Technical conditions for cyanidation

During cyanide gold extraction, the cyanide concentration, oxygen concentration, alkalinity and time required to complete the dissolution should be controlled. For the first three parameters, the concentration of the first parameter, the method of measuring the concentration, and the cause of the consumption should be emphasized in the operation, so that these parameters are controlled within the selected numerical range. In addition, efforts should be made to reduce energy consumption, add appropriate levels of lead oxide and examine several other minor parameters.

I. Cyanide operation time and its condition control

The so-called cyanide operation time refers to the time required for the operation of processing an appropriate amount of a batch of raw materials. Cyanide leaching was stirred job comprises dissolving the above slurry was stirred for 4h most of the gold, the gold telluride 72h complete decomposition of the ore. Leaching leaching can take up to 5 days or longer.

In fact, the allowable cyanide time is only to maximize the extraction rate of gold, and it is impossible to extract all the gold in the raw materials. This is because:

(1) In terms of dissolution rate, the leaching efficiency of gold is very low when the leaching process is near the end point. In the end, the cost of extracting 1% of gold may be higher than the actual value of recycled metals.

(2) In terms of time, if the final leaching time of reducing the batch of raw materials is used to treat another batch of raw materials, the total amount of raw materials can be increased in the same time, so that the production of metal can be increased by an adult.

The following factors also indicate the need to reduce the final leaching time and final gold extraction rate for cyanidation operations.

(1) During the whole cyanide decomposition, the surface area of ​​the gold particles is continuously reduced, the coarse particles are thinned, and the fine particles are completely dissolved, so that the gold content in the ore is getting lower and lower. Therefore, the cyanidation operation can first dissolve 60% to 70% of the gold in the ore, and then the slurry is sent to the cyanide grinding process for grinding.

(2) As the dissolution continues, the solution can diffuse into the gold dissolved in the crack, but the dissolution rate of the gold particles in the crack is rather slow, because the stirring can only play a role in slowly conducting the medium. At this time, increasing the stirring strength does not accelerate the dissolution process of gold in the crack.

(3) Although the gold concentration does not increase to the extent that hinders the dissolution of gold in practice, the cyanidation time is long, and the increase in the concentration of gold in the solution affects the progress of the dissolution reaction.

(D) will be generated into the iron-containing, sulfur and iron ball mill lined with wear iron iron sulfide, stibnite, etc. for a long time in the surface of the gold cyanide-insoluble mineral film, block or impact gold Dissolved.

2. Oxygen concentration and its control during cyanidation

During the cyanidation operation, oxygen is supplied by aeration into the slurry. The solubility of oxygen in the cyanide solution is one of the main factors determining the effect of cyanide gold extraction.

(1) The concentration of oxygen in the solution

The solubility of oxygen in an aqueous cyanide solution varies with temperature and pressure on the liquid surface. In a special equipment, the solubility of oxygen in the solution depends mainly on the local atmospheric pressure on the liquid surface of the equipment and the salt concentration of the solution during operation. Generally, the highest solubility of oxygen in water is in the range of 5 to 10 mg ∕L (5 to 10) × 10 ^{- 4} %).

Under normal conditions, the cyanidation operation does not require control of high solution temperatures (except for the necessary conditions to prevent slurry freezing), nor does it require an increase in oxygen pressure (eg, using high pressure air or supplying oxygen in a closed vessel). Instead, the oxygen in the slurry is saturated by the aeration of the mixer impeller or by the supply of compressed air. If the Pachuca air agitation leaching tank is used, the exhaust gas is blown into a concentrated slurry tank containing 12 to 16 m depth by a blower, and a high concentration of oxygen is supplied into the slurry.

(2) Determination of oxygen concentration

The concentration of oxygen in the cyanide solution can be determined in a number of ways. The commonly used methods are:

1. White's colorimetric method. The alkaline pyrogallol solution was added to the cyanide solution, and the presence of oxygen was confirmed when the solution turned brown, and then the colorimetric determination of the oxygen content was carried out.

2. Weinig's fast capacity method. The indigo disulfonate was used as an indicator and titrated with dithionite.

3. Solid electrode polarography and Bechman oxygen electrode direct insertion. This is two methods for measuring oxygen concentration in modern times. As long as the electrode is slightly inserted into the slurry, the oxygen content corresponding to the sample of the clear solution can be obtained.

In view of the difficulty in determining the absolute concentration of oxygen in a solution, a rapid method is generally employed to determine the percentage of oxygen saturation in the solution. This readily available value allows for a more efficient comparison of the actual changes in oxygen concentration of the solution between the various devices due to different aerations. The oxygen content in the cyanide solution can be determined from a sample of a solution of a particularly saturated oxygen through which air bubbles pass through the apparatus, using standard methods.

(three) oxygen consumption

At the cyanide plant, the amount of oxygen that the ore needs to consume is not known. The main loss of oxygen in the total consumption is during the grinding and classification process. In addition, although the total amount of oxygen supplied by the agitation is known (the same air is supplied), these oxygens are generally not always available. This is because the supplied air is dispersed in an approximate small bubble distributed throughout the slurry, but most of it may later escape to the atmosphere. Therefore, the amount of oxygen actually used is greatly different from the amount of oxygen supplied to the air. The standard consumption of oxygen may be only 4 to 5 kg 矿t ore.

The rate at which oxygen is transported from the gas phase to the liquid phase is reduced by the thickness of the slurry. The transfer rate in viscous pulp is much slower than in water. Therefore, some plants use dilute slurry as much as possible to increase the solubility of oxygen.

The main oxygen-consuming substances in the ore are gold, iron, and sulfide.

1, gold. The oxygen consumed by gold during the dissolution process is only a small fraction of the total oxygen consumption. If the gold content per ton of ore is 8g, the amount of oxygen required to dissolve the gold can be determined by the Elsner reaction:

4Au+8NaCN+O ₂ +2H ₂ O 4NaAu(CN) ₂ +4NaOH

Calculate:

4Au+8CN ^- +O ₂ +2H ₂ O 4Au(CN) ₂ ^- +4OH ^-

O ₂ ∕g·t ^{- 1} = 8× =0.32

Therefore, the amount of oxygen consumed when gold is dissolved is only one ten thousandth of the actual supply.

2. Metal iron. The metal iron in the ore is mainly derived from the mechanical wear of the grinding equipment lining and the iron ball (about 0.5 to 2.5 kg per ton), which also consumes a certain amount of cyanide and oxygen. In closed-circuit operation, the oxidation of metallic iron may first form an iron oxide film on the surface of the gold particles to prevent or prevent further dissolution. However, the oxidation of metallic iron can inhibit itself and only oxidize the surface.

3. Sulfide. Sulfide in the slurry is the main consumer of oxygen. Pyrite is almost always present in gold ore, and its oxidation rate is controlled by chemical action at room temperature. Pyrite and pyrrhotite often consume significantly more than stoichiometric amounts of active oxygen. In order to obtain a satisfactory cyanidation effect, the ore should be calcined before cyanidation. Alternatively, most of the sulfides are removed by enrichment by appropriate aeration, and are often one of the important measures to increase the cyanidation effect.

After extensive research on the mechanism of oxidation activity of sulfide minerals in aqueous solution, JT Woodcock pointed out that the reaction of pyrite in aqueous solution can be expressed by the following formula:

4FeS ₂ +16OH ^- +15O ₂ 8SO ₄ ^{2 -} +4Fe(OH) ₃ +2H ₂ O

The final product shown by the reaction formula is sulfate ion and iron hydroxide, but the intermediate product includes ferrous ion and thiosulfate, and thiocyanate and ferricyanide complex formed in the presence of cyanide. The reaction process is controlled by chemical action, which tends to inhibit itself, producing iron hydroxide precipitate only on the surface of the mineral, and a small amount of pyrite can actually act as an oxidant.

4. Control of oxygen concentration. Since the distribution of air in the mixer is observable, many plants use either manual air conditioning valve adjustments. EK Penrose et al. measured data for six Delphus mixers. Each mixer has a diameter of 10m and a depth of 5.4m. The air supply of each mixer is 14m ³ ∕min, 85kPa, and the oxygen saturation rate of the solution stirred by each mixer (see Table 1):

Table 1 Oxygen saturation rate of the solution stirred by each mixer

A. Gold (King) cited Pachuca air stirring leaching tank and mechanical mixer stirring solution oxygen concentration changes as shown in Figure 1.

Figure 1. Relationship between changes in oxygen concentration and gold dissolution in Pachuca air mixers and mechanical mixers (Woodcock, 1949)

3. Cyanide concentration and its control during cyanidation

(1) Cyanide used as gold for cyanide extraction

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 2 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 2 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.

(2) Concentration of cyanide in 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 .

(3) 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.

(4) 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 produces 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 ^- ratio shown in Fig. 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 2 CN cyanide generated at 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 3) 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 3 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 measured by ES Liv and JA Wulf are shown in Table 4. 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 ^-

Table 4 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.

4. The concentration of alkali and its control during cyanidation

During the cyanidation operation and during the storage of the cyanide-containing solution, it is necessary to maintain a certain concentration of free alkali in the solution, which is referred to in the literature as "protecting the base". It neutralizes the mineral acid produced during the process, prevents the decomposition loss of cyanide and produces HCN gas, and obtains the pH required for the gold to dissolve normally.

Lime is a common base used in most factories. It is cheap and contributes to the agglomeration of solid materials in the slurry, which accelerates the concentration and filtration of the slurry.

(1) The amount and alkalinity of lime

The lime concentration limit in the solution is about 0.15% CaO. The concentration during normal operation ranges from 0.002% to 0.012% CaO. At this time, the corresponding pH is 9-12. The step number plant is operated under the condition of “negative alkalinity” (ie, the solution is close to slightly acidic), and the concentration of free cyanide ions is titrated after the addition of phenolphthalein. The operation of some plants is carried out under high alkalinity conditions, which contributes to the decomposition of the telluride.

Most plants operate under high alkali conditions in order to reduce the loss of cyanide. It is advantageous to use a solution of low pH if some of the sulfides in the ore are more susceptible to oxygen in high pH solutions.

In order to quickly dissolve gold in certain minerals, the pH of the solution should be maintained at least at a level of 9 (Figure 3). Of course, in order to rapidly decompose several tellurides in the ore, it is more necessary to operate under high pH conditions.

Figure 3 The dissolution rate of several natural gold and pure gold in different pH solutions

In short, the CaO content of the solution must be determined or the pH of the slurry determined based on the specific conditions of the ore.

Further, when water containing a magnesium salt, the concentration of CaO in the solution should be maintained at 0.002% or less, to avoid excess lime solution generating the magnesium salt Mg (OH) ₂ precipitates. In addition, high alkalinity may contribute to the settling speed of the slurry in the concentrating equipment .

(2) Determination of alkalinity

The correct method for determining the alkalinity of the slurry is, in principle, the addition of an acid titration to the sample to a pH of about 10 as the end point. However, in order to eliminate the normal interference of cyanide, the classical method is to take the sample with sulfuric acid or oxalic acid standard droplets to check pH 8.3 with phenolphthalein test paper as the end point. The response is:

CaO+H ₂ SO ₄ CaSO4+H2O

CaO+(COOH) ₂ H ₂ O Ca(COO) ₂ +2H ₂ O

The important thing here is to eliminate the interference of cyanide on titration. Since many cyanides, especially zinc cyanide complexes, interfere with the measurement at pH 8-9, the results are inaccurate, so the cyanide should first be decomposed to form HCN for removal.

In pure lime liquor, there is a specific ratio between the percent concentration of CaO and the pH. However, in the practice of cyanidation, the ratio between the percentage of CaO and the pH is likely to change greatly due to the constant change of the composition of the solution. However, the measurement can be used to find out the changes between the two to meet the needs of the operation process.

(3) Lime consumption

The lime consumption range reported in the literature ranges from 0.25 to 5 kg CaO per ton of ore to 15 kg CaO per ton of pyrite concentrate calcine. The amount of CaO per unit weight of slaked lime [Ca(OH) ₂ ] and quicklime (CaO) is different, and should be distinguished when it is used in production and reading. In practice, CaO entering the solution increases the consumption of lime due to the precipitation of calcium sulphate, which is often difficult to avoid in many plants. The main reasons for the increase in lime consumption include:

1. CO ₂ brought in from the air when inflating into the mixer;

2. Acidic sulfides formed by oxidation of other sulfides in the pyrite field;

3. Use acidic substances brought in water and certain metal (magnesium, aluminum ) ions;

4. The reaction of certain specific substances in the ore with alkali.

(4) pH control

The pH of the control solution is achieved by controlling the lime concentration of the solution. In most factories, the solution sample is taken every 1~2h, and the lime concentration is controlled by manually adjusting the lime addition amount of the lime feeder according to the measurement result. It is feasible to automatically measure and automatically control the pH of the slurry using a glass electrode, but in the high pH range, the measurement results are often unsatisfactory.

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