U.S. patent application number 11/908109 was filed with the patent office on 2008-08-28 for producing calcium cyanide at a mine site using easily transportable starting materials.
This patent application is currently assigned to Nevada Chemicals, Inc.. Invention is credited to John T. Day.
Application Number | 20080203809 11/908109 |
Document ID | / |
Family ID | 36992034 |
Filed Date | 2008-08-28 |
United States Patent
Application |
20080203809 |
Kind Code |
A1 |
Day; John T. |
August 28, 2008 |
Producing Calcium Cyanide At A Mine Site Using Easily Transportable
Starting Materials
Abstract
The present invention is a method by which the mine can obtain a
supply of calcium cyanide for use in its leaching operations.
Specifically, one preferred method within the scope of the
invention involves producing hydrogen cyanide directly at a mine
site using formamide as a starting material. Alternative methods of
producing hydrogen cyanide at the mine site using easily
transportable starting materials, such as methanol and urea, are
disclosed. The hydrogen cyanide is neutralized at the mine site
with slaked lime (Ca(OH).sub.2) to form the calcium cyanide.
Inventors: |
Day; John T.; (Sandy,
UT) |
Correspondence
Address: |
KIRTON AND MCCONKIE
60 EAST SOUTH TEMPLE,, SUITE 1800
SALT LAKE CITY
UT
84111
US
|
Assignee: |
Nevada Chemicals, Inc.
Sandy
UT
|
Family ID: |
36992034 |
Appl. No.: |
11/908109 |
Filed: |
March 10, 2006 |
PCT Filed: |
March 10, 2006 |
PCT NO: |
PCT/US06/08816 |
371 Date: |
September 7, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60661182 |
Mar 11, 2005 |
|
|
|
Current U.S.
Class: |
299/5 |
Current CPC
Class: |
C01C 3/0212 20130101;
C01C 3/0241 20130101; C22B 11/08 20130101; C01C 3/08 20130101; C01C
3/0204 20130101 |
Class at
Publication: |
299/5 |
International
Class: |
E21B 43/28 20060101
E21B043/28 |
Claims
1. A method for producing an aqueous solution of calcium cyanide
directly at a mine site, the method comprising: obtaining a supply
of formamide at the mine site; reacting the formamide at the mine
site to produce a quantity of hydrogen cyanide; and contacting the
quantity of hydrogen cyanide at the mine site with slaked lime to
produce a supply of calcium cyanide.
2. The method according to claim 1 wherein the slaked lime
comprises primarily calcium hydroxide.
3. The method according to claim 1 further comprising the step of
diluting the calcium cyanide to concentrations that may be used in
at the mine site in at least one mining operation.
4. A method for producing an aqueous solution of calcium cyanide
directly at a mine site, the method comprising: obtaining a supply
of ammonia at the mine site; obtaining a supply of methanol at the
mine site; reacting the ammonia and methanol at the mine site to
produce a quantity of hydrogen cyanide; and contacting the quantity
of hydrogen cyanide at the mine site with slaked lime to produce a
supply of calcium cyanide.
5. The method according to claim 4 wherein the ammonia is obtained
by decomposing urea.
6. The method according to claim 4 wherein the slaked lime
comprises primarily calcium hydroxide.
7. The method according to claim 4 further comprising the step of
diluting the calcium cyanide to concentrations that may be used in
at the mine site in at least one mining operation.
8. A method of mining precious metals using a calcium cyanide
solution comprising: producing the calcium cyanide solution at a
mine site from at least one starting material; leaching an area of
interest at the mine site with the cyanide solution; collecting a
pregnant solution which contains a quantity of precious metal ions
bound to cyanide anions as an aqueous complex; and recovering the
precious metal ions from the pregnant solution.
9. The method of mining according to claim 8 wherein the producing
step comprises decomposing formamide to form hydrogen cyanide and
neutralizing the hydrogen cyanide with slaked lime to form the
calcium cyanide solution.
10. The method of mining according to claim 8 wherein the producing
step comprises creating the cyanide solution using methanol and
urea as the starting materials.
11. The method of mining according to claim 8 wherein producing
step comprises producing a quantity of hydrogen cyanide that is
then converted into a solution of calcium cyanide using lime or
slaked lime.
12. A method for producing calcium cyanide directly at a mine site,
the method comprising: obtaining a supply of the starting materials
at the mine site for use in an Andrussow process for producing
hydrogen cyanide; reacting the starting materials at the mine site
to produce a quantity of hydrogen cyanide; and processing the
quantity of hydrogen cyanide at the mine site to produce a supply
of calcium cyanide.
13. The method according to claim 12 wherein the processing step is
accomplished by contacting the hydrogen cyanide with slaked lime to
produce the calcium cyanide.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims the benefit of U.S. Provisional
Application No. 60/661,182, filed Mar. 11, 2005.
BACKGROUND OF THE INVENTION
[0002] The present invention relates to producing cyanide for
mining applications. More specifically, the present invention
relates to producing cyanide for mining applications at the mining
site using starting materials that are readily transportable.
[0003] Aqueous solutions of cyanide anions (or "cyanide") are used
extensively in the mining industry as a mechanism for extracting
precious metals, such as gold and silver, from the earth.
Generally, these aqueous cyanide solutions contain sodium cyanide,
potassium cyanide, or cyanide salts of other alkali metals.
Although miners have tried to find other means of winning the gold
from the ores, cyanide extraction continues to be the only
cost-effective mining technique. Accordingly, it is believed that
cyanide will remain the material of choice for precious metal
mining.
[0004] Cyanide mining is usually accomplished in either a heap
leaching or vat leaching operation. In these leaching operations,
ore is contacted with a basic, dilute solution of cyanide. The
solutions of cyanide typically have a concentration of less than
1000 ppm. In these basic conditions, the cyanide will react with
the solid metal and oxygen in the air to form a soluble, aqueous
complex, which is generally in the form of [Ag(CN).sub.2].sup.- or
[Au(CN).sub.2].sup.-. Once the leaching process is completed, the
aqueous complex of the precious metal is then collected and
concentrated so that the precious metal may be extracted and
reduced into its metallic state. The cyanide used in the leaching
process is also recycled for further leaching and/or disposed of in
accordance with governmental regulations. During the process, some
cyanide oxidizes and some is lost for further use.
[0005] Today the majority of the precious metal mining industry
receives its cyanide for these mining processes either in the form
of a liquid solution or in the form of dry sodium cyanide
briquettes. The briquettes are generally manufactured in large
integrated chemical manufacturing sites. Several main producers of
alkali cyanide worldwide are:
(1) E.I. du Pont de Nemours and Company (which has cyanide
facilities in Memphis, Tenn. and Texas City, Tex.); (2) Degussa AG
(a German company that has cyanide facilities in Wessling,
Germany); and
(3) Tae Kwang Industrial Co., Ltd. (Seoul, South Korea);
(4) Tongsuh Petrochemical Corporation, Ltd., (Seoul, South Korea);
and
[0006] (5) Australian Gold Reagents Pty. Ltd., (Kwinana, Western
Australia).
[0007] Generally, the product of choice for precious metal mines
using the cyanide leaching process is sodium cyanide (NaCN). This
product is usually manufactured from the neutralization of hydrogen
cyanide (HCN) and caustic soda (NaOH). The HCN is produced either
on purpose (primarily through the Andrussow Process or BMA Process)
or from the reaction to produce acrylonitrile (in which one of the
byproducts of this reaction is HCN). There is some production of
hydrogen cyanide from other processes, but this is limited and is
generally not a source for NaCN. Once formed, the NaCN is then
dried and made into briquettes to assist in shipping the product
over greater distances. However, if the NaCN is produced close to
the mine site, the product of choice for the mine is usually a 30%
solution of NaCN.
[0008] To produce HCN through the Andrussow Process, the raw
materials are natural gas (methane, CH.sub.4) and ammonia
(NH.sub.3). These gasses are reacted at elevated temperatures
(2000.degree. F.) over a platinum-rhodium catalyst. The reaction is
endothermic requiring the addition of heat. The reaction
essentially involves the ammoxidation of methane, e.g. partial
oxidation affected by the addition of oxygen (air). HCN is produced
in this reaction as one of the products.
[0009] In some instances this HCN product is separated from the
other product gases by first removing ammonia (using either a
sulfuric acid scrubber or phosphoric acid). The HCN is then
absorbed into water, which is distilled to obtain pure HCN. Upon
removal of undesired impurities, like ammonia, carbonates,
formates, etc. the HCN can be used in other downstream
manufacturing processes including the production of NaCN which is
dried. In other applications the HCN product gases are contacted
directly with caustic soda to produce a NaCN solution. A NaCN
solution is produced when the purchasing mine is within a distance
suitable for freight logical shipping of the solution rather than a
dry NaCN product, such as in the gold fields of Australia, South
Africa, Uzbekistan, China and/or the United States.
[0010] When the acrylonitrile process is used to produce HCN, the
HCN is essentially a byproduct of the acrylonitrile. However, in
order to comply with current environmental/safety regulations, the
manufacturing plant must treat and/or dispose of this HCN. Thus,
these plants often choose to convert the HCN into NaCN and sell
this additional product to the mining industry.
[0011] Despite the market for cyanide, there are growing concerns
in the mining industry that supplies of cyanide will not be
available at a commercially viable price in the future. It is
anticipated that older facilities and technology currently used to
manufacture hydrogen cyanide will become obsolete, thereby reducing
the amount of HCN that is currently being produced and sold. Thus,
the overall cost of cyanide is expected to increase in the
future.
[0012] Cyanide is defined as an extremely hazardous chemical. In
fact, in the public's view, the use of cyanide is considered a
threat to the environment and a possible tool for the use of
terrorists. In the United States, some railroads will not transport
cyanide. It is becoming more difficult to ship cyanide
internationally. Exporting manufacturers must currently obtain
export licenses from governmental agencies which define legitimate
end users and location of use. Many international mining operations
are concerned that future restrictions/regulations may restrict
shipping cyanide across the oceans, shipping cyanide through ports
in less developed regions of the world, and/or transporting cyanide
over poorly maintained and/or less-traveled roads. Such additional
restrictions will make shipping cyanide prohibitively expensive (or
even impossible) and may jeopardize the commercial viability of
existing or future mines.
[0013] Even with developed countries, there is a strong movement
from lobbyists to decrease the use of cyanide in mines. Although it
is expected that these lobbyists will mandate additional safeguards
on mines, an outright ban of cyanide is not expected. However,
there are certain states in the United States that have outlawed
under certain conditions the use of cyanide, namely Wisconsin and
Montana. Other states restrict the use of cyanide for new mining
operations. If additional states follow this course, cyanide mining
of precious metals will become a commercial impossibility.
[0014] Finally, there is a perceived likelihood in the mining
industry that the traditional large manufacturers of cyanide may
seek to exit the cyanide market. ICI and FMC, both previous
manufacturers of cyanide, have already left the market. There are
some possible reasons why cyanide manufacturers may desire to stop
their cyanide production. First, cyanide manufacturing may not be
considered a core business element of some large manufacturers.
Second, cyanide manufacturers are concerned about the safety,
security and environmental hazards regarding cyanide. Specifically,
these producers are concerned that they may be held liable if a
disaster regarding cyanide ever occurred. Also, many producers seem
to believe that cyanide is becoming a "commodity" chemical.
Typically, chemicals are either classified as being a "specialty"
product or a "commodity" product. As a chemical moves from being a
specialty chemical to a commodity, the large,
technologically-innovative companies often exit the market for this
product so that they can focus their efforts on new products with
higher profit margins. Thus, it should not be surprising if these
large producers begin to slowly exit the cyanide market.
[0015] Given the present climate regarding cyanide, it would be
advantageous for mines to find a new source for alkali cyanide that
is commercially viable in the long-term. It would also be
advantageous to provide a method for mines to obtain alkali cyanide
without having to endure the difficulties associated with
transporting alkali cyanide, including future risk of government
prohibition of cyanide transportation. Such a method and process is
disclosed herein.
BRIEF SUMMARY OF THE INVENTION
[0016] This invention provides a method by which a supply of
cyanide is produced directly at a precious metal mine. This supply
of cyanide may then be used by the mine in its leaching operations
to extract the precious metal from the earth. The ability to
produce cyanide at the mine site may provide the mine with
long-term stability and/or commercial advantage. It may act as a
type of "insurance" against future cyanide supply disruption and
thereby protect the substantial investment associated with the
mining operation. Because mining operations typically have a short
lifespan of about 8-10+ years, a controlled supply of cyanide would
be very desirable.
[0017] Generally, the mine will manufacture the supply of cyanide
using starting materials that are readily available and easily
shipped. As used herein, the term readily available means that the
starting materials are commercially available and not subject to
special or restrictive government regulation of control and
handling. As used herein, the term easily shipped means the
starting materials may be shipped through conventional and
commercially available shipping means without special or
restrictive government regulation of the shipping. In one presently
preferred embodiment, the starting material is formamide which can
be decomposed into HCN and water. Other starting materials may also
be used. Once the HCN has been formed, the producer will then
neutralize the HCN with an inorganic base to produce a supply of
cyanide anions. In some embodiments, the preferred inorganic base
will be lime (CaO) or slaked lime (Ca(OH).sub.2) because this
product is much lower cost compared to NaOH, readily available, and
is often already used by the mine in other processes. Thus, when
reacted with lime or slaked lime, the cyanide will be in the form
of dissolved Ca(CN).sub.2.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
[0018] In order that the manner in which the above-recited and
other features and advantages of the invention are obtained will be
readily understood, a more particular description of the invention
briefly described above will be rendered by reference to specific
embodiments thereof which are illustrated in the appended drawings.
Understanding that these drawings depict only typical embodiments
of the invention and are not therefore to be considered to be
limiting of its scope, the invention will be described and
explained with additional specificity and detail through the use of
the accompanying drawings in which:
[0019] FIG. 1 is a flow diagram of a method of producing a supply
of calcium cyanide at the mine site according to the present
invention;
[0020] FIG. 2 is a flow diagram of another method of producing a
supply of calcium cyanide at the mine site according to the present
invention;
[0021] FIG. 3 is a flow diagram of another method of producing a
supply of calcium cyanide at the mine site according to the present
invention;
[0022] FIG. 4 is a flow diagram of a method of mining precious
metals that uses calcium cyanide that was produced at the mine
site;
[0023] FIG. 5 is a schematic flow diagram of a process for
producing a supply of calcium cyanide; and
[0024] FIG. 6 is a schematic flow diagram of a process for
producing a supply of calcium cyanide similar to the process of
FIG. 5, but with recycle stream of the produced calcium cyanide to
increase its concentration.
DETAILED DESCRIPTION OF THE INVENTION
[0025] The presently preferred embodiments of the present invention
will be best understood by reference to the drawings, wherein like
parts are designated by like numerals throughout. It will be
readily understood that the components of the present invention, as
generally described and illustrated in the figures herein, could be
arranged and designed in a wide variety of different
configurations. Thus, the following more detailed description of
the embodiments of the method of the present invention, as
represented in FIGS. 1 through 6 is not intended to limit the scope
of the invention, as claimed, but is merely representative of
presently preferred embodiments of the invention.
[0026] Referring now to FIG. 1, a method 120 of the present
invention is illustrated in a flow diagram. The method 120
represents a method that allows a precious metal producer to
manufacture cyanide directly at the mine site. Such on-site
production of cyanide may provide the mine with long-term
stability, commercial viability, and/or other advantages. For
example, in some embodiments, this on-site production of cyanide
may provide the mine with long-term stability in that the mine's
cyanide supply no longer depends upon manufacturers that might exit
the marketplace. Likewise, such production of cyanide may give the
mine more stability in that its cyanide supply is not influenced by
difficulties in shipping cyanide which may develop throughout the
world. Finally, such on-site production of cyanide may also reduce
the mine's operating costs in that the mine will no longer be
forced to pay shipping costs, taxes, fees, and/or other charges
that may be associated with cyanide transportation. This includes
elimination of the hazards associated with shipping cyanide to a
mine site. As a result, this method of on-site cyanide production
may provide significant advantages.
[0027] The method 120 includes the step of obtaining 122 a supply
of formamide (HCONH.sub.2). Formamide is a relatively expensive
chemical that is commercially available in liquid form from a
variety of sources including the BASF Aktiengesellschaft company
located in Ludwigshafen, Germany, the Kemira Oyj Industrial
Chemicals company located in Helsinki, Finland, and other small
producers. Formamide is much less hazardous and is subject to far
fewer restrictions than is cyanide. Presently formamide is not
viewed as a terrorist threat and may be shipped throughout the
world without much difficulty.
[0028] Once the supply of formamide has been obtained, the supply
of formamide may be decomposed 124 at the mine site to produce a
quantity of HCN. As used herein, the term "at the mine site" refers
to any location that is associated within a nominal distance to a
precious metal mine. Thus, in some of the presently preferred
embodiments, this phrase means that the decomposing and
neutralizing steps of the method 120 are accomplished either on the
same property and/or on property that is proximate or contiguous
with the parcel from which the precious metals are being extracted.
In some cases, there may be multiple mining operations within a few
miles. In such cases, there may be only one facility to manufacture
the cyanide that supports the multiple mining operations.
[0029] In general, the heterogeneous catalytic reaction step that
converts the formamide into HCN is relatively straightforward.
HCONH.sub.2(g).fwdarw.H.sub.2O(g)+HCN(g)
The reaction takes place in the vapor phase at elevated temperature
and low pressures and it is essentially quantitative. The exact
methods and processes by which this reaction may occur are
described in U.S. Pat. No. 4,693,877 entitled "Cleavage of
Formamide to give Hydrocyanic Acid and Water." This patent, which
is now expired, is expressly incorporated herein by reference.
[0030] Further information and descriptions of the conversion of
formamide into HCN are described in the following patents and book
excerpts: [0031] U.S. Pat. No. 2,042,451; [0032] U.S. Pat. No.
2,529,546; [0033] U.S. Pat. No. 2,534,000; [0034] U.S. Pat. No.
2,604,380; [0035] U.S. Pat. No. 3,702,887; [0036] U.S. Pat. No.
4,745,207; [0037] U.S. Pat. No. 4,693,877; [0038] German Patent No.
DE 476,662; [0039] German Patent No. DE 477,437; [0040] German
Patent No. DE 498,733: [0041] German Patent No. DE 561,816; [0042]
German Patent No. DE 944,547; [0043] German Patent No. DE
1,000,796; [0044] German Patent No. DE 1,211,612; [0045] German
Patent No. DE 2,445,168; and. [0046] James A Kent, ed., RIEGEL'S
HANDBOOK OF INDUSTRIAL CHEMISTRY, 8th Ed., Van Norstrand Reinhold
Company, Inc., New York, 1983, pp. 203-209.
[0047] Each of the above-referenced patents, as well as the
above-cited treatise, is expressly incorporated herein by
reference. While some of the above-referenced documents or patents
describe the decomposition of formamide on a micro-scale, these
reactions can be easily scaled for large-quantity, industrial
applications.
[0048] Once the formamide has been decomposed to form the quantity
of HCN, the HCN may then be neutralized at the mine site to produce
an aqueous solution of calcium cyanide, step 126. The aqueous
solution of calcium cyanide may then be diluted or concentrated for
use in the mining operations. In general, this processing step may
involve one of more of the following steps: [0049] contacting the
HCN gas with a solution or slurry of slaked lime (Ca(OH).sub.2) to
produce the aqueous solution of calcium cyanide; [0050] diluting
the formed cyanide solution to concentrations that are appropriate
for the mine's leaching processes (such as 1000 ppm, etc.).
[0051] FIG. 5 contains a general schematic representation of a
process 150 of producing calcium cyanide from the decomposition of
formamide. As shown in FIG. 5, a supply of formamide 152 is
provided to a vaporizer 154 which vaporizes the formamide. The
formamide stream 156 passes through a heat exchanger 158 to heat
the formamide to a temperature suitable for the catalytic
decomposition reaction which occurs in a catalytic contactor 160.
The formamide is decomposed into hydrogen cyanide gas and water
vapor. These gases 162 are introduced into a contactor 164 together
with calcium hydroxide 166, slaked lime, to neutralize the HCN and
form Ca(CN).sub.2. The neutralized product stream 168 is introduced
into a separator 170 to which a vacuum pump 172 is connected for
removal of gases 174. The separator 170 includes a liquid barrier
176 is permeable to gases, but not liquids. The aqueous solution of
Ca(CN).sub.2 product stream 178 is then recovered.
[0052] In some applications, it may be desirable to concentrate the
Ca(CN).sub.2 thus formed. FIG. 6 contains a general schematic
representation of a process 180 similar to process 150 except that
a portion of the aqueous solution of calcium cyanide is recycled
back to the contactor for further contact with HCN gas. The aqueous
solution of Ca(CN).sub.2 product stream 178 is passed through a
heat exchanger 182 to capture heat from the product stream 178. The
concentrated aqueous solution of Ca(CN).sub.2 184 is removed for
use at the mine site. Additional Ca(OH).sub.2 186 may be added to
the recycle stream 188, as needed, to react with the HCN. Barren
stream 190 may also be added to the recycle stream 188. The barren
stream 190 usually contains a small amount of aqueous cyanide.
Recycling the barren stream in this manner may allow the aqueous
cyanide to be efficiently used. Some fresh water may be added to
the recycle stream or the barren stream 190 may be replaced by a
fresh water stream. However, fresh water usually contains dissolved
carbonate which may react with calcium to form calcium carbonate, a
scaling problem. The barren stream 190 would have less dissolved
carbonate than fresh water, reducing any calcium carbonate scaling
problem.
[0053] As noted above, lime or slaked lime is the inorganic base
for converting the HCN into a slurry or solution of calcium cyanide
(Ca(CN).sub.2). Lime may be converted to slaked lime by contact
with water in a slaker. As is known in the art, lime comprises
primarily calcium oxide, but it may contain measurable amounts of
other compounds including silica, alumina, and/or other metal
cations including magnesium, iron, etc. Thus, when lime or slaked
lime is used as the inorganic base, the main product will be
calcium cyanide; however, given the other ingredients in lime,
there may be measurable quantities of other impurities containing
magnesium, iron, silica, alumina, etc. The presence of such
impurities is not detrimental to the applications proposed herein.
In fact, the ability to use an inorganic base, like lime, of lower
purity and cost is a distinct advantage in accordance with the
invention.
[0054] Lime is a presently preferred inorganic base because it is
substantially less expensive than caustic soda (NaOH) or other
inorganic bases. Generally, caustic soda is often not produced in
areas close to the mine and thus, caustic soda must be shipped long
distances as a 50% aqueous solution or in some cases as flaked
caustic soda. Such shipping can be prohibitively expensive. On the
other hand, lime is generally produced locally (i.e., in areas
close to the mine) and is substantially cheaper to ship and
purchase. In fact, many mines will already have a supply of lime or
slaked lime on-hand because lime is often added to the leaching
solution to ensure that the leaching solution maintains a pH range
of about 9 to about 10.5. Lime is also often used to neutralize
effluent from autoclave or roasting processes at the mine. In this
manner, the lime needed to neutralize the HCN may already be
available at the mine site.
[0055] It should also be noted that the reaction equipment
necessary to produce the calcium cyanide directly at the mine site
could be further designed as a pre-fabricated product. In other
words, the reaction equipment used to produce the cyanide may be
manufactured and assembled using readily available parts and
fittings, and then shipped (via freight, skids, etc.) to the mine
site. Once this pre-fabricated reaction equipment arrives, it may
be quickly set up and used by the producer to manufacture the
calcium cyanide. Moreover, when the producer finishes its cyanide
production, the reaction equipment could then be sold or shipped
off to another mine, etc.
[0056] Because of the simplicity of the formamide decomposition
process described above, the reaction processing equipment may be
easily scaled to meet desired production requirements. In addition,
the reaction processing equipment is relatively simple to operate.
This will be particularly advantageous when operated in remote
locations where skilled labor is expensive and not widely
available.
[0057] Referring now to FIG. 2, a flow diagram of a second method
220 of the present invention is illustrated. The method 220 again
represents a method that allows cyanide to be produced directly at
the mine site. As noted above, this production of the cyanide at
the mine site may provide the mine with significant advantages.
[0058] Unlike the method 120 outlined above, the first step in the
method 220 involves obtaining 222 the starting materials for the
Andrussow process for producing HCN. As described above, these
starting materials include natural gas and ammonia. However,
because these starting materials may not be readily available at
the mine site or may be expensive and/or difficult to ship,
methanol (CH.sub.3OH) and urea (CO(NH.sub.2).sub.2) may be used as
the starting materials. Method 240, shown in FIG. 3, is
substantially the same as method 220, except that obtaining step
242 replaces step 222. Step 242 includes obtaining a supply of
methanol and urea at the mine site as starting materials to prepare
HCN. Other embodiments may also be made in which the starting
materials are a combination of some of the above-recited reagents
such as natural gas and urea or methanol and ammonia.
[0059] Urea is available in large quantities and it is used as a
fertilizer throughout the world. It is shipped and handled as a
non-hazardous material. Methanol is also a commodity chemical in
the world and can be shipped easily. Neither urea nor methanol is
known to be a terrorist threat.
[0060] Urea is known to decompose into ammonia. For example, U.S.
Pat. Nos. 5,252,308, 2,797,148 and 3,718,731 describe ways in which
urea may be decomposed into ammonia. These patents are incorporated
herein by reference and show the methods/reaction conditions by
which these reactions may take place.
[0061] Again, once the proper starting materials have been
obtained, the next step in the method 220 is to react 224 the
starting materials at the mine site to form a quantity of HCN. Such
reactions are known in the art. For example, information regarding
the reaction of ammonia and methanol to produce HCN is also known.
Methanol along with propylene and ammonia are currently used to
produce HCN over catalysts at lower temperatures as part of the
process for forming acrylonitrile (which forms HCN as a byproduct).
In these reactions, excess ammonia is reacted with methanol to
produce a greater amount of HCN. As is known in the art, such
reactions may involve both fixed and fluidized beds operating at
lower temperatures. In fact, the following patents, which are all
incorporated herein by reference, teach specific information that
may be useful in performing the reaction of ammonia/urea with
methanol/methane to produce HCN: [0062] U.S. Pat. No. 5,158,787;
[0063] U.S. Pat. No. 3,911,089; [0064] U.S. Pat. No. 4,485,079;
[0065] U.S. Pat. No. 5,288,473; [0066] U.S. Pat. No. 3,716,496;
[0067] U.S. Pat. No. 3,988,359; [0068] U.S. Pat. No. 4,511,548;
[0069] GB Patent No. 718112; [0070] GB Patent No. 913836; [0071]
Japanese Patent No. JP 74-87,474; [0072] Japanese Patent No. JP
79-08655; [0073] Japanese Patent No. JP 78-35232; and [0074] German
Patent No. DE 1,143,497 (no oxygen present).
[0075] Once the supply of HCN has been formed, the next step is to
neutralize 226 the HCN at the mine site to produce the calcium
cyanide. The processes for the step 226 are similar and/or
identical to the methods and processes associated with the
processing step 126 described above in conjunction with FIG. 1. For
the sake of brevity, however, this discussion will not be repeated;
rather, the reader may simply examine the previous disclosure for
more information.
[0076] It should also be noted that lime is not widely used as the
inorganic base of choice for the Andrussow process due to the
possibility that some of the calcium cations (or other metal
cations) will react with carbon dioxide and form a calcium
carbonate precipitate (or some other type of precipitate).
Moreover, Ca(CN).sub.2 has not been a desirable alkali cyanide
because it cannot be dried and formed in briquettes, unlike NaCN.
In addition, Ca(CN).sub.2 can only be concentrated to about 15-17%
by weight, which is substantially more dilute than concentrated
NaCN solutions. Thus, shipping costs for Ca(CN).sub.2 are
significantly greater than shipping costs for NaCN. Another reason
Ca(CN).sub.2 is not a desirable alkali cyanide because it
decomposes readily at higher temperatures and higher concentrations
than NaCN. As a result, cyanide manufacturers have generally
avoided making Ca(CN).sub.2. However, according to the present
invention, the ability to manufacture an aqueous Ca(CN).sub.2
solution directly at a mine site, as needed for direct use,
eliminate the foregoing disadvantages.
[0077] Referring now to FIG. 4, a flow diagram of a different
method 320 of the present invention is illustrated. The method 320
is a method for mining precious metals using calcium cyanide. The
first step of this method involves producing 322 the calcium
cyanide at the mine site using at least one starting material. This
producing step may be accomplished using either the methods 120,
220, 240 outlined above. Accordingly, the starting material(s) may
be formamide, methane and ammonia, methanol and urea, etc.
[0078] The next step in the mining process 320 is to leach 324 the
ore with the cyanide solution. The way in which such leaching may
be performed in known in the art. Once this leaching 324 has been
done, the pregnant solution is then collected 326. The pregnant
solution is the solution that is gathered after the leaching has
been accomplished. This pregnant solution will contain a quantity
of precious metal ions bound to cyanide anions as an aqueous
complex. Again, the precise methods/processes for gathering this
pregnant solution are well known in the art and are currently being
practiced at all precious metal mines that use cyanide.
[0079] Finally, the last step 328 in the method 320 is recovering
the precious metal from the pregnant solution. The exact method for
recovering 328 the precious metal will depend on a variety of
factors including cost, availability of reagents, etc. Of course,
all of these specific methods/steps associated with recovering the
precious metal are well known in the mining industry and fall
within the scope of the present invention.
[0080] The present invention may be embodied in other specific
forms without departing from its structures, methods, or other
essential characteristics as broadly described herein and claimed
hereinafter. The described embodiments are to be considered in all
respects only as illustrative, and not restrictive. The scope of
the invention is, therefore, indicated by the appended claims,
rather than by the foregoing description. All changes that come
within the meaning and range of equivalency of the claims are to be
embraced within their scope.
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