NEW CYANIDE FREE TECHNOLOGY FOR ELECTROCHEMICAL EXTRACTION OF GOLD FROM SULFIDE ORES
Candidate of technical sciences, senior researcher, associate professor Oryngojin Ye. S.
Mining Institute after D.A.Kunaev
Development of gold-mining industry of Kazakhstan is connected with the insufficient involvement of sulfide ores in mining and processing, which are complex in composition and persistent in gold extraction. Technologies of sulfide ores processing applied at Kazakhstan plants are distinguished with high material W labor costs; they are multi-operational and not environmentally safe. Therefore, search and creation of new effective technologies for gold-containing ore processing is exceptionally important economic assignment.
With the view of solution of this problem, the National Engineering Academy of the Republic of Kazakhstan has developed new electrochemical, cyanide free technology for gold leaching from persistent sulfide-containing raw material. The technology has successfully passed a stage of semi-industrial tests resulting in the production of gold in the solution by means of the plant manufactured in Stepnogorsk city. The next stage of development was the elaboration of technology for gold extraction from sulfide-containing raw material by combining the processes of electrochemical leaching and sorption in one technological device.
The idea of combination included the creation of U-shaped electrochemical reactor, in which one branch is separated for cathode and anode chambers with interelectrode cation-exchange partition, and the other branch of the device is designed for airlift mixing of the ore grinded till the class 0,074 mm in 20-30% sodium chloride solution. At that the process of electrochemical gold leaching and sorption is combined with the gold concentration on the cathode covered with coal fiber.
To implement the idea, the flowchart and the chart of its hardware design have been developed. Hardware chart including two consequently working U-shaped devices for electrochemical sorption gold leaching is shown in the picture 1.
Beforehand, the studies by means of devices of box type have been realized.
The operation of devices of electrochemical leaching of box type with the use of cation-exchange membranes for the separation of cathode and anode chambers revealed the opportunity of their application, but in short-term operation – in pulp environments the membranes turned out to be short-lived and in several cycles showed cracks in the membrane MKK-1.
The gold came to the cationite into the cathode chamber separated from the anode chamber with the ion-exchange membrane only in the form of anion complex and the process rate was limited by the ion diffusion rate through the cation-exchange membrane that led to the increase of leaching time. The pulp after the electric treatment was discharged from the anode chamber, i.e. from the place where gold proper transformed into solution that stipulated the multi-stage process of the electrochemical leaching.
Based on the experienced gained, basically new structure of interelectrode partition was developed. The partition consists of two corrosion plates, in each of which holes are drilled angularly. Hydrogen extracted at cathode, chlorine at anode, as a result of electrochemical reaction, in this case cannot arbitrarily mix with each other any more, since the inclined holes result in the limited access to the zone of opposite sign; recovery of oxidized gold with hydrogen is minimized. At the same time, relatively free access for the pulp from one chamber to another is ensured, that leads to involvement of the whole volume of the pulp in the electrochemical process and helps to discharge the de-gilded pulp from any chamber.
1- scales; 2 - bunker; 3 - auger feeder; 4 - chute; 5 - conditioner; 6 - sizing screen; 7 - mill; 8 - thickener; 9 - repulper; 10 – device for electrosorption leaching; 11 - reactor; 12 - pump; 13 - nutsch filter; 14 – settling tank; 15 - arrester; 16 – tempering cabinet; 17 - burdening table; 18 – gold-melting oven; 19 - mould; 20 – laboratory scales
Picture 1 - Hardware chart of electrochemical, sorption gold leaching
As an anode, graphite was used, as a cathode, coal-fiber material based on graphite with the cover made of filter fabric. Such structure of cathode was explained by the need to regularly regenerate it. The cover made of filter fabric ensuring free access for solute gold to a cathode hinders the ingress of particles of ore material that significantly facilitates the process of regeneration of coal fiber.
The first test pachuca for electrochemical leaching was remade in conformity with the idea described above; cathode and anode chambers were equipped with new partitions and established at one branch of the U-shaped device, and the second branch served for the airlift mixing of the pulp. The total volume of both branches of the device is – 200 dm3.
The second 250 dm3 device was made of polyethylene pipe, 330 mm in diameter. The laboratory tests proved the efficiency of use of coal fiber as a cathode that allowed the simplification of the gold extraction flowchart. The process of ion exchange gold extraction for ionite resins with numerous auxiliary operations for resin cleaning from the sludge and regenerating solution was need no more. There was no need in the assembly for preparation of desorption solution, in the use of sizing screens to separate the resin before and after desorption etc.
To carry out the tests, sulfide raw material containing 11,0 g/t of gold was used – tails of gravity concentration of sulfide ores of “Altyn Alyans – 3” Ltd in amount of 2 t.
Semi-industrial test of the new technology was carried out in the following regime:
Voltage – 4,2 – 4,6 v;
Anode current density – 780-820 à/m2;
Pulp density – 1,35 kg/dm3;
S:L Relation = 1 : 3;
NaCl concentration – 200 – 220 g/dm3;
Duration of electrochemical treatment is 6 – 8 hours.
During the test, the value pH was noted at the output of pulp. Pulp sampling was taken every hour and was averaged for 8 hours; the averaged sample was analyzed for gold content.
As a coal cathode, coal fiber of the class ÍÒÌ – 100 was used. The current was supplied to the coal fiber through the graphite framework on which the coal fiber was coiled and then glass fiber cloth. Consumption of coal fiber per every cathode was 20 g. The results of the tests are given in the table 1. They show that: concentration of gold in liquid phase of waste pulp at the output from the second pachuca is 0,02–0,03 mg/dm3, that corresponds to 99,5% of its extraction. The results of the tests showed that involvement of one or two additional pachucas in the process flowchart ensures an opportunity to minimize the loss of gold in the liquid phase.
During the first series of tests at the duration of electrotreatment of 8 hours, the residual content of gold cleansed cakes amounted to ~ 1,7 g/t, that corresponds to 84% of gold extraction. Reduction of process time till 5 hours results in reduction of gold extraction by 9-10%.
The use in the tests relatively poor raw material did not allow the achievement of maximum capacity of electrocoal – the coal cathode with the capacity of 22 g/kg of gold was separated for regeneration. The rest of the gold was most probably left as a material in process.
Coal cathode was washed with water to cleanse from sludge, was dismantled from the graphite framework and treated with aqua regia at S:L relation = 1:5. After this treatment the coal fiber was washed with water, the wash water was mixed with gold-containing eluate, diluted with water by ~1,5 – 2 times. After the solution was settled during one hour, the sediment of the liquid phase was separated by means of the filtering. Out of the filtrate, the coffee like gold-containing sediment was extracted by means of treatment with the sodium sulfite; and then by means of melting 0,35 g of gold was produced.
The analysis of the remnants of the coal fiber after its treatment with aqua regia and washing with water showed that they contained up to 8 g/kg of gold. Relatively high residual capacitance of coal fiber proves the advisability of its direct treatment by means of burning and consequent extraction of gold from the ash, since the operation of cleansing of coal fiber from the sludge is quite labor intensive.
In whole, generalizing the results of the research of the developed technology, it is possible to state that the chart of combination of operations of electrochemical leaching and electrosorption with the use of coal fiber material is quite effective and operative and after certain upgrade can be recommended for implementation in industrial practice of gold production.