During cyanide gold extraction, the cyanide concentration, oxygen concentration, alkali concentration and the 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 reasons for the consumption should be focused on in the operation, so that these parameters are controlled within the selected index value range. In addition, efforts should be made to reduce energy consumption, add appropriate levels of lead oxide and examine several other minor parameters. Therefore, the amount of oxygen consumed when gold is dissolved is only one ten thousandth of the actual supply. [next] Mixer number 1 2 3 4 5 6 Cyanide saturation rate /% 35 45 45 55 90 100 3) Cyanide concentration control 1 Used as cyanide for cyanide gold 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 in the unit mass of cyanide, as well as the valence of the metal element constituting the cyanide and the relative molecular mass of cyanide. Table 1 lists the properties of these four cyanides and the relative ability to dissolve gold with 100% KCN. In the selection of ammonium compounds, 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  Relative solubility of gold in the properties of four cyanides name Molecular formula Relative molecular mass Valence Relative solvency of KCN (with KCN of 100 ) Relative consumption when equal solvency Stable sequence of solution Sodium cyanide NaCN 49 1 132.6 49 2 Potassium cyanide KCN 65 1 100 65 1 Calcium cyanide Ca ( CN ) 2 92 2 141.3 46 4 Ammonium cyanide CH 4 CN 44 1 147.7 44 3 Although cyanide gold extraction initially used almost exclusively KCN, modern gold extraction uses NaCN and sometimes Ca(CN) 2 because of its low relative ability to dissolve gold and its high price. [next] 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 the ferricyanide ion is 10 -37 , which is minimal compared with [Cu(CN) 4 ] 3 -about 10 -2 . [next] Due to the CO 2 introduced 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. e) Consumption caused by zinc minerals: Zinc minerals are soluble in cyanide solution, but the dissolution rate of zinc in the raw materials is small. The dissolution rates of zinc minerals measured by ESLeaver and JAWoolf in cyanide solution are shown in Table 2. In the cyanide leaching process with free cyanide ions and oxygen, the dissolution of zinc leads to an increase in cyanide consumption: [next] Table 2  Dissolution rate of zinc minerals in cyanide solution Mineral name Component Sample containing zinc /% Zinc dissolution rate /% Zinc silicate ZnSiO 4 1.22 13.1 Heterogeneous mine (ZnOH) 2 SiO 3 1.19 13.4 Sphalerite ZnS 1.36 18.4 Zincification 3ZnCO 3 · 2H 2 O 1.36 35.1 Red zinc mine ZnO 1.22 35.2 Sphalerite ZnCO 3 1.22 40.2 Note: The test conditions are to grind various zinc minerals to -100 mesh, add -100 mesh quartz to prepare a sample containing about 1.25% zinc, and leaching in 0.2% NaCN solution for 24 hours, the ratio of solid to liquid is 1:5. f) 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. 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 .
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1) Cyanide time control The so-called cyanide operation time refers to the time required for the operation of processing a proper amount 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 may take 5d 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 approaches 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, thereby greatly increasing the yield of the metal.
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 fine, 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%~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 diffuses into the gold dissolved in the crack, but the dissolution rate of the gold particles in the fracture is rather slow, because the agitation only plays 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 in practice the gold concentration does not increase to the extent that hinders the dissolution of gold, the cyanide time is long, and an increase in the concentration of gold in the solution affects the progress of the dissolution reaction.
â‘£, 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, impede or affect the dissolution of gold .
2) Oxygen concentration control 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. [next]
Under normal conditions, the cyanidation operation does not require control of high solution temperatures (except for the necessary conditions to prevent slurry freezing) and does not require an increase in oxygen pressure. The oxygen in the slurry is saturated only by the aeration of the mixer impeller or by the supply of compressed air. If the Pachuca air agitation leaching tank is used, the air is blown into a concentrated slurry tank containing 12 to 16 m depth by a blower to supply a high concentration of oxygen to 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:
a) White's colorimetric method: The alkaline pyrogallol solution is added to the cyanide solution, and the presence of oxygen is confirmed when the solution turns brown, and then the colorimetric determination of the oxygen content is performed.
b) Winergen's rapid volume method: using an indigo disulfonate as an indicator, titrated with dithionite.
c) Solid-state electrode polarography and Beckman (1970) oxygen electrode direct insertion: This is two methods for determining 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.
3 oxygen consumption. In 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, although the approximate small bubbles are dispersed throughout the slurry, most of them 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 4~5kg/t ore.
The rate at which oxygen is transported from the gas phase to the liquid phase is reduced by the concentration of the slurry. The transfer speed 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.
a) Oxygen consumption of 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 calculated according to the Elsner reaction formula: 
b) Metal iron consumption: The metal iron in the ore is mainly from the mechanical wear of the grinding equipment lining and the iron ball (about 0.5~2.5kg·t -1 ), and it also needs to consume 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 is self-repressing and only oxidizes its surface.
c) Sulfide oxygen consumption: 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.
JTWoodcock has studied a large number of oxidation activity mechanisms of sulfide minerals in aqueous solution and pointed out that the reaction of pyrite in aqueous solution can be expressed by the following formula:
4 FeS 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 mineral surface, and a small amount of pyrite can actually act as an oxidant.
d) Control of oxygen concentration: Since the distribution of air in the mixer is observable, many plants use any manual air-conditioning valve adjustment. EKPenrose 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 is:
The change of oxygen concentration in the stirred solution of the Pachuca air-mixing leaching tank and mechanical mixer cited by A.King is shown in Fig. 1. [next]
The sodium cyanide used in each plant is different. The purity of NaCN commonly used in production is 94%~98%. The two solid sodium cyanide used in the Australian mines are good. The use of these two solid sodium cyanide can reduce transportation and storage costs. One of them is a white block, containing about 98% by mass of NaCN; the other is a small piece, which consists of sodium cyanide, sodium chloride, free base and carbon, and contains about 48% by mass. NaCN. Canada and South Africa use a liquid sodium cyanide containing approximately 30% by weight of NaCN, which works well.
2 The concentration of cyanide in the solution. The solubility of sodium cyanide in water is more than 30%, far exceeding the range of any concentration 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 cyanide solution is usually in the range of 0.02% to 0.1% (determined by silver nitrate droplets), and the concentration in the diafiltration solution is in the range of 0.03% to 0.2%. However, it should be noted that the true concentration of free cyanide in the solution is usually smaller than the titration value because the titration value includes such complexes as [Zn(CN) 4 ] 2- and [Cu(CN) 4 ] 3- Cyanide in the middle.
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 the cyanide present in the sample reacted with silver to form a silver cyanide complex, the human silver nitrate was 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 -
Therefore, it is preferred to determine the cyanide in the solution as "total cyanide", ie cyanide including free cyanide and copper and zinc complexes, and cyanide which may exist as other compounds, but excluding thiocyanide ( CNS - ). The method of determination is that the sample of the clear solution is acidified, and the HCN is volatilized by distillation and trapped in the NaOH solution, and then titrated with a silver nitrate standard.
The iodometric method measures the CN - concentration in the solution without interference from Cl - and Ag + and also allows the sample 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 cannot be grasped, it should 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 was prepared at a rate of KI 35 g per liter plus I 2 13 g, and the standard concentration was calibrated with a known concentration of Na 3 AsO 3 or Na 2 S 2 O 3 solution. The quantitative reaction of the iodometric method is:
CN - +I 2 =====CNI+I -
When the iodine standard c (0.5I 2 ) = 0.1 mol/L, the sodium cyanide component p (NaCN) = 2.450 g/l.
Analytical procedure: Take about 50mL of NaCN concentration of sample (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 typically 0.1 mL).
For turbid samples, add CCl 4 3-4mL to vigorously shake and titrate 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 of the iodometric method is 0.12% to 1.34% lower than that of the silver 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, usually 250 to 500 g. The consumption of sodium cyanide in pyrite concentrate and calcine is 2-6kg/t, resulting in such high sodium cyanide consumption due to:
a) Self-decomposition of cyanide: When the solution is adjusted, the cyanide in the solution will slowly decompose to form carbonate and ammonia. But this loss is not important.
b) 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, the ratio of cyanide decomposition to HCN and CN- is shown in Fig. 2. When pH is 7, cyanide almost completely forms HCN; when pH is 12, cyanide is almost completely dissociated into CN - . The cutoff value of both is about pH=9.3. [next]
The volatilization loss of HCN mainly occurs during vacuum filtration of slag and degassing of mother liquor.
c) 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+ formed by oxidation can form a ferricyanide complex with cyanide ions at pH=9-10. At pH=11~12, 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.
d) Consumption caused by copper minerals: The dissolution rate of copper minerals in cyanide solution as determined by ESLeaver and JAWoolf proves that many copper minerals are soluble in cyanide liquor. 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
2Zn+8CN - +O 2 +2H 2 O→2[Zn(CN) 4 ] 2- +4OH -
g) Mechanical loss of cyanide: The amount of 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 solution samples at 1h or 2h for titration with silver nitrate standard solution, and control the cyanide concentration in the cyanide solution by adjusting the amount of cyanide feeder according to the titration results. [next]
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.
GTWOimrod (1974) et al 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, the Wuhan Institute of Instrumentation and Research has developed the DWH-201 “total cyanide†measuring instrument with a measuring range of (0.0026-260)×10 -6 and a maximum relative error of ±10%.
Hedley et al. pointed out that the molar ratio of total cyanide ion concentration in solution to cyanide ion concentration of copper-forming cyanide complex must exceed 4:1, and the dissolution rate of gold can be satisfactory.
4) Alkali concentration control During the cyanidation operation and during storage of the cyanide-containing solution, it is necessary to maintain a certain concentration of free alkali in the solution. This is called "protective base". It neutralizes the acid produced during the cyanidation process, prevents the decomposition of cyanide and the production of HCN gas, and the pH required to dissolve the gold normally.
Lime is a common base used in most cyanide plants. It is cheap and contributes to the agglomeration of solid materials in the slurry, which accelerates the concentration of the slurry and facilitates filtration.
1 lime dosage and alkalinity. The lime concentration limit in the solution is about 0.15% CaO. The CaO-containing range during normal operation is 0.002% to 0.012%. At this time, the corresponding pH is 9~120. A few cyanide plants are operated under the condition of “negative alkalinity†(ie, the solution is close to slightly acidic). After the addition of phenolphthalein, the free cyanide ion concentration is titrated. The operation of some cyanide plants is carried out under high alkalinity conditions, which contributes to the decomposition of the telluride.
Most cyanide plants operate under high alkalinity 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). In summary, the CaO content of the solution must be determined or the pH of the slurry determined based on the specific conditions of the ore. [next]
2 Determination of alkalinity. The correct method for determining the alkalinity of the slurry is, in principle, the addition of acid to the sample to pH = 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 →CaSO 4 +H 2 O
CaO+(COOH) 2 H 2 O→Ca(C00) 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 determination at pH 8-9, the results are inaccurate, so the cyanide should first be decomposed to form HCN for removal.
In pure lime, there is a specific ratio between the concentration of CaO and the pH. However, in the practice of cyanidation, the ratio between the concentration 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. Typically lime consumption ranges from 0.25-5 kg ​​CaO per ton of ore to 15 kg CaO per ton of pyrite concentrate calcine. The amount of CaO contained in the unit mass 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 formation of calcium sulfate precipitates, which is often difficult to avoid in many cyanide plants. The main reasons for the increase in lime consumption are:
a) When inflating the mixer, CO 2 is introduced from the air;
b) oxidation of pyrite or other sulfides to form acid sulfides;
c) using water to bring in acidic substances and certain metal (magnesium, aluminum ) ions;
d) Some specific substances in the ore react with the base.
(A) Control of pH. The pH of the control solution is controlled by controlling the lime concentration of the solution. Most cyanide plants take the solution sample every 1-2 hours, and manually adjust the lime addition amount of the lime feeder according to the measurement result to control the lime concentration. 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.