Evaporative Treatment Of Liquids By Submerged Combustion

Abstract

Method and apparatus for the evaporative treatment of liquid usable in the electrolytic refining of copper in which fuel is burned in an open-bottomed otherwise closed chamber in which the bottom of the chamber is below the surface of a bath of said liquid, the interior surfaces of the chamber are cooled by application of cooling media to the chamber exterior, the interior surfaces of the chamber are periodically flushed by steam to remove accumulation thereon and the precipitated matter is removed from the bath.

Description

This invention relates to the evaporative treatment of liquids to concentrate them and remove matters dissolved therein. Such treatment may be carried out merely to concentrate, purify or re-condition the liquid, or merely to recover the dissolved matter, or with both of those ends combinedly in view.

The invention is applicable generally to the treatment of liquids for the purposes indicated. However, it is presently expected that the invention will be particularly advantageous in the treatment of electrolyte as used in the electrolytic refining of copper; to re-condition the electrolyte for re-use and to recover from it matters of commercial importance which accumulate in it as a result of its sustained use in the copper refining process. Because of this expectation the invention will be further described herein primarily in terms of that particular use but is not to be regarded as restricted thereto.

In the electrolytic refining of copper the most commonly used electrolyte is a solution of sulphuric acid and copper sulphate and the copper anodes usually contain nickel as an impurity at levels less than 1 percent.

This Ni content is, of course, freed during the electrolytic migration, but if it is not removed from the electrolyte it will accumulate and increasingly contaminate the deposited cathode copper by occlusion of electrolyte during deposition. It is thus necessary for the Ni concentration in the electrolyte to be kept sufficiently low to ensure no more than an acceptable proportion (for example, 2 ppm) of Ni in the cathode copper. There are, of course, a number of other soluble impurity elements, such as arsenic, iron and calcium, which are not wanted in the electrolyte and these may be removed along with, or prior to the removal of, the Ni content but the present discussion is largely in terms of the Ni ingredient because its recovery (in the form of NiSO.sub.4) is of considerable commercial importance.

Thus the main objects of the present invention, in its application to the electrolytic refining of copper are:

A. the efficient refining of copper.

B. the recovery in marketable form of a valuable Ni by-product, and

C. the achievement of objectives (a) and (b) in a manner and with apparatus which are simple yet highly effective by comparison with the prior art.

The purification of an electrolyte, viewed with sufficient generality as to be common to the prior art and the present invention, comprises two stages. The first stage accomplishes the removal of copper and arsenic, electrolytically or otherwise, and thus prepares the feed liquor for the second stage, viz. the concentration process to which this invention applies. This stage two feed liquor consists of a solution of sulphuric acid, nickel sulphate, iron sulphate, calcium sulphate and small proportions of other materials.

It will be understood that the function of the purification circuit is to keep the concentration of alien matters (that is, matters such as nickel sulphate which are valuable but unwanted in or alien to the electrolyte) in the electrolyte in actual use in the electrorefining circuit, down to an acceptable level; and hence only a sufficient fraction of the refining electrolyte must be treated in the purification circuit so as to remove an amount of impurity equal to the input of that impurity to the refinery. It is a feature of the present invention that, because of its efficiency, purification plant may be on a relatively small scale compared with the prior art. In an experimental practicing of this plant it was found that a satisfactory alien matter concentration was not exceeded, with the continuous withdrawal of 3 gals/min. from a total refinery pump output of 1,620 gals/min.

The purification circuit is, in effect, a subcircuit in parallel with the refining circuit; the purification circuit has a liquid intake pipe which branches from the output pipe of the refinery pump and a discharge pipe by which finally purified electrolyte is fed back into the intake pipe leading back to the refining circuit pump.

It will be appreciated that any second stage purification in essence consists in heating the electrolyte so to concentrate the liquid by driving off excess water and to bring the salts dissolved in the electrolyte to a state of saturated solution so that upon subsequent cooling a satisfactory removal of the dissolved salts is effected. The crystals are precipitated partly in the evaporator and partly by subsequent cooling and thus may be removed and the concentrated liquid, freed of at least its most damaging impurities, can then be returned to the refining circuit for re-use.

It might appear from this that second stage purification is a relatively simple procedure, but that is not so; it has long been a severe problem in the industry and many different expedients have been adopted in the past, but so far as we are aware none of these prior expedients, although they are still widely in use, has been entirely satisfactory. Thus it is an object of this invention to remedy or substantially remedy the prior art shortcomings.

One commonly used second stage purification incorporates the use of an ordinary tubular heat exchanger applying heat by means of steam or heated gases. This is satisfactory up to a point, but it rapidly becomes inefficient through accumulation of solid encrustations, in that keeping one of the heat exchange surfaces (that having liquid under treatment in contact with it) clean is a major task which virtually prohibits continuous operation.

Another common method is to heat the solution in heavy steel or cast iron pans by means of flame heating underneath the pan, but this again involves the same disabilities as just referred to in connection with tubular heat exchangers, and in an endeavour to avoid these disabilities it has been proposed to direct hot gases into the solution in order to transfer heat directly to it. The direct heating method, however, necessitates the use of nozzles or the like dipping into the solution, or other devices to introduce the hot gases thereto, but this method does not avoid the build up of solid sulphate or like deposits on the nozzles or the like and the intense local heat to which the encrustations are then subjected causes the sulphates (nickel and iron, for example) to decompose giving off sulphur trioxide. This escapes into the atmosphere where it becomes an obnoxious pollutant and, in general, this is one of the more serious effects of some of the prior art methods known to us, in that the hot gases leaving the evaporating system, by reason of air pollution, are a serious disadvantage.

A further disability of the prior methods involving encrustation of solids on the heating or heat transfer equipment, is that their presence is highly detrimental in point of thermal efficiency, because an insulating layer of salts crystallises or settles out in the heat exchange vicinity. This in turn causes inefficiency of heat transfer with consequent excessive heating of the exchange medium and consequent reaction with the free H.sub.2 SO.sub.4 present in, and in continuous supply to, the salt layer and, as previously indicated, the salt layer and entrapped H.sub.2 SO.sub.4 are heated beyond decomposition temperature with unavoidable SO.sub.3 generation.

It is a further object of the present invention to avoid the disabilities recited above by use of a method and apparatus of high thermal efficiency which reduces discharge of SO.sub.3 or other pollutant matters to the atmosphere to a level which is acceptable having regard to the laws concerned to prevent atmospheric pollution, while enabling efficient recovery of values, such as nickel sulphate and/or the effective elmination of other impurities.

A further advantage due to the present invention is that it permits second stage purification to be carried out continuously during the process of electrolytic refining and the first stage purification of the electrolyte involved in the refining process.

In its broadest aspect the invention consists in a method of evaporatively concentrating a liquid and removing dissolved matter therefrom, comprising the steps of: burning fuel in an open-bottomed, otherwise closed, chamber whereof the bottom rim is below the surface of a bath of said liquid so that the products of combustion are discharged directly into the liquid, cooling the interior surfaces of said chamber by application of cooling media to the chamber exterior, flushing said interior surfaces sufficiently to clear therefrom solid particles tending to accumulate thereon, and removing solid matter precipitated in said bath.

One embodiment of the invention defined in the preceding paragraph will now be described with reference to the accompanying drawing, in which similar reference indicate corresponding parts, and in which:

FIG. 1 shows a plan view of a combustion chamber,

FIG. 2 shows, in front elevation and partly in section, a view along the line 2--2 of FIG. 1 and,

FIG. 3 shows, in side elevation, a tank containing a bath of liquor in which three of said combustion chambers are partially submerged.

Upon referring to the drawings, it will be seen that a combustion chamber 1 surrounded by a jacket 2 terminates in a skirt 3 in which a plurality of slots such as 4 is formed. A mounting flange 6, fixed to the outer wall of said water jacket 2, permits the chamber 1 to be mounted with respect to a tank hereinafter described. Above the roof 7 of the chamber is mounted anair manifold 8 from which a refractory spacer 9 separates the burner tile 10 which communicates with an oil burner 11. Beneath the roof 7, a steam spray ring 12 is mounted with a plurality of steam nozzles 13 depending therefrom. Said chamber is adapted to be partially immersed in said bath so that liquor therein rises to the level 14.

The medium used to flush the interior surfaces such as 15 of the chamber 1 may be water directly sprayed on to those surfaces, but is preferably steam emitted from the nozzles 13 which by condensation on the cooled inner surfaces of the chamber provides a film-like stream of water which runs down the walls and acts as a solvent or parting layer which prevents solids from building up as encrustations on that surface. Obviously the addition of flush water, in any form, to the bath of liquid under treatment increases the amount of water to be driven off so that the required level of concentration of the liquid is maintained. Therefore, it is preferable for the flush medium to be applied sparingly, for example, by short intermittent blasts of steam directed towards the walls of the chamber.

A presently preferred apparatus for carrying out the invention comprises the chamber 1 having a cylindrical wall as shown, with its open bottom terminating the skirt 3 being adapted to dip into a bath of the liquid to be subjected to second stage purification. The oil burner 11 employs any suitable liquid fuel such as diesel distillate injected via suitable nozzles thereinto together with the air necessary for its combustion. Because the fuel is liquid, it may be burnt under controlled conditions so that the products of combustion will be virtually oxygen and ash free. The jacket 2 is supplied with a coolant such as water circulated continuously therein by means of a suitable pump which derives the water from a cooling tower or the like.

The circulation rate of the cooling water is preferably such as will maintain the chamber walls at a temperature such that the effluent temperature of the cooling water does not exceed 85.degree.C. The effluent heat of the cooling water may be advantageously employed by using it in a heat exchanger as pre-heating counter current to the liquor leaving the first stage of purification on its way to the second stage. The roof of the combustion chamber may be water cooled in much the same manner as just described with reference to the chamber walls, but, for preference, roof cooling is effected by use of the manifold 8 as a pre-heating feed duct for the air necessary for fuel combustion in the burner chamber.

Discharge of steam within the chamber may be carried out in several different ways, but for preference, the tubular steam spray ring 12 is mounted in the chamber 1 immediately below the roof 7 thereof. The outside diameter of this ring is preferably such as to be almost equal to the internal diameter of the chamber. Steam is fed to the ring from any conventional source of supply and the ring is furnished with a plurality of nozzles 13 by which the steam is directed onto the walls and roof of the chamber, the nozzles being sufficiently closely spaced as will ensure that the condensate curtain formed on the inner wall of the chamber is circumferentially continuous before it reaches the bottom rim of the container which dips into the liquor to be treated. The preferred steam supply is saturated steam at a nozzle pressure of about 100 lbs/ins.sup.2.

The preferred bath has a liquid capacity equal to 450 gallons, and a feed liquor input of about 3 gals. per min. The internal dimensions of the chamber 1 are preferably about 20 inches in height and 26 inches in diameter are under these conditions a suitable rate of steam infeed is intermittently discharged into the chamber in the form of short bursts each of about 1 min. duration alternating with inactive intervals of 2 hours duration.

The skirt 3 of the chamber is preferably multitudinously slotted or of sawtooth shape so that the combustion gases will enter the bath as a large number of small streams, thus to reduce what might otherwise be excessive bath turbulence, and for preference the submerged skirt is surrounded by an open bottom annular duct or inverted channel 5 having a circumferential outer wall which is similarly slotted or sawtoothed to provide a still greater multiciplicity of outlets through which combustion gases are discharged into the bath liquor.

The operation is preferably conducted continuously at a constant concentration in the bath so that the rate of addition of feed liquor just balances the rate of evaporation of water plus the runoff of concentrated liquor. In practice this is preferably achieved by continuous addition of feed liquor and intermittent discharge of concentrated liquor so that the level in the bath alternates between upper and lower limits. In the preferred method of operation, the concentration of sulphuric acid is maintained above that which will corrosively attack the steel components.

By intermittent agitation (e.g. by means of a suitably designed rabble and drive 20) those salts which precipitate in the bath are maintained in suspension and are thus removed with the concentrated liquor. This hot saturated solution plus precipitate from the bath is then cooled thereby precipitating the major part of the salts which are to be removed. The cooled liquor is separated from the precipitated salts by settling, filtration or the like, and the purified liquor is directed back into the intake pipe leading to the refining circuit.

As shown in FIG. 3, a tank 16 contains a bath 17 of liquor which rises to the level 14. This tank receives at a level near its roof via the inlet 18 the liquor to be purified, and at its lower or floor level has a concentrated liquor take-off pipe 19. Means may be provided for ensuring that the level of concentrated liquor in the bath is maintained substantially constant and that the level is maintained above that at which the combustion gases are discharged into it. The tank 16 includes a roof plate 21 to which a plurality of (in this case, three) chambers such as 1 are attached by means of the mounting flange 6. Each said chamber is fitted through an opening in the roof 21 which is equipped with a flue or the like for taking off the exhaust gases after they have bubbled their way through the bath 17.

Claims

We claim:

1. Apparatus for evaporatively concentrating a liquid to recover matters dissolved therein, comprising:

a. a tank to contain the liquid to be concentrated,

b. an open-bottom chamber, consisting of a side wall and a roof, mounted on said tank so that its open bottom is submerged below the surface of the liquid in said tank,

c. a fuel burner mounted on said roof and arranged to direct the products of combustion produced into said chamber through an aperture in said roof,

d. means for injecting fuel and air for the combustion thereof into said burner,

e. a steam distribution header mounted in said chamber immediately below said roof,

f. means to feed steam into said header, and

g. a plurality of outlet nozzles on said header by which steam is directed onto the walls and roof of the chamber, and which are so spaced along said header as to ensure that a condensate curtain formed on the interior of said wall is circumferentially continuous before reaching, said chamber walls and/or roof being jacketed to provide for circulation of a coolant about said chamber for producing said condensate curtain.

2. The apparatus of claim 1 wherein said rim is slotted or of saw-toothed shape whereby combustion gases may enter said bath as a large number of small streams to reduce bath turbulence, and wherein the submerged rim is surrounded by an open-bottomed annular duct having a circumferential outer wall which is similarly slotted or saw-toothed to provide further outlets through which said combustion gases are discharged into said bath liquor.

3. The apparatus of claim 1 wherein said bath is in the form of a tank which receives liquor to be purified near the level of the roof of said tank and which possesses a concentrated liquor take-off pipe at or near the floor level of said tank, and means which maintain the level of concentration liquor substantially constant whereby said last-mentioned level is maintained above said chamber open bottom at which said combustion gases are discharged, said tank including a roof plate which separates a plurality of said chambers, and said roof having an opening therein constituting an intake end of a flue adapted to take off exhaust gases after the latter have bubbled their way through the bath liquor.

4. The apparatus of claim 1 wherein said means for injecting fuel and air defines a refractory duct, said duct being furnished with nozzles for injection thereinto of fuel and air.