiments of the invention were selected for data collection to
demonstrate the effectiveness of a two-stage precipitation process
with a selected "field separation" process according to the
invention. Equipment limitations prevented collecting data for the
DAF and vortex separation embodiments, but it is known in the art
that these technologies can effectively remove fine particles.
[0073] The first step of the method of the invention as employed to
remove heavy metals from a water stream is hydroxide precipitation.
Over a number of laboratory tests from all types of water, the
first step precipitation resulted in removal efficiencies of
95.7-98.5% with an average removal efficiency of 96.8%. This
removal level lowered the dosage requirements for advanced metal
precipitants by 89%. The main goal of this testing was to prove
that lesser quantities of treatment chemicals would be used, while
better metal removal levels would be attained following the
practices of the present invention.
[0074] Chelated copper wastewater samples, taken from the printed
circuit board industry, were selected to demonstrate the
effectiveness of the present invention. Five samples were
separately tested, first using the conventional one-step sulfide
process, and then using the two-step process of the present
invention. The results -of the tests, showing the copper content of
the samples in ppm before and after treatment, are as follows:
TABLE-US-00001 One- and Two-step precipitation data Original
Sulfide Ferrous 1.sup.st Step 2.sup.nd Step Copper Precipitant
Chloride Copper Copper (ppm) (ppm) (ppm) (ppm) (ppm) Sample 1
One-step process 16.4 100 100 0.12 N/A Two-step process 16.4 10 10
0.70 0.01 Sample 2 One-step process 69.3 250 360 0.05 N/A Two-step
process 69.3 50 50 2.87 0.01 Sample 3 One-step process 25.2 150 200
0.47 N/A Two-step process 25.2 10 10 0.73 0.06 Sample 4 One-step
process 46.4 150 300 0.31 N/A Two-step process 46.4 10 20 0.70 0.09
Sample 5 One-step process 21.9 100 200 1.01 N/A Two-step process
21.0 25 50 4.41 0.19
[0075] The above data shows that although the precipitants were
added in lesser quantities in practice of the two-step process of
the present invention, lower copper concentrations in the water
sample were nonetheless achieved. Thus, better water quality as
well as substantial cost savings result from use of the present
invention. The metal precipitants added for the second step of the
precipitation process react with dissolved metals that did not
precipitate in the first step. The above data shows how effective
the two-step precipitation process (hydroxide and sulfide) is over
the one-step precipitation process (sulfide only).
[0076] The metal precipitant used for the second step precipitation
for the tests was an inorganic sulfide and a small amount of a
ferrous salt, e.g., ferrous sulfate, was added to lower the metals
in the treated water. The sulfide precipitates had a
characteristically small particle. They were too fine to easily
settle by gravity and required the use of an organic
flocculant.
[0077] Conventional separation methods can be used, as discussed in
detail below, but a preferred mode of practice of the invention
employs a magnetic separator and a magnetic seed material in the
separation step.
[0078] Magnetic seeding is a technology for the enhanced removal of
magnetic and non-magnetic particulates from liquids. It involves
the addition of a small amount of magnetic seed particles,
preferably magnetite (a naturally occurring iron oxide); however,
other materials showing magnetic susceptibility, such as iron
powder, are suitable and are within the scope of the present
invention. These magnetic seed particles are caused to attach to
non-magnetic metal sulfides by the use of a flocculating polymer
(preferably an anionic polymer). Then a magnetic separator removes
the flocculated particles that have high magnetic susceptibility
derived from the magnetic seed material. This magnetic seeding
technique is applicable to a wide range of liquid wastes including
contaminated groundwater, process waters, municipal wastewater, and
industrial wastewater.
[0079] The effectiveness of magnetic seeding with magnetite is
determined by measuring the level of suspended particles (ppm)
before treatment and after treatment with a magnetic separator. The
magnetic separator can comprise any of a wide variety of devices
producing a magnetic field effective to apply a magnetic force to
water-borne particles exhibiting magnetic properties; in the
preferred embodiment (again, see FIGS. 5-10, below) permanent
magnets are used for reasons of cost with no interruption to
operation for cleaning.
[0080] Tests were performed to determine the effectiveness of the
inventive process to remove fine particles from water. The present
invention was specifically tested against gravity clarification
without the use of magnetite.
[0081] The following data shows how effective the use of a magnetic
seed and a magnetic separator according to the invention is on a
variety of water samples, as compared to a simple gravity-settling
clarification process. Anionic flocculants (Stockhausen A3040L)
were added to all samples. Total suspended solids (TSS)
determination using Hach DR 2010 equipment was the measure of how
effectively fine particles were removed. The industrial wastewaters
and potable water were first treated with metal precipitants that
added to the amount of fine particles to be removed. No metal
precipitants were used with the municipal and storm water samples
because these waters already contained a large quantity of fine
particles and ordinarily do not require metal removal.
TABLE-US-00002 Clarification Magnetic Separator Initial TSS Final
TSS Initial TSS Final TSS Potable water 8 3 8 4 Storm water 550 19
550 8 Municipal wastewater 154 10 154 4 Industrial wastewater 1 67
17 67 13 Industrial wastewater 2 220 9 220 6 Industrial wastewater
3 160 21 160 1
[0082] All final samples were allowed to settle for one minute
before TSS readings were taken. The magnetic separator samples were
then treated with a bar magnet to remove any remaining TSS.
[0083] As is apparent from this data, addition of magnetite and
providing a magnetic separation step according to the present
invention improved the effectiveness of the separation with respect
to each sample (except for the potable water sample), and in some
cases the improvement was well over 100%. In particular, note that
the method of the invention was effective in removing particulates
(essentially dirt) from storm water, that is, without any preceding
treatment step.
[0084] The present inventor then determined whether recirculating
the collected magnetic particles had any adverse effects on the
collection of new magnetic particles. The precipitate collected
from each previous laboratory sample was added to each subsequent
sample. This recirculation of solids improved the flocculation of
the tested sample. The water was visibly clearer and the dissolved
metal levels were lower.
[0085] Although not intending to be bound thereby, the inventor
theorizes that this recirculation of solids lowers the level of
dissolved metals in the wastewater because the reaction has
additional time to go to completion and the recirculated metal
sulfide precipitates absorb additional dissolved metals. The
improved flocculation is a result of having more solids present to
increase the number of collisions between particles, which improves
flocculation.
[0086] Tests were performed to determine the effectiveness of
enhanced flotation using expanded plastics to remove fine particles
from water. The process of the present invention was implemented
essentially as above, and was also specifically tested against
gravity clarification.
[0087] The following data compares the effectiveness of using
expanded plastics according to the invention to remove fine
contaminant particles from a water stream to a simple
gravity-settling clarification process. These tests were performed
on a variety of different samples of water, as listed below. The
total suspended solids (TSS) in each sample were measured using
Hach DR 2010 equipment to determine how effectively fine particles
were removed. The industrial wastewaters and potable water were
first treated with sulfide metal precipitants that formed
particulates, thus adding to the amount of fine particles to be
removed. No metal precipitants were added to the municipal and
storm waters because these waters already contained a large
quantity of fine particles and ordinarily do not require metal
removal.
[0088] The expanded plastics (in this case EPS), was added to each
sample in the amount of 1 percent by weight. For comparison
purposes, ferrous sulfide was added at a concentration of
approximately 50 ppm. The flocculant used comprised a cationic
polymer added to the EPS at a dose of 20 ppm and an anionic polymer
added to the water stream containing the ferrous sulfide
particulates at a dose of 10 ppm.
TABLE-US-00003 Clarification Expanded Polystyrene Initial TSS Final
TSS Initial TSS Final TSS Potable water 8 3 8 5 Storm water 550 19
550 12 Municipal wastewater 154 10 154 8 Industrial wastewater 1 67
17 67 12 Industrial wastewater 2 220 9 220 7 Industrial wastewater
3 160 21 160 14
[0089] The clarifier samples were allowed to settle for one minute
and the EPS samples were allowed to float for one minute before TSS
readings were taken.
[0090] As is apparent from this data, seeding the water with
expanded plastics and flocculating polymers according to the
invention improved the effectiveness of the separation with respect
to each sample; in some cases, the improvement was well over
100%.
[0091] It will be apparent to those skilled in the art that
allowing the samples to settle longer in the clarifying tanks would
have led to improved results. However, as noted, it is generally
the case that some fraction of the particulates are not removed in
the clarifying process; accordingly, deliberately enhanced
flotation and removal according to the invention can yield
substantially improved results regardless of the amount of settling
time provided in a clarifying tank.
[0092] While improvement in clarification is important, it is also
important to be able to process water quickly. The biggest drawback
to clarification by settling is that it takes large-capacity
equipment to be able to process reasonable quantities of water, as
the required residence time in a clarifier may range from 30
minutes to several hours. If a clarifier is being used, it is
important to maintain a slow and non-turbulent flow to allow the
flocculated particles to settle. Contrary to this, the present
invention allows rapid flow because the buoyant flocculate will
float very rapidly and completely. The residence time to remove the
fine particles with this invention is in the order of one minute as
compared to the 30-minute minimum required for clarification by
gravity settling without added ballast.
[0093] Another test was performed to evaluate the effectiveness of
separating the EPS particles that have been bound in-line to the
metal precipitates from the treated water. This was easily
accomplished by discharging the flow into a separation tank. The
EPS floated on the surface of the water while the clear treated
water was withdrawn from the bottom of the tank. The EPS floating
on the surface of the water in the tank formed a mat which removed
any metal precipitates that had not been bound to the EPS
in-line.
[0094] In conclusion, the laboratory tests discussed above show
that an expanded plastic material such as EPS can be effectively
attached to fine contaminant particles with the use of flocculating
polymers, preferably anionic and cationic polymers, one flocculated
with the metal precipitates and the other bound to the EPS. The
combined particles floated on the water's surface and were easily
removed. The resulting water was clear of suspended solids to the
naked eye, and resulted in low suspended solids as measured by Hach
equipment. Mixing under high shear conditions easily separated the
EPS and particulates bound thereto, allowing the EPS to be reused
over and over again. The shear mixing to separate the EPS from the
fine particles did not noticeably affect the buoyancy of the
EPS.
[0095] Tests were performed to prove that the metal precipitates
were attached to the EPS in the floating mat by molecular forces
rather than by in-depth filtration, that is, by molecular forces
rather than by filtration in a filter such as a sand filter, where
the particles are trapped in the interstitial spaces between the
sand particles, but are not attached to the sand, so that if the
bed is disturbed the particles will come free. Three samples were
prepared for testing. One sample constituted the blank and
contained EPS treated with a cationic polymer, one sample contained
EPS treated with an anionic polymer, and the third sample contained
EPS treated with a cationic polymer. Then a sample of water
containing a metal hydroxide flocculated with an anionic polymer
was passed through the sample containing the EPS with the anionic
polymer, and an equal quantity of water containing a metal
hydroxide flocculated with an anionic polymer was passed through
the sample containing the EPS treated with the cationic polymer.
The water that percolated through the EPS floating mats were tested
for Total Suspended Solids to determine the EPS's ability to remove
the metal precipitates. Then the floating beds of EPS were gently
agitated to see if the metal precipitates would be released back
into the water. The following data was collected:
TABLE-US-00004 TSS (after TSS bed agitation) Blank (EPS treated
with 13 13 a cationic polymer) Sample 1 (EPS treated with an 191
>3800 anionic polymer plus metal hydroxides) Sample 2 (EPS
treated with a 10 8 cationic polymer plus metal hydroxides)
[0096] In summary, the EPS treated with a cationic polymer was able
to collect all the metal precipitate with no breakthrough (Sample
2). The TSS reading was actually less after the metal hydroxides
were added. The Sample 1 containing the anionic polymer had
breakthrough of the metal hydroxides. After agitating the beds,
almost all of the metal hydroxides went back into the water for the
sample containing the anionic polymer. However, the sample
containing the cationic polymer (Sample 2) still retained all the
metal hydroxides even after the bed was agitated.
[0097] FIGS. 1-4, described in the following, schematically
illustrate various process arrangements that may be used to
implement the present invention. Again, the preferred mode employs
magnetic separation techniques and expanded plastics flotation, but
the other techniques mentioned are within the scope of the
invention, and may be preferred in various circumstances.
Magnetic Separation: FIG. 1
[0098] In this implementation of the invention, water first enters
through pipeline (37) into a pH adjustment tank (4) including a
high speed mixer (2), in which the pH is adjusted with either acid
(1) or caustic (3) to the optimum pH for metal hydroxides to form.
In most cases the acid is sulfuric acid, and the caustic is usually
sodium hydroxide or lime. For mixed metal solutions, the pH that
removes the most metals is selected. The water then flows through a
pipeline (5) into a flocculation tank (8) where an anionic polymer
(6) is added to flocculate the hydroxide precipitate. A slow speed
mixer (7) aids the formation of floc. pH adjustment and hydroxide
formation thus comprise a first precipitation step. The water and
metal hydroxide precipitates then flow through a pipeline (9) into
a clarifier (10) where the metal hydroxide precipitates settle out
of the wastewater and flow through a pipeline (30) and into a
sludge settling tank (29). From here the slurry flows through
pipeline (31) and is pumped (32) through a pipeline (33) to a
filter press (34) for dewatering. The dewatered sludge is
discharged to a hopper (35) and disposed of or recycled. The
filtrate from the filter press (34) flows back through pipeline
(11) and is treated for metal removal.
[0099] A metal precipitant is added at (12), at the discharge from
the clarifier (10) The metal precipitant can be any of a variety of
materials. The most common are sulfides, either organic or
inorganic, but in a few cases, other precipitants like borohydride
can be added; the criterion is simply that an insoluble compound is
formed, thus causing the second precipitation step to occur. A
ferrous compound may be added, as a metal precipitant or in order
to cause other precipitants to be more effective, by breaking
chelate bonds.
[0100] The metal precipitates then flow through a pipeline (14) to
a flocculating tank (15). Here an anionic flocculating polymer (17)
is added to flocculate the metal precipitate with the recycled
magnetic seed material coming from pipeline (26). It is preferred
to add the metal precipitant first, to produce a completed reaction
with the heavy metals, and then to add the polymer flocculant, This
gives the precipitated particles the maximum time to attach to the
magnetite with the aid of the polymer flocculant. A slow speed
mixer (16) aids in the flocculation process. The flocculate then
flows through pipeline (18) into a separator tank (19). In this
embodiment, separation is accomplished by a combination of gravity
and magnetic separation. More specifically, a large fraction, on
the order of 90% (depending on the area of the settling tank and
the average residence time) of the flocculated particles, again
comprising the precipitated metals to be removed having been bonded
by the flocculating polymer to the magnetite, forms a heavy
precipitate that settles to the bottom of the separator tank as a
dense sludge by gravity; the remaining fraction, principally
lighter fine particles, is swept into the upper region of the
separator tank by the water flow.
[0101] These particles are captured by a magnetic separator (20) to
prevent their discharge through pipeline (36). A scraper (21) is
installed in the separator tank (19) for cleaning separator (20)
when it becomes heavily laden with magnetic particles. FIGS. 5 and
6, discussed below, further detail these components. The particles
scraped from the magnetic separator (20) tend to clump with one
another because of a slight magnetic charge, forming heavier
particles, and then settle to the bottom of the separator tank (19)
and are discharged through pipeline (22). A pump (23) then pumps
the sludge to a magnetic seed cleaning tank (25). Here the magnetic
seed material is separated from the metal precipitates. The
magnetic seed material flows through pipeline (26) and is reused in
the process. The metal precipitates flow through pipeline (28) into
the sludge settling tank (29) for subsequent dewatering and
disposal. Substantial improvements on the FIG. 1 apparatus are
discussed below in connection with FIGS. 5-10.
Dissolved Air Flotation: FIG. 2
[0102] In this implementation of the invention, dissolved air
flotation (DAF) techniques are used in either or both precipitation
step(s); in the example, DAF separation replaces the magnetic
separation used in the second precipitation stage of the process of
FIG. 1. DAF techniques are preferred where the materials to be
removed are light, e.g., oil or grease, or low-density
particulates. Water enters through pipeline (30) into a pH
adjustment tank (4) in which, impelled by a high-speed mixer (2),
the pH is adjusted with either acid (1) or caustic (3) to the
optimum pH for metal hydroxides to form. For mixed metal solutions,
the pH that removes the most metals is selected. The water then
flows through a pipeline (5) into a flocculation tank (8) where a
polymer (preferably anionic) (6) is added to flocculate the
hydroxide precipitate. A slow-speed mixer (7) aids the formation of
floc. The water and metal hydroxide precipitates then flow through
a pipeline (9) into a clarifier (10) where metal hydroxide
precipitates settle out of the wastewater and flow through a
pipeline (21) into a sludge settling tank (22). From here the
slurry, that is, the hydroxide particles in a slurry with water,
flows through a pipeline (23) and is pumped by a pump (24) through
a pipeline (25) into a filter press (26) for dewatering. The
dewatered sludge is discharged to a hopper (28) and disposed of or
recycled. The filtrate from the filter press flows through a
pipeline (11) back to the discharge point of the clarifier for
metal removal. At the discharge from the clarifier (10), metal
precipitant (12) and ferrous material (13) are added. The
flocculated particles flow through a pipeline (14) and into a
flocculation tank (16). Here a polymer (preferably an anionic
polymer) is added at point (15) and the solution allowed to mix
with the aid of a slow-speed mixer (17). The flocculated particles
then flow through a pipeline (18) into separation tank (19), in
this embodiment configured as a dissolved air flotation (DAF)
device. Air is injected at point (28), which attaches to the
flocculated particles causing them to rise to the top of the
separation tank (19), as a froth containing the "sludge" to be
removed. The sludge-containing froth then flows through pipeline
(20) into the sludge settling tank (22) and clean water from the
separation tank (19) flows out the bottom and discharged through
pipeline (29). As above, the sludge collected in the sludge
settling tank (22) flows through pipeline (23) and is pumped (24)
through pipeline (25) and into a filter press (26) for dewatering.
The dewatered sludge is discharged into a collection hopper (27)
and disposed of, while the filtrate flows through pipeline (11)
into the discharge from the clarifier (10) for metal removal.
Vortex Separation: FIG. 3
[0103] In this implementation of the invention, vortex separation
is employed instead of magnetic separation in a two-step
precipitation process. Again, water first enters through pipeline
(29) and into a pH adjustment tank (4) in which the pH is adjusted
with either acid (1) or caustic (3) to the optimum pH for metal
hydroxides to form; a mixer (2) may be provided to ensure proper
mixing. For mixed metal solutions, the pH that removes the most
metals is selected. The water then flows through a pipeline (5)
into a flocculation tank (8) where a polymer (6) (preferably
anionic) is added to flocculate the hydroxide precipitate. A slow
speed mixer (7) aids the formation of floc. The water and metal
hydroxide precipitates then flow through a pipeline (9) to a
clarifier (10) where metal hydroxide precipitates settle out of the
wastewater and flow through a pipeline (21) into a sludge settling
tank (22). From here the. slurry flows through pipeline (23) and is
pumped (24) through a pipeline (25) to a filter press (26) for
dewatering. The dewatered sludge is discharged to a hopper (27) and
disposed of or recycled. The filtrate flows through pipeline ( 11)
from the filter press back to the discharge point of the clarifier
(10) for metal removal. At the discharge from the clarifier (10),
metal precipitant (12), and ferrous material (13) are added to
precipitate any residual metals. The metal precipitates flow
through a pipeline (14) and into a flocculation tank (16). Here a
polymer (17) (preferably an anionic polymer) is added and the
solution allowed to mix with the aid of a slow-speed mixer (15) to
increase the floc size. The flocculation formed then flows through
a pipeline (18) into separation tank (19), in this case comprising
a vortex separator. The vortex separator is nothing more than a
vertical cylindrical tank (19), with the water entry is arranged so
that water enters tangentially and swirls up to the top. The
heavier particles congregate in the center of the tank, settle to
the bottom under the influence of gravity, and can readily be
withdrawn. The clean water rises to the top of the separation tank
(19) and is discharged through pipeline (28) and discharged, while
the heavy particles settle to the bottom of the tank (19). The
sludge from the bottom of the tank (19) then flows through pipeline
(20) into the sludge settling tank (22). Here the precipitates
settle to the bottom and are discharged through a pipeline (23) and
are pumped (24) through pipeline (25) into a filter press (26) for
dewatering. The dewatered sludge is discharged to a hopper (27) and
disposed or recycled. Magnetite can be used in the process to
promote rapid settling in the separation tank (19). When cost
justified, the magnetite can be recycled by shearing the bond
between the metal sulfides and magnetite with high-speed agitation
and returning the magnetite back to the flocculation tank (16).
Expanded Plastics Flotation: FIG. 4
[0104] In this embodiment, a novel expanded plastic flotation
technique is used to separate the precipitated particles or other
fine contaminant particles from the water stream. Again, water
first enters through pipeline (35) into a pH adjustment tank (4)
including a high speed mixer (2), in which the pH is adjusted with
either acid (1) or caustic (3) to the optimum pH for metal
hydroxides to form. For mixed metal solutions, the pH that removes
the most metals is selected. The water then flows through a
pipeline (5) into a flocculation tank (8) where an anionic polymer
(6) is added to flocculate the hydroxide precipitate. A slow speed
mixer (7) aids the formation of floc. The water and metal hydroxide
precipitates then flow through a pipeline (9) into a clarifier (10)
where metal hydroxide precipitates settle out of the wastewater and
flow through a pipeline (28) and into a sludge settling tank (27).
From here the slurry flows through pipeline (29) and is pumped (30)
through a pipeline (31) to a filter press (32) for dewatering. The
dewatered sludge is discharged to a hopper (33) and disposed of or
recycled. The filtrate from the filter press (32) flows back
through pipeline (11) and is treated for metal removal. At the
discharge from the clarifier (10), metal precipitant (12) and, if
desired, a ferrous compound (13) are added. The metal precipitate
particles then flows through a pipeline (14) to a flocculating tank
(15). Here an anionic flocculating polymer (17) is added to
flocculate the metal precipitate. A slow speed mixer (16) aids in
the flocculation process. The flocculate then flows through
pipeline (18) into a separator tank (19). In this embodiment of the
invention, the separator tank (19) contains a floating bed of
granulated expanded polystyrene (EPS). The EPS has a positive
charge from the addition of a cationic polymer at point (24). The
flocculated metal precipitates from the flocculation tank (15) have
a negative charge from the addition of an anionic polymer at point
(17). Accordingly, when the metal precipitate comes into contact
with the EPS, their opposite charges attract, causing them to stick
together; the buoyancy of the expanded plastic material causes the
agglomerated particles to float to the top of the liquid in tank
(19). Clean water is discharged through pipeline (34), near the
bottom of tank (19). The dirty EPS is withdrawn by way of a
pipeline (20) located in the separation tank (19) with its inlet
just below the floating EPS and is pumped by pump (21) through a
pipeline (22) to a cleaning tank (25). The action of the pump (21)
causes the metal precipitates to be sheared away from the EPS. The
cleaned EPS goes back into the process through pipeline (23) to be
used over again and the metal precipitate sludge flowing through
pipeline (26) goes into the sludge settling tank (27) for eventual
dewatering and disposal. The same process can be used to separate
out particulates from a water stream, e.g., silt from storm runoff;
again, a flocculant polymer can be used to attach an expanded
plastic material to the particulates, which are then removed by
flotation, as above.
[0105] FIGS. 5-9 illustrate schematically a preferred magnetic
separation unit 98. FIG. 5 shows the magnetic separation unit 98 in
plan view, FIG. 6, an elevation of the unit 98, and FIGS. 7 - 9 are
cross-sections taken along lines 7-7, 8-8, and 9-9, respectively,
of FIG. 5. FIG. 10 shows an exploded perspective of a component
thereof.
[0106] Magnetic separation unit 98 unit is-useful in the FIG. 1
embodiment of the invention, which removes non-magnetic pollutant
particles from water using magnetic seeding technology. These
particles may included heavy metals, precipitated after the
two-stage process discussed in detail above, particles precipitated
after a single-stage precipitation, or particulates present in the
water stream, e.g., dirt--more specifically, silt, clay, organic
waste, inorganic waste, metal precipitates, etc., in storm water,
wash water, or the like. Briefly, removal of non-magnetic pollutant
particles is accomplished by binding the pollutant particles to a
magnetic seed material (preferably magnetite, ferrosilicon, or
other known ferromagnetic compounds) using an organic flocculating
polymer. A substantial fraction of the resultant magnetic particles
are then removed by settling out, and the remainder by a permanent
magnet disk collector, employed as discussed below in connection
with FIG. 10.
[0107] Referring now to the detailed illustration of the apparatus
shown in FIGS. 5-9, a stream of water to be treated flows into a
flocculation chamber (72) at an inlet (70). A magnetic seed
material (e.g., magnetite, ferrosilicon, hematite, ferrite,
zero-valent iron, or others) and a flocculating polymer (e.g.,
Stockhausen 3040L) are added to the water stream in chamber 72. A
floc mixer (87), driven by a motor (73) is provided to ensure the
seed material and flocculant thoroughly contact the pollutant
particles, so that a "floc", comprising the combination of the
magnetic material and the particles to be removed, held together by
the organic flocculating polymer, is efficiently formed. The floc
exhibits ferromagnetic properties, so that it is attracted to a
magnet. The water stream including the floc then flows out through
a drain at the bottom of the flocculation chamber (72) and into a
settling chamber (74), as indicated by arrows (81). Because of the
high density of the floc, most of the flocculated particles will
settle to the bottom of the settling chamber (74) and flow back
into the flocculation chamber (72). More specifically, the bottom
of the settling chamber (74a) is open to the flocculating chamber
(72), and is angled, so the particles will move towards the
flocculating chamber (72) and are swept away by the mixing action
of the floc mixer (97). The water stream, having been cleaned,
enters a trough (81) and then exits unit (98) at (78), while the
flocculated particles that do not settle by gravity but remain
suspended in the water stream are collected on a magnetic disk
collector (76), due to the ferromagnetic properties of the seed
material. As discussed in detail in connection with FIG. 10 below,
the combined particles are scraped off the magnetic disk collector
(76) and fall to the bottom of the settling chamber (74). The
reason they are not re-entrained back onto the magnetic disk
collector (76) after having been scraped therefrom, is that the
particles possess a small magnetic charge that causes them to clump
together after collection on the disk collector (76) and scraping
therefrom. These clumps are heavy and settle rapidly to the bottom
of the settling chamber (74), even though individual particles may
not thus settle. Clean water, free of pollutant particles and
magnetite, flows through the magnetic disk collector (76) and into
a trough (81) and then into the outlet (78).
[0108] Over time, the magnetite becomes dirty with the precipitates
and other fine pollutants and has to be cleaned so it can be
reused. This is accomplished by collecting the dirty magnetite in
the flocculation chamber (72) on a magnetic drum roller (80),
mounted so as to be partially submerged in the flocculation chamber
(72). The magnetic drum roller (80) is shown in more detail by FIG.
8. As illustrated, drum roller (80) (as well as other rollers
discussed below) are driven by motor (79), through a gear train
located above the water line to minimize the effects of abrasive
magnetite on the gears. The combined particles adhere to the
magnetized drum (80). Drum (80) may comprise a stationary inner
ferromagnetic cylinder, lined with magnets, disposed within a
rotating plastic outer cylinder. As drum (80) rotates, the
magnetite rotates with it until it reaches a scraper (82), held
firmly against the surface of rotating drum (80). The dirty
magnetite then is scraped off the magnetic drum roller (80) by
scraper (82), and then falls into a trough (83) at the front (in
FIG. 5, that is) of the flocculating chamber (72), and flows
downwardly, as indicated by arrows (83a) into a shear mixing
chamber (93), in which is located a shear mixer, comprising a
single speed motor (84), a mixer shaft (84a) and high shear mixer
blades (89) . The shear mixer uses mechanical forces to break the
flocculation bond between the pollutant particles and the
magnetite. The sheared liquid then flows (91) out of the shear
chamber (93) and onto the surface of another, similarly-constructed
gear-driven magnetic drum roller (86), which collects the clean
magnetite. A second scraper (88) scrapes the clean magnetite into
the flocculation chamber (72) for reuse. See FIG. 7. A semicircular
trough 86a serves as a housing around the drum roller (86) and
prevents the sludge that is not collected on the drum roller from
going back into the floc tank (72). Accordingly, the non-magnetic
pollutant particles not collected by the magnetic drum roller (86)
flow out of the unit (98) at an outlet (90) for dewatering and
disposal, typically after processing by a conventional sludge
dewatering system, employed to form a solid filter cake.
[0109] FIG. 10 shows an exploded perspective view of a set (66) of
three disks; several similar sets (66) of such disks, which are
disposed such that the top of the disks are just above the level of
the water in settling tank 74 (FIGS. 5 and 6), comprise the
magnetic disk collector (76). As noted, FIG. 9 shows an end view of
the assembly (76). Each of the disks of each set (66) is made of
plastic or other suitable non-ferromagnetic material such as 300
series stainless steel. Each set (66) comprises two outer wear
disks (42, 44) which rotate freely around a fixed shaft (46), and a
fixed inner magnet disk (48) which is attached to shaft (46) by a
key (50) and does not rotate. A drive gear (40), which is mounted
above the water, is driven by motor (79) and drives at least the
wear disks (42, 44) of each set of disks by meshing with gear teeth
formed on their peripheries. The outer wear disks (42) and (44) are
secured to a ring gear (62), rotating freely on the periphery of
inner magnet disk (48), e.g., by screws (60); ring gear (62) is
driven by drive gear (40), so that the outer wear disks (42) and
(44) rotate together with ring gear (62).
[0110] In use, wear disks (42, 44) are closely juxtaposed to the
inner magnet disk (48), which contains permanent magnets (52),
retained in recesses in disk (48), so that magnetic particles in
the water, that is, the flocculated pollutant particles and
magnetite or other ferromagnetic particles, are attracted to and
retained on the outer surfaces of outer wear disks (42, 44), that
is, on their surfaces not juxtaposed to inner disk (48). 0099 A
sector (54), e.g., the lower quadrant of the inner magnet disk (48)
does not contain any permanent magnets (52), providing an area for
scrapers (56) engaging the outer surfaces of the outer wear disks
(42, 44) to clean the magnetic disk collector; that is, because the
magnetite is not urged against the surfaces of the wear disks (42,
44) by the permanent magnets (52) in these areas, cleaning the
magnetic disk collector by scraping becomes easier. It is
preferable to form radially-extending score marks (58) on the outer
surfaces of the wear disks (42, 44) to cause the magnetite to move
to the region of the scrapers (56) as the wear disks (42, 44)
rotate; if these surfaces are smooth, the magnetite tends to remain
in the vicinity of the permanent magnets (52) and not move with the
disks, and is thus not amenable to being removed by the scrapers
(56). If the outer wear disks (42, 44) are formed of plastic,
suitable score marks can be made by hand using a sharp implement or
knife blade; if the disks are, for example, non-magnetic stainless
steel, presumably a machine operation would be required. Having a
sector void of magnets on the lower quadrant of the inner magnet
disk (48) and having score marks (58) on the outer wear disks
(42,44) are important to the design of the magnetic disk collector
(66) because of the use of NdFeB permanent magnets, as is
preferred. The NdFeB permanent magnets are extremely powerful,
having a coercive field strength of 900-1000 kA/m. By comparison,
in Nilsson patent U.S. Pat. No. 3,980,562, the preferred magnets
have a coercive field strength of 100-200 kA/m.
[0111] Thus, the operation of the magnetic separation unit of FIGS.
5-9 can be summarized as follows: A stream of water containing
particles to be removed therefrom is admitted to flocculation
chamber (72) and is there mixed with particles of a ferromagnetic
material and with a flocculating polymer, under mixing conditions
such that a floc is formed. The mixing conditions are adjusted
according to a fine balance between too much agitation, which
breaks the floc, and insufficient agitation to keep the magnetite
suspended. The magnetite needs to be in suspension in order to get
good contact with the flocculating polymer in the floc tank and
with new particles entering the tank. When this flow leaves the
agitated floc tank into the settling tank the floc settles out, and
slides down inclined wall 74a, back into the floc tank. More
specifically, the water and floc leave the flocculation chamber
(72) through an underflow (81) into the settling chamber (74). The
flow rate through the settling chamber is such that most of the
particles settle out and back into the flocculation chamber (72); a
typical Surface Overflow Rate (SOR) is less than about 10 gallons
per minute per square foot of surface area of the settling chamber
(74). At this SOR, a smaller fraction of the floc remains suspended
in the water stream, but is separated therefrom in magnetic disk
collector 76 (detailed in FIG. 10); the "clumped" floc then settles
and is removed as with the major portion having settled previously.
Cleaned water is discharged through a trough (81) and outlet (78) .
Floc is attracted to the drum roller (80) and scraped therefrom by
scraper (82) into trough (83); the magnetite is separated from the
remainder of the floc in high-shear mixing chamber (93) and
returned to the floc chamber (72) for reuse, while the pollutants
and flocculating polymer are discharged at (90), for dewatering and
disposal.
[0112] An advantage of this magnetite cleaning system is that the
magnetite (or other magnetic seed material) is removed as a wet
solid or heavy sludge, rather than as a slurry; the sludge requires
much less further processing (e.g, dewatering) for reuse than would
a slurry, and the process is accordingly less costly.
[0113] While several preferred embodiments of the invention have
been disclosed, those of skill in the art will recognize that
numerous modifications, enhancements and improvements thereto are
possible without departure from the scope of the invention.
* * * * *