Heap leaching of gold from the destructured oxidized ore of the Belsu deposit, Republic of Kazakhstan

Heap leaching technology is increasingly being used to extract metals from poor ores, especially in the gold mining industry and is characterized by low capital costs compared to other methods. The purpose of this work is to develop modes and indicators of heap leaching of gold from the destructured oxidized ore of the Belsu deposit, Kazakhstan. The material composition of the ore has been studied, studies of percolation and pelletizing modes have been carried out. For the first time, studies on heap leaching for oxidized ore of the Belsu deposit (Kazakhstan) were conducted. It has been established that gold is well extracted by heap leaching from ore crushed to -25 mm and -12 mm after preliminary pelletizing. The degree of dissolution of gold from ore with a size of -25 mm and -12 mm has similar values – 74.41% and 76.57%, respectively. The leaching tails contain 0.478 g/t and 0.440 g/t gold. The consumption of sodium cyanide did not exceed 0.47 kg/t. The expected extraction of gold into the Dore alloy will be 66.60-68.53% with an initial gold content of 1.87-1.88 g/t in the ore. According to the developed technology, an industrial plant for heap leaching of gold from oxidized ore of the Belsu deposit was built.


Introduction
Heap leaching technology is increasingly being used to extract metals from poor ores, especially in the gold mining industry and is characterized by low capital costs compared to other methods.For many decades, the demand for heap leaching has been growing due to its environmental advantages [1][2][3][4][5].The long-term practice of foreign plants using this technology confirms their high technical and economic efficiency [4][5][6][7][8][9][10].
However, a big problem is the destructured and clay ores, which reduce the seepage of the gold-dissolving solution, which leads to decrease of gold extraction [11][12][13][14][15]. Based on the extensive experience of heap leaching operations, agglomeration of crushed ore can be successfully considered and used as a pretreatment of ore that contains a significant amount of fines and clay minerals [14][15][16][17][18][19][20][21].
The purpose of this work is to develop modes and indicators of heap leaching of gold from the destructured oxidized ore of the Belsu deposit, Kazakhstan with recommendations for use on an industrial scale.

Experiment 2.1. Materials
The gold-bearing ore of the Belsu deposit in the Abay district, Abay region, Republic of Kazakhstan was studied (figure 1).The technological sample is represented by two types of ores in terms of physical structure: destructured weathering crust (sometimes clumped together) and strong quartz-containing pieces of ore, the number of which is much smaller by weight.

Methods
According to the results of assay analysis, the average gold content in the ore is 1.88 g/t.The chemical composition of the ore is shown in table 1.
The industrial value in the sample is gold.Other metals are contained in insignificant quantities and are out of interest for industrial production.Arsenic is practically absent, and the antimony content was 0.082%.The sample belongs to a poor sulfide type of ore (Sulfide < 1%).According to the degree of oxidation, the sample is assigned to the oxidized zone.The results of wet sieve analyses of a crushed sample with a size of -25 mm and -12 mm are shown in figures 2-4.
The visual determination of a large amount of destructured ore in the sample was confirmed by the results of the sieve analysis.According to the physical condition, the initial ore sample (and even more crushed ore) is highly destructured.The amount of fine grade (-2.5 mm) in the crushed sample of -25 mm and -12 mm was 82-83%.This will create serious problems for leaching solutions to seep through the ore stack during heap leaching.

Laboratory tests of cyanide leaching of gold.
To evaluate the type of the gold in the ore, standard bottle tests were carried out on crushed ore with a grain size of 90% of the -0.071 mm class.To assess the change in the degree of dissolution of gold with an increase in the size of the ore, an additional bottle test is carried out on crushed ore with a size of -2.5 mm.Laboratory studies on cyanide leaching were carried out in a bottle agitator with a rotation speed of 30 rpm.The modes of leaching were as follows: the mass of ore samples for each experiment was 300 g, the ratio Solid:Liquid = 1:2, pH 10-11, the concentration of sodium cyanide was 0.100%, the duration of leaching was 24 hours.During the leaching process, the concentration of sodium cyanide and the pH of the solution were monitored, reagents were added if necessary.

Determination of percolation characteristics of the initial non-pelletized ore.
In world practice, when the ore size is less than 12 mm, as a rule, preliminary pelletizing of the ore with cement is carried out before stacking it.Pelletizing is carried out in agglomerating devices of various types.All this requires certain material costs and increases the cost of gold production [6,14,15,19].In this regard, the possibility of using non-pelletized raw materials was tested by determining its percolation properties.
To test the possibility of using non-pelletized ore for heap leaching, percolation tests of ore samples with a size of -25 and -12 mm, i.e. the size that will be used during column tests, were carried out.
Percolation tests were carried out in a column with a diameter of 97 mm and a height of 515 mm.The column was filled with 5.0 kg of ore, its compaction was performed by shaking, water was poured from the bottom up, the degree of shrinkage of the ore was determined after soaking, after holding for 2 hours and additional shaking, the maximum percolation rate of water through the ore layer was determined.

Research on the technology of heap leaching of gold.
Studies on the technology of heap leaching of gold were carried out in columns.Heap leaching of gold in columns was carried out in a closed cycle: leaching of gold from ore with alkaline cyanide solutions; sorption of dissolved gold with a sorbent; return of the solution to leaching after adjusting the concentration of sodium cyanide and pH.
Anion exchange resin AM-2B was used as a sorbent for the extraction of dissolved gold from productive solutions.The choice of ion exchange resin as a sorbent is due to the presence of a large number of fine sludge particles in the ore of the Belsu deposit.When using activated carbon, thin sludge particles will have a negative effect on resin, reducing its sorption activity.
The installation for conducting column tests included 2 columns for leaching gold from ore: • for ore size -25 mm: the diameter of the leaching column is 260 mm; the height of the column is 2420 mm; the height of the ore layer in the column is 2420 mm.The mass of ore in the column by dry weight is 136.97 kg.• for ore size -12 mm: the diameter of the leaching column is 253 mm; the height of the column is 2410 mm; the height of the ore layer in the column is 2410 mm.The mass of ore in the column by dry weight is 137.46 kg.Solutions of 0.05% (0.5 g/l) sodium cyanide were used as a leaching solution, the pH was maintained in the range of 10.5-11.0due to the addition of sodium hydroxide.

Results and discussion
The results of bottle tests of cyanide leaching of gold are presented in table 2. The results of bottle tests of cyanide leaching of gold from crushed ore (90% -0.071 mm) confirmed that the gold in the ore is mainly in free form, the degree of dissolution of gold from the crushed sample reached 94.5-95%.
From a crushed sample with a size of -2.5 mm, 84.62-84.94%(average 84.79%) of gold is dissolved, i.e. it decreases by 10%, but this decrease is not drastic and assumes fairly good indicators for heap leaching.
The consumption of sodium cyanide for interaction with minerals was 0.84-1.80kg/t.The results of percolation tests of crushed ore are shown in table 3. Table 3. Results of tests on percolation of crushed ore.

Name of indicators -25 -12
Degree of shrinkage of the ore after soaking, % 0.42 0.63 Degree of shrinkage of the ore after soaking and shaking with water, % 0.84 1.25 Percolation rate of water through the ore layer, m/h 0.01 0.01 The degree of shrinkage of the ore after soaking and shaking with water for both sizes met the requirements of the guidelines (no more than 10%), and the percolation rate of water through the ore layer is many times lower than the regulated one (at least 10 m/h).Ore without preliminary pelletizing is not suitable for heap leaching.
Pelletizing modes have been developed to conduct the heap leaching process.Portland cement of the M-400 D20 GOST 10178-85 grade was used as a binder for pelletizing ore.The results of tests to determine the optimal consumption of Portland cement are shown in tables 4, 4. All regulatory parameters of percolation for ore with a grain size of -25 mm and -12 mm are satisfied by the following modes: portland cement consumption of 18 kg/t; water consumption of 190.7 l/t (for a grain size of -25 mm) and 196.5 l/t (for a grain size of -12 mm); humidity of granules is about 18%.
Two column tests were carried out, with preliminary pelletizing of ore with a consumption of Portland cement of 18 kg/t.Figure 5 shows the obtained granules.
The results of studies on heap leaching of gold from pelletized ore are presented in figures 6-8.
To dissolve gold, 31 leaching cycles were required for ore with a grain size of -25 mm and 32 cycles for ore with a grain size of -12 mm.The total amount of productive solution to achieve complete leaching was 2.192-2.296m 3 /t.The consumption of sodium cyanide did not exceed 0.47 kg/t, the consumption of sodium hydroxide was 0.150-0.147kg/t.The gold content in the leaching tailings is 0.478 g/t and 0.440 g/t.Based on the data obtained, the gold balance was calculated during heap leaching of gold from the ore of the Belsu deposit (table 6).From ore, gold is satisfactorily dissolved during heap leaching.According to the balance, the degree of dissolution of gold from ore was 74.41-76.57%.
The obtained high technological indicators of column tests indicate that the oxidized ore of the Belsu site can be efficiently processed by heap leaching with preliminary pelletizing with cement.
According to the research results, technological regulations have been developed and an industrial plant for heap leaching of gold from the ore of the Belsu deposit has been built.

Economic of production
The total investment in the organization of the production site at the Belsu field amounted to 8 million US dollars.At the market price of gold of 1800 US dollars per 1 troy ounce, the total revenue will be 147.7 million US dollars.All in-sustaining costs 1042 US dollars per troy ounce IOP Publishing doi:10.1088/1755-1315/1254/1/01209410 of gold.The project implementation period with an annual output of 500 thousand tons of ore will be 6 years.The net present value for the period of the project implementation at a discount rate of 15% will amount to 24.8 million US dollars.The investment return period is 1 year.

Conclusions
The gold-bearing ore of the Belsu deposit is extremely destructured.The amount of class -2.5 mm limiting percolation during heap leaching was 82-83%.Processing of ore of such granulometric composition by heap leaching technology is impossible.
We have developed ore processing modes of the Belsu deposit with preliminary pelletizing.Cement consumption was 18 kg per 1 ton of ore.The degree of dissolution of gold from ore with a size of -25 mm and -12 mm has similar values -74.41% and 76.57%, respectively.The leaching tails contain 0.478 g/t and 0.440 g/t gold, respectively.The consumption of sodium cyanide did not exceed 0.47 kg/t.The consumption of sodium hydroxide was 0.150-0.147kg/t during gold leaching.The volume of passed solutions for complete leaching of gold was 2.192-2.296m 3 /t of ore, which characterizes a fairly good kinetics of gold dissolution.The expected extraction of gold into the Dore alloy during processing of the ore of the upper oxidized zone under industrial conditions will be 66.60-68.53%with an initial gold content of 1.87-1.88g/t in the ore.
For industrial conditions of heap leaching, it is recommended to use the ore of the Belsu deposit with a size of -25 mm with its preliminary pelletizing with cement.The developed technology made it possible to efficiently process this ore and obtain gold in the form of marketable products -the Dore alloy.The economic calculation confirmed the effectiveness of the developed technology, the technology has been introduced into industrial production.

Figure 1 .
Figure 1.General view of the ore deposit.

Figure 3 .
Figure 3. Change in gold content for ore of various sizes by class.

Figure 4 .
Figure 4. Distribution of gold for ore of various sizes by class.

Figure 6 .
Figure 6.Dependence of gold extraction on the duration of leaching.

Figure 7 .
Figure 7. Dependence of gold extraction on the amount of solution passed.

Table 1 .
Chemical composition of gold-bearing ore of Belsu deposit.

Table 2 .
Results of bottle tests of cyanide leaching of gold from samples of various sizes.

Table 4 .
Determination of optimum Portland cement consumption for pelletising -25 mm crushed ore.

Table 5 .
Determination of optimum Portland cement consumption for pelletising -12 mm crushed ore.