US3063921A - Method of and apparatus for the electrolytic extraction of gold from a gold-bearing solution - Google Patents

Description

Nov. 13, 1962 A. LEIBOWlTZ 3,063,921

METHOD OF AND APPARATUS FOR THE ELECTROLYTIC EXTRACTION OF GOLD FROM A GOLD-BEARING SOLUTION Unite States 3,063,921 METHQD OF AND APPARATUS FOR THE ELEC- TROLYTIC EXTRACTION OF GOLD FRGM A GOLD-BEARING SOLUTION Alec Leihowitz, Johannesburg, Transvaal, Republic of South Africa, assignor to Rand Mines Limited, Johannesburg, Transvaal, Republic of South Africa, a limited-liability company of the Republic of South Africa,

and Alec Leibowitz, Hugh McLelian Husted, Lionel Siderslry, and Kenneth Neil Kitsch, a partnership, Jo-

hannesburg, Transvaal, Republic of South Africa Filed Apr. 20, 196i), Ser. No. 23,427 Claims priority, application Republic of South Africa Aug. 6, 1957 6 Claims. (Cl. 2ll4113) This invention relates to a method of electrolytic extraction of gold from a gold-bearing solution, and this application is a continuation-in-part of patent application Serial Number 752,233, filed July 31, 1958, now abandoned. The term gold bearing solution means an aqueous solution of a gold solvent, such as cyanide, which has been in contact with gold and has caused its dissolution. The said solution may either be clarified or it may contain comminuted ore from which the gold has been wholly or partially dissolved, or is in process of being dissolved, but in practice it will invariably contain surplus gold solvent.

In the past, attempts to extract gold electrolytically from a gold-bearing solution which had been obtained after cyanide treatment of the comminuted gold-bearing ore, have been unsatisfatcory in the presence of such ore. It has been assumed by those skilled in the art that failure has been due to:

(a) The difliculty experienced in obtaining an adherent deposit of gold on the cathode.

(b)The abrasion of the deposited gold by the ore passing over a stationary cathode.

(c) Cathode corrosion.

(d) Inability to treat large quantities.

The electrolytic extraction of gold from a clarified solution has been unsatisfactory hitherto for the last two reasons enumerated above.

It is known that, if a clarified gold-bearing solution is passed rapidly through stationary cathodes made of iron gauze, the actual extraction of the gold by an electrolytic process becomes more satisfactory. Owing to the rapid passage of the solution through the cathode, the current density can be increased to an extent which will increase the rate of deposition without producing a powdery deposit of gold on the cathodes. It is believed that the efiiciency of the extraction is such that only a small residue of gold remains unextracted. However, in order to obtain satisfactory results by this method, a large number of cathodes is required and the solution has to pass through them very rapidly indeed. The result is that the size and complexity of the plant required oppose the practical application of the method.

It is an object of the present invention to provide a satisfactory electrolytic method of extraction which minimizes the above disadvantages.

It is also an object of the invention to provide a method for the electrolytic extraction of gold by means of which substantially complete extraction may be effected.

It is another object of the invention to provide a method for the electrolytic extraction of gold by means of which substantially complete extraction may be effected in the'prese'nce of comminuted ore contained in the goldbearing solution.

According to one aspect of the invention, there is provided a method of electrolytically extracting gold from a gold-bearing solution which comprises the step of moving a liquid-pervious cathode, the entire deposition zone fihfifill Patented Nov. 13, 1952 of which is always immersed in the solution, continuously through the solution while a direct electric current flows between an anode and the cathode, at a speed which is high enough to deposit an adherent metallic layer and low enough to prevent substantial resolution and dislodgrnent of the deposited gold.

According also to the invention, the method above defined is applied to the electrolytic extraction of gold from a gold bearing solution in the presence of commi nuted ore.

If the electrolytic extraction is carried out in a vessel in which gold bearing comminuted ore from which the gold is in process of being dissolved is present, and if substantially complete dissolution is required, it has been found essential to aerate the solution and to agitate it by independent mechanical means in the event that aeration and/or cathode movement do not themselves perform the function of agitation. At the same time, it is equally essential to prevent air bubbles from flowing over the surface of the moving cathode or from being entrapped thereby. This may be achieved, for example, by bafiles so arranged that they do not prevent the pulp from moving freely with the cathode.

It is theoretically possible to achieve satisfactory results by imparting a reciprocating movement to the cathode, provided the leading edges of the cathode structure, which exist by reason of the fact that thecathode is liquid pervious, are caused to face the direction of movement. It is preferred, however, to rotate the cathode in the solution.

According also to the invention, the initial speed of movement of the cathode is reduced during the extraction process.

According to another aspect of the invention, there is provided apparatus for carrying out the method abovedefined comprising a tank to contain the solution and a cathode and an anode, which anode may, if desired, be provided by the tank itself. Said anode must be substantially parallel to the cathode and substantially at least as long as and substantially co-extensive with the longitudinal extent of the deposition zone of the cathode. Said cathode is liquid-pervious, having its entire deposition zone always immersed in the solution during operation, and is so constructed that any point on its mean peripheral surface moves at substantially the same speed. Also included are means for moving the cathode and means for causing a direct current to flow between the anode and the cathode.

If the electrolytic extraction is taking place in a tank which also contains gold bearing comminuted ore from which the gold is in process of being dissolved, and if substantially complete dissolution is required, the tank must be provided with an inlet through which air under pressure may be introduced to aerate and agitate the solution and also with battle means to prevent air bubbles from flowing over the surface of the cathode and from being entrapped thereby, said bafile means being arranged so as not to prevent the pulp from moving freely with the cathode.

The cathode may conveniently comprise a gauze cylinder. The longitudinal elements of the moving gauze structure will, as will be appreciated, have both leading and trailing edges. Such cylinder may, for example, be supported on a spider and caused to rotate about a vertical axis. Alternatively, it may be supported on rollers and driven by means located externally of the tank. The gauze may be made of any electrically conductive metal of adequate structural strength such as, for example, stainless steel or copper. The advantage of constructing the cathode of stainless steel is that this material is inert and, in consequence, corrosion which might occur as a result of intermittent use of the apparatus or during the deplating stage referred to below is avoided.

It has been found that iron anodes are corroded as a result of exposure to air and/ or water, and/ or as a result of the electrolytic action, and/or as a result of impurities introduced by the comminuted ore. If the tank is made of a conducting material which is not unduly affected by such corrosion, then the tank itself may serve as the anode. Otherwise, an independent anode may be used. Such independent anode may comprise open-ended cylinders suspended in the tank, without being in electrical contact therewith, to a minimum depth such as will enable them to surround the deposition zone of the rotating cathode. The anodes may be constructed from any suitable resistant conducting material such as, for example, hard carbon, graphite, or peroxidized lead. Unperoxidized lead will, of course, become peroxidized in the tank due to the anodic electrolytic effect.

The invention and the manner in which the same is to be performed will now be further described, purely by way of example, with reference to the accompanying drawing in which:

FIG. 1 is a diagrammatic cross-sectional elevation of apparatus suitable for the electrolytic extraction of gold in accordance with the invention.

FIG. 2 is a cross-section on line IIIl of FIG. 1.

Referring to the drawing, 1 is a tank to contain goldbearing pulp. 2 is a stainless steel gauze cathode in the form of a cylinder supported by a spider 3 and rotatable on a vertical axis by means of a variable speed prime mover 4 through a reduction gear 5. 6 is a stationary lead anode in the form of an open-ended cylinder disposed around the cathode 2.

The anode 6 is parallel to the cathode 2 and as long as and co-extensive with the longitudinal extent of the cathode 2. 7 is an inlet for compressed air and 8 a baffie which prevents air bubbles from flowing over the surface of the cathode or from being entrapped thereby. This prevents resolution of the gold due to interference with the uniformity of the current flowing between the anode and cathode. 9 is a source of DC. current and 10 an outlet for draining the tank.

In tests conducted, the tank was 4 feet high and 15 inches in diameter. The cathode was inches diameter and 6 inches high, its effective surface area being 70 square inches. The voltage applied was 12 volts and the current 1.4 amperes. The initial gold content was 1 dwt. per liquid ton and after 24 hours the gold residue was 0.04 dwt. per liquid ton. The density of the pulp was 1.6.

During removal of the gold from the solution from 1 dwt. down to 0.2 dwt., the speed of rotation of the cathode was 200 r.p.rn. and from 0.2 dwt. down to 0.04 dwt., the speed was reduced to 120 rpm.

It has been found that if a dark, friable deposit of gold on the cathode is obtained, which type of deposit is not required, it is an indication that the speed of rotation of the cathode is too low and should be increased in order to obtain an adherent deposited layer. Also, a low speed will mean that extraction proceeds but slowly i.e. that the efiiciency of extraction is low. It might therefore be supposed that, compatible with the current density employed, the higher the speed of rotation of the cathode the better. It has been found, however, that this is not the case. Two factors impose a practical upper limit on the speed of rotation. These are resolution of the deposited gold and, in the case where extraction takes place in the presence of comminuted ore, abrasion.

As regards resolution, it is known that when gold particles are dissolved conventionally in a very dilute cyanide solution, the speed of movement of the gold particles is a determining factor in the rate of solution. H. A. White (Journal of the Chemical, Metallurgical and Mining Society of South Africa, vol. 35, pp. 1-11) reaches the 4 conclusion that gold particles of 260 mesh size will dissolve at the rate of 8.8 i

milligrams per square centimetre of surface per 24 hours, V being the average velocity of particle settlement in centimetres per second, and l the average length of a particle in microns.

Tests with a solution containing 1 dwt./ton of gold showed that the effective degree of extraction was initially much more satisfactory at reasonably high cathode speeds. Thus, with a peripheral speed of 30 feet per minute, the extraction rate was 0.045 dwts. per ton per hour, Whereas with a speed of 300 feet per minute it was 0.20 dwts. per ton per hour, an increase in the extraction rate of 340%.

However, when the gold concentration becomes low, the reverse is found to occur. With a solution containing 0.2 dwts. per ton and a peripheral speed of 30 feet per minute, the extraction rate was 0.013 dwts. per ton per hour. Raising the speed to 300 feet per minute lowered the extraction rate to 0.008 dwts. per ton per hour, a decrease of about 38%.

Visual examination indicated that at high speeds, and when the gold concentration had reached a low level, resolution of the gold which had been deposited at the higher concentrations on the trailing edges of the vertical members of the cathode gauze took place.

If, therefore, the deposit on the trailing edges of the cathode structure begins to be removed owing to resolution, especially at low concentrations, it is an indication that the speed of rotation is too high. Hence, in the tests, the reduction in speed when the concentration fell to 0.2 dwts. per liquid ton.

As regards abrasion during extraction from a solution containing comminuted ore, it is in practice impossible to grind or mill ore fine enough entirely to eliminate abrasion of the deposited gold at high cathode speeds. It is therefore essential that the electrolyte should contain the gold solvent so that any abraded gold can redissolve prior to redeposition. Furthermore, although it has been established that a high cathode speed is effective in improving the rate of extraction, this approach must be moderated in view of the amount of abrasion which this high speed can cause.

In practice, therefore, it is necessary in order to obtain substantially complete extraction, to strike a balance between too high a speed which will cause material resolution, and also abrasion if extraction is taking place in the presence of comminuted ore, and too low a speed which will induce a loose, powdery deposit and poor extraction efficiency.

It has been shown that while a relatively high speed may be employed for rapid initial extraction, that speed must be reduced in order to inhibit resolution at low concentrations. The reduction in speed will bring with it the benefit of a reduced tendency to abrasion and the final speed to obtain substantially complete extraction must be associated with the elimination of abrasion.

Cathode movement is essential to obtain good admixture of the pulp or electrolyte and to establish maximum contact between the cathode surface and the gold ions in solution. It is therefore necessary to obtain maximum movement within the scope of abrasion and resolution limitations. To effect this, constructional design must aim at minimizing abrasion. The cathode movement will cause the pulp to flow with it, thereby reducing impact between the ore particles and the cathode surface. To maintain this stream of pulp flowing with the moving cathode, it is essential that no obstacle exists in the tank which will divert this flow from or against the direction of cathode movement.

As regards resolution from the trailing edges of the cathode structure, it has also been found that it can be minimized by increasing the voltage, reducing the thickness of the cathode and/or providing anodes both inside and outside the cathode. If anodes are provided inside the cathode, they may be caused to rotate therewith.

In the test described with reference to the drawing, perfect adherence of the gold to the cathode was obtained and it was convincingly demonstrated that there was no tendency for the gold to be removed by abrasion.

After extraction has proceeded to the desired extent, the deposited gold may be transferred from the cathode to concentrating members. This may be done by gradually removing the cathode from the tank while the current supply is still connected and Washing the cathode down with a water spray to prevent redissolution of the gold in the solution adhering to it. Thereafter, the cathode is transterred to an aqueous cyanide solution having concentrating members of electrically conductive material located in it. The electrolytic process is then reversed by changing the polarity of the current flowing through the cathode so that the gold is deplated from the cathode and deposited on the concentrating members. If desired, the cathode may be deplated after the treatment of each batch of gold-bearing solution, but it is possible for successive depositions to take place on the cathode before it is deplated.

I claim:

1. A method of electrolytically extracting gold from a gold-bearing solution which comprises the steps of moving a liquid-pervious cathode, which is always immersed in the solution, continuously through the solution while causing a direct current to flow between an anode and the cathode, at a speed of movement of the cathode which is high enough to deposit an adherent metallic layer on the cathode and low enough to substantially prevent resolution and dislodgment of the deposited gold; and

adjusting the speed of movement of the cathode to a lower level during the extraction process to compensate for depletion of gold from said solution.

2. A method of electrolytically extracting gold from a gold-bearing solution in the presence of comrninuted are which comprises the steps of moving a liquid pervious cathode, which is always immersed in the solution, continuously through the solution while causing a direct current to flow between an anode and the cathode, at a speed of movement of the cathode which is high enough to deposit an adherent metallic layer on the cathode and low enough to substantially prevent resolution and dislodgment of the deposited gold; and then adjusting the speed of movement of the cathode to a lower level during the extraction process to compensate for depletion of gold from said solution.

3. The method of claim 1, in which the cathode is caused to rotate in the solution.

4. The method of claim 1, in which the solution is an aqueous cyanide solution.

5. The method of claim 2, in which the cathode is caused to rotate in the solution.

6. The method of claim 2, in which the solution is an aqueous cyanide solution.

References Qited in the tile of this patent UNITED STATES PATENTS 640,718 Tatro et a1 Jan. 2, 1900 1,015,546 Davis Jan. 23, 1912 1,251,302 Tainton Dec. 25, 1917 1,344,681 Dalziel June 29, 1920 OTHER REFERENCES Electro-Analysis by Edgar F. Smith, 4th edition 1907 (page 51

Claims (1)

1. A METHOD OF ELECTROLYTICALLY EXTRACTING GOLD FROM A GOLD-BEARING SOLUTION WHICH COMPRISES THE STEPS OF MOVING A LIQUID-PREVIOUS CATHODE, WHICH IS ALWAYS IMMERSED IN THE SOLUTION, CONTINUOUSLY THROUGH THE SOLUTION WHILE CAUDING A DIRECT CURRENT TO FLOW BETWEEN AN ANODE AND THE CATHODE, AT A SPEED OF MOVEMENT OF THE CATHODE WHICH IS HIGH ENOUGH TO DEPOSIT AN ADHERENT METALLIC LAYER ON THE CATHODE AND LOW ENOUGH TO SUBSTANTIALLY PREVENT RESOLUTION AND DISLODGMENT OF THE DEPOSITED GOLD; AND ADJUSTING THE SPEED OF MOVEMENT OF THE CATHODE TO A LOWER LEVEL DURING THE EXTRACTION PROCESS TO COMPENSATE FOR DEPLETION OF GOLD FROM SAID SOLUTION.

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