Cyanide concentration during cyanidation and its control

1. Cyanide used as gold for cyanidation

Cyanide used in cyanide gold extraction includes alkali metal cyanide and alkaline earth metal cyanide. Commonly used are sodium cyanide, potassium cyanide, ammonium cyanide and calcium cyanide. The relative ability of each cyanide to dissolve gold is determined by the amount of cyanide per unit weight of cyanide, as well as the valence of the metal element constituting the cyanide and the molecular weight of the cyanide. Table 1 lists the properties of the four cyanides and the relative ability to dissolve gold with 100% KCN. When choosing cyanide, factors such as their relative ability to dissolve gold, stability, price, and the effect of impurities contained on gold dissolution must be considered.

Table 1 Properties of four cyanides and their relative solvency to gold

Although Cyanide gold was initially used almost exclusively for KCN, it is sometimes used in modern gold extractions using NaCN and sometimes Ca(CN) ₂ because of its low relative ability to dissolve gold and its high price.

The sodium cyanide used in each plant is different, and the purity of NaCN commonly used in production is 94% to 98%. The two solid sodium cyanide used in the Australian mines are good. The use of these two solid sodium cyanide reduces transportation and storage costs. One of them is a white block, containing about 98% NaCN; the other is a small piece, which consists of sodium cyanide, sodium chloride, free base and carbon, and contains about 48% NaCN. The use of a liquid sodium cyanide containing approximately 30% NaCN in Canada and South Africa is also very effective.

Second, the concentration of cyanide in the solution

The solubility of sodium cyanide in water is above 30%, far exceeding any concentration range required for cyanidation practice. Under normal operating conditions, a balance should be achieved between the dissolution rate of gold and the consumption of cyanide. The concentration of sodium cyanide in the cyanidation solution is usually in the range of 0.02% to 0.1% (determined by silver nitrate droplets), and the concentration in the diafiltration leaching solution is in the range of 0.03% to 0.2%. It should be noted, however, that the true concentration of free cyanide in the solution is usually smaller than the titration value because the titration value includes cyanide in a complex such as Zn(CN) ₄ ² and Cu(CN) ₄ ² .

Third, the determination of cyanide concentration

The method of determination of cyanide, the sample is taken clear solution, potassium iodide as an indicator in a given standard solution with silver nitrate. The response is:

2NaCN+AgNO ₃ =NaAg(CN) ₂ +NaNO ₃

AgNO ₃ + KI=AgI↓+KNO ₃

That is, silver and cyanide react to form a silver cyanide complex. When all of the cyanide present in the sample reacted with silver to form a silver cyanide complex, the further dropped silver nitrate reacted with iodine to form a silver iodide precipitate, indicating the end point of the titration.

This method can determine the true concentration of all free cyanide ions in pure cyanide solution, but there will always be undissociated NaCN or Ca(CN) _{2 in} any cyanide solution. Moreover, the total solution used for cyanidation Contains copper cyanide salt and zinc cyanide salt, and they can be liberated from cyanide. Taking copper cyanide anion as an example, its dissociation reaction is:

Cu(CN) ₄ ^{3 -} Cu(CN) ₃ ^{2 -} +CN ^-

Cu(CN) ₃ ^{2 -} Cu(CN) ₂ ^- +CN ^-

Cu(CN) ₂ ^- CuCN+CN ^-

The zinc cyanide anion can also release cyanide from the explanation. Therefore, the law does not indicate the end point exactly. However, the dissociation constant of ferricyanide complex ions is ^10-37, and Cu (CN) ₄ ^{3 -} compared to about 10 ^-2, the former effect is minimal.

Therefore, it is preferred to determine the cyanide in the solution to determine "total cyanide", i.e., free cyanide and cyanide as a copper, zinc complex, and cyanide which may exist as other compounds. But does not include thiocyanide (CNS ^- ). The method is determined by adding acidified acid to the sample of the clear solution, and the HCN is volatilized by distillation and trapped in the NaOH solution, and then determined by using silver nitrate standard droplets.

The concentration of CN in the solution was determined by iodometric method and was not interfered by Cl ^- and Ag ⁺ , and the sample was allowed to be slightly turbid. The iodine standard solution is easier to store than the silver standard solution, the concentration is stable, and the operation is simple. When the end point judgment is not grasped, it can be processed continuously several times until it is satisfied. The tester used this method for on-site analysis of heap leaching. When the ore composition was not too complicated, the results were consistent with the results of the silver salt titration method. Therefore, this method is especially convenient for production monitoring, sodium cyanide quality testing and cyanide liquid preparation.

The iodine standard solution is prepared in an amount of KI 35 g per liter plus I2 13 g, and the standard concentration is calibrated with a known concentration of Na ₃ AsO ₃ or Na ₂ S ₂ O ₃ . The quantitative reaction of the iodometric method is:

CN ^- +I ₂ =CNI+I ^-

When iodine standard solution C ( When I ₂ )=0.1000 mol/L, p(NaCN)=2.450 g/L.

Analytical procedure: Take about 50mL of sample NaCN concentration (100mL of low concentration sample, can measure 0.02g ∕L NaCN concentration), put it in 500mL volumetric flask, add NaHCO ₃ 1-2g to control alkalinity, under shaking The iodine standard droplets were set to the end of the pale yellow color consistent with the blank test (the blank is generally 0.1 mL).

For turbid samples, add 3 to 4 mL of CCl ₄ and shake vigorously until the organic layer is light reddish purple. If you can't grasp it, you can add the original sample to the organic layer to fading, and then use the iodine standard to determine the droplet. This operation can be repeated multiple times until the end point is judged to be satisfactory.

According to the comparison of the four samples, the measurement result by the iodometric method is 0.12% to 1.34% lower than the silver amount method.

Fourth, the consumption of cyanide

During the cyanidation operation, the consumption of cyanide per ton of ore is in the range of 250 to 1000 g of ore, usually 250 to 500 g. The consumption of sodium cyanide in pyrite concentrate and calcine is 2-6 kg∕t. The high sodium cyanide consumption caused by this is due to:

(1) Self-decomposition of cyanide. When the solution is adjusted, the cyanide in the liquid slowly decomposes to form carbonate and ammonia. But this loss is not important.

(2) Hydrolysis to form HCN. As the pH of the solution decreases, cyanide typically hydrolyzes to form volatile HCN with loss. The response is:

NaCN+H ₂ O HCN↑+NaOH

When the pH in the solution increases, cyanide decomposes into free cyanide ions in the solution. In different pH solutions, decomposition of cyanide HCN and CN ^- Figure 1 ratio. It can be seen from the figure that when the pH is 7, cyanide almost completely forms HCN; when the pH is 12, cyanide is almost completely dissociated into CN ^- . The cutoff value of both is about pH 9.3.

FIG 1 CN cyanide generation in different pH solutions ^- and HCN ratio (Willis, 1948)

Owing to the CO ₂ carried in the air, the acidic substances brought in the water, and the inorganic salts (such as carbonates) contained in the ore, or the products formed by the oxidation of the sulfide minerals, the acid solution is lowered to lower the pH, and if necessary, the operation is performed. The water used should be treated with alkali first.

The volatilization loss of HCN mainly occurs during vacuum filtration of slag and degassing of mother liquor.

(3) Consumption caused by iron sulfide. Iron sulfide can consume oxygen, alkali and cyanide in the solution. Pyrite is usually not very active, but pyrrhotite is usually quite lively. In the reaction, Fe ^{2 +} produced by oxidation can form a ferricyanide complex with cyanide ions at pH 9-10. At pH 11 to 12, the oxidized sulfur easily forms thiocyanate. However, the cyanide consumed by the pyrrhotite can be reused after adding lead oxide or other lead salt to the solution.

(4) Consumption caused by copper minerals. The dissolution rate of copper minerals in cyanide by the measurements of Lesver and Woolf (Table 2) demonstrates that many copper minerals are soluble in cyanide. Among these copper minerals, the presence of chalcopyrite has little effect on cyanidation. However, when the ore contains a small amount of copper carbonate, the cyanide consumption is too large, and the cost is increased, so that the cyanidation treatment cannot be used. The decomposition reaction of copper carbonate in cyanide solution is as follows:

2CuCO ₃ +8NaCN 2Na ₂ Cu(CN) ₃ +(CN) ₂ ↑+2Na ₂ CO ₃

That is, every 1 mol (molecular) copper carbonate consumption

=1.117 mol (molecular) NaCN.

Table 2 Dissolution rate of some copper minerals in cyanide solution ¹

1 Test conditions: various copper minerals were separately ground to -0.15mm (100 mesh), with -0.15mm quartz sand to prepare a 0.2% copper sample, leached in 0.1% NaCN solution for 24h, solid materials Accounted for 9%.

(5) Consumption caused by zinc minerals. Zinc minerals are soluble in cyanide liquor, but the dissolution rate of zinc in the feedstock is small. The dissolution rates of zinc minerals in cyanide solution determined by ES Liv and JA Wulf are shown in Table 3. In the cyanide leaching process with free cyanide ions and oxygen, the dissolution of zinc leads to an increase in the consumption of cyanide:

2Zn+8CN ^- +O ₂ +2H ₂ O 2Zn(CN) ₄ ^{2 -} +4OH ^{- Â} 

Table 3 Dissolution rate of some zinc minerals in cyanide solution ¹

1 Test conditions: Various zinc minerals were separately ground to -100 mesh, and -100 mesh quartz sand was added to prepare a sample containing about 1.25% zinc, and leached in 0.2% NaCN solution for 24 hours, and the solid-liquid ratio was 1:5.

(6) Consumption caused by arsenic and antimony minerals. Arsenic minerals react in cyanide to form S ^- , AsS ^{3 -} , CNS ^- , S ₂ O ₃ ^{2 -} , AsO ₃ ^{3 -} and AsO ₄ ^{3 -} . Bismuth minerals also react to form similar substances. Most of these materials are easily formed at a pH greater than 11, and the dissolution rate of gold is lowered. However, the addition of lead nitrate (0.15-0.75 kg ∕t) at pH 9-10 helps to eliminate the harmful effects of these minerals.

(7) Mechanical loss of cyanide. The magnitude of the mechanical loss of cyanide depends on the total amount of wash water, the final slurry concentration, the manner of final solid-liquid separation, and the cyanide content of the final residue. When discarding cyanide-depleted residues, consideration should be given to minimizing this loss of cyanide.

5. Control of cyanide concentration

Most factories take manual solution titration with silver nitrate standard solution every 1 or 2 hours, and control the cyanide concentration in the cyanide solution by manually adjusting the amount of cyanide feeder according to the titration result.

In Canada, continuous automatic titration was used in 1964 to determine the free cyanide ion concentration in solution. A device for the determination of copper cyanide complex in cyanide solution was developed in 1969.

GTW Omrod (1974) reported that South Africa successfully used silver electrodes and reference electrodes to indicate the cyanide concentration during the Kegold system leaching operation. In recent years, Wuhan Instrument and Meter Research Institute has developed DWH-201 "total cyanide" measuring instrument, the measuring range is (0.0026 ~ 260) × 10 ^-6 , the maximum relative error is ± 10%.

Hedley et al. pointed out that the molar ratio of total cyanide ion in solution to cyanide ion concentration in copper cyanide complex must exceed 4:1, and the dissolution rate of gold can be satisfactory.

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