CH 2 (CN) 2 +OH - ==== [CH(CN) 2 ] - +H 2 0
The ions complex with gold to form Au[CH(CN) 2 ] into the solution. The complex is larger than the gold cyanide ion, and often exceeds the inner pores of the carbonaceous particles, so that the adsorption rate of carbon is lowered, so The dinitrile leaching gold from the carbonaceous ore can achieve a higher leaching rate. Using 0.05% malononitrile and enough lime to make a slurry with a pH of 9. When the ore contains 0.2% organic carbon, 83% gold can be leached with malononitrile, and the conventional cyanidation leaching rate is only 67%. When the ore contains 0.3% organic carbon, the gold leaching rates of the two methods are 56% and 33%, respectively. If a resin slurry process is used at the same time, 61.7 kg of Ionac A-300 anion exchange resin is added to the slurry, the gold leaching rate can be increased from 83% to 95%, and the gold malononitrile complex adsorbed on the resin is strongly inorganic. The acid can be completely eluted.
As part of the US Bureau of Mines research program, three organic nitrile compounds (malononitrile, acetonitrile, and cyanoacetamide) were tested to replace sodium cyanide. Several leaching agents were tested for an oxidized ore, two carbonaceous ores and one sulphide ore. The test results show that malononitrile is the most successful one of organic leaching agents. The leaching of oxidized ore is similar to that of sodium cyanide, and the gold leaching rate is about 80%. When the carbonaceous ore was ineffective by leaching carbonaceous ore with sodium cyanide, the leaching rate with malononitrile was 24%. When the slurry method was used for the leaching of the slurry, the recovery of sodium cyanide was 60%, and the recovery of malononitrile was 80%. The leaching rate for sulphide ore is the same at 80%. As measured by LD50 (lethal dose), malononitrile is 6 times less toxic than sodium cyanide.
It has been reported that some derivatives related to malononitrile, such as α-hydroxynitrile, lactonitrile and mandelic acid nitrile, have a leaching rate of gold in the carbonaceous ore ten times higher than that of the cyanidation method. The content of the base is evaluated for its gold dissolution effect, and it is most advantageous to use a lactamonitrile having a higher nitrile group. Further cyanoacetamide, cyano blue ethyl acetate, malononitrile bromine, dichloro malononitrile etc., at dosages up to 1% gold extraction was 75% to 90%, in addition to the advantages of organic nitriles Carbonaceous ore is more suitable, but also has the advantages of cheap price and sufficient source.
The nitrile dinitrile method was proposed by the US Bureau of Mines and obtained a US patent. The treatment of carbonaceous ore is slightly stronger than the cyanidation method, but the superiority is not outstanding, and the toxicity and volatility of malononitrile are and recovering gold from the solution when a simple zinc, aluminum or magnesium powder substitutions are not effective, so that the process for industrial production of too early.
Cyanide acid method and α-hydroxynitrile method for leaching gold 1) Cyanide bromine acid method for leaching gold Cyanuric acid is prepared by mixing bromine water and potassium cyanide solution in a factory preparation room:
2KBr+KBrO 3 +3KCN+3H 2 S0 4 —→ 3BrCN+3K 2 S0 4 +3H 2 0
Cyanuric acid is only decomposed in an alkaline solution, but its reaction to dissolve gold is carried out in a neutral or slightly acidic solution as follows:
BrCN+3KCN+2Au —→ 2KAu(CN) 2 +KBr
This method has been used in gold ore processing tellurium Australia Kalgoorlie, and for Hannan Si. Star Gold Factory and Kirkland Lake Gold Factory. Although this method has long been replaced by other more economical processes, it is still effective to leach gold from gold-containing compounds and sulfide concentrates.
2) α-Hydroxynitrile Leaching of Gold α-Hydroxynitrile (or cyanohydrin) includes 2-hydroxypropionitrile and α-hydroxyisobutyronitrile. These reagents are stable in acidic aqueous solutions but slowly hydrolyzed to cyanide in an alkaline solution. This cyanide dissolves gold in the presence of oxygen. It is recommended as a solvent for gold because it is an intermediate product in the production of other products and is inexpensive. Furthermore, due to its usefulness can be used at relatively low pH (7-9) conditions, especially for treatment of antimony-containing ores luminance. The use of alpha-hydroxypropionitrile reagents supplied by the American Cyanamide Company is described in the E. L. Carpenter patent, but the scope of use of such reagents requires further investigation.
Immersion of gold by thiocyanate method 1) Immersion of gold by thiocyanate thiocyanate is an organic and inorganic compound containing a -SCN group. Thiocyanate and its salts are collectively referred to as thiocyanide. Thiocyanate is similar to cyanic acid and exists in the structure of HS-C=N and HN=C=S. The latter form is called isothiocyanate and produces isothiocyanate, which shows clearly Characterizes organic compounds.
Inorganic cyanides and thiocyanates similar halides, because most metal (other than lead, mercury, silver and copper salts) are water soluble, and forms a complex with an excess of a wide variety of thiocyanate, For example [Pt(SCN) 4 ] 2- and [Pt(SCN) 5 ] 2- . Thiocyanates therefore include soluble thiocyanates and insoluble thiocyanates. The properties of soluble thiocyanate are as follows: [next]
Non-oxidizing dilute acid does not react with thiocyanate. When concentrated sulfuric acid and thiocyanate act, a yellow reaction occurs when it is cold; when heated, the reaction is intense, and oxygen sulfur carbon [COS] is released; when it is ignited, a blue flame occurs.
KSCN+2H 2 S0 4 +H 2 O —→ KHS0 4 +NH 4 HSO 4 +COS↑
If the concentration of sulfuric acid is higher, COS, HCOOH, CO 2 , SO 2 and the like will be generated, and sulfur will be deposited.
Silver nitrate in the thiocyanide solution produces a white curd-like silver thiocyanate precipitate. In analytical chemistry, a red blood compound is formed with Fe 3+ to indicate the end point; silver thiocyanate and concentrated sulfuric acid boil, there is black Ag 2 S precipitate formation. Iron salt was added to a solution of thiocyanate to produce red blood compounds. The drilled salt reacted with concentrated thiocyanate to form a deep blue, less stable Co(SCN) 4 2- ion, but no precipitate formed. The copper salt is added to the thiocyanide solution to form a black copper thiocyanate precipitate. This compound is generally unstable and eventually forms cuprous thiocyanate. Mercury nitrate meets thiocyanate to form white thiocyanate precipitate. This precipitate is extremely insoluble in water, but soluble in excess potassium thiocyanate solution to form thiocyanate complex; The acid salt forms a gray to black precipitate. When the mercury nitrate solution is added dropwise to the still concentrated thiocyanate solution, the precipitation of gray metal mercury first appears. If more mercury is added, the pure mercury is present. The formation of white mercury thiocyanate precipitates, and if a very dilute potassium thiocyanate solution is added to the extremely dilute mercury nitrate solution, the white mercury thiocyanate precipitate can be directly obtained. The benzidine and copper ions are added to the thiocyanate solution to form a dark blue brewing compound due to oxidation of the benzidine.
The dilute nitric acid and thiocyanate decompose under heating conditions, and a red reaction occurs, and NO and HCN are released; concentrated nitric acid can decompose thiocyanate to form NO, CO 2 and sulfuric acid. In acidic solution, when using potassium permanganate titration thiocyanate, cyanide and thiocyanate decomposed into sulfuric acid, thiocyanate oxidation of hydrogen peroxide may be hydrocyanic acid and sulfuric acid. Manganese dioxide is added to the thiocyanate solution to form cyanogen sulfide. In an acidic solution of thiocyanate, zinc can reduce it to hydrogen cyanide and hydrogen sulfide.
In difficult leaching pyrite flotation concentrate containing copper pyrite roasting and leaching of gold, tin Pang Tao Chen Jianxun made of tests that contribute thiocyanate method from a refractory ore, fine Gold is obtained in ore or roasting, but more work is needed to produce the thiocyanate method.
2) Leaching gold from gold or silver pyrite flotation concentrates The thiocyanate ions have a large complexing ability with Au (I, II), such as Au(I)-CNS - system 1g β 2 =25,Au (III)-CNS - system 1g β 2 =42, which is smaller than the complexing ability of CN - ion and Au(1) (1g β 2 =38.8), but it is more complex than thiourea ( 1g β 1 = 4.519,1g β 2 = 5.763,1g β 3 = 6.14) is much larger, and therefore as wet thiocyanate gold leaching agent is possible. According to the relevant constant and potential-pH diagram (as shown in the figure below), it can be seen that Mn0 2 acts as an oxidant, which can oxidize gold to Au(CNS) 2 - cation ions and dissolve. Its reaction
[next]
Mn0 2 +4H + +2e - ==== Mn 2+ +2H 2 0
Au +2CNS+e - â†â†’Au(CNS) 2 -
When pH < 4, the following reactions can also occur:
Mn0 2 +4H + +4CNS - —→ Mn(CNS) 2 +2H 2 0+(CNS) 2
(CNS) 2 +2Au+2CNS - â†â†’ ZAu(CNS) 2 -
Since Au and MnO 2 are solid substances, direct electron transfer redox reaction difficult, Mn0 2 first CNS - ions into water-soluble (CNS) 2, from which then again oxidized to a soluble gold complex ions In theory, it is easier to carry out. In order to oxidize the CNS - ion to (CNS) 2 , it is preferred that the leaching solution has a high acidity. Therefore, when the pH is 1 to 2, the leaching rate and the leaching rate of gold are favorable.
The minerals studied were pyrite flotation concentrates containing gold and silver. The chemical composition was: Au 59.3g/t, Ag 144g/t, Fe 21.27%, Cu 1.3%, Pb 1.7%, As 15%. Ca 2%, Mg 0.5%, Si 16.26%, total sulfur 34.8%. Also containing a trace of Ba, Ti, V, Mo, W; free of arsenic and tellurium.
3) Factors affecting gold, who and the rate of acceptance 1 The effect of pH on leaching gold and silver. The results of pH on the leaching rate of gold and silver are shown in Table 1.
pH | 9.2 | 7.5 | 6.5 | 5.5 | 5.0 | 4.1 | 3.0 | 2.5 | 2.2 | 1.5 |
Au leaching rate /% Ag leaching rate /% | 41.82 50.69 | 49.24 64.86 | 55.98 71.94 | 66.00 73.80 | 69.81 75.90 | 74.80 86.60 | 85.60 86.80 | 89.25 87.00 | 91.06 89.07 | 93.03 90.05 |
It can be seen from Table 1 that as the pH of the leaching agent is lowered, the leaching rate of gold is gradually increased; when the pH reaches 4, the pH is continuously lowered, and although the leaching rate of silver is improved, it is not as obvious as gold. This is directly related to the increase in acidity and the increase in the oxidation of MnO 2 . In addition, when the pH is <3, the CNS - ion can form a deep red ion with the Fe 3+ ion Fe(CNS)S x 3- , which causes the surface of some pyrite to dissolve, causing the exposure of the hidden gold particles, which is beneficial to The increase in gold and silver leaching rate.
2 The effect of the concentration of ammonium sulphate. It has been observed experimentally that as the concentration of thiocyanate increases, the leaching rate of gold and silver increases. This is because the concentration of CNS - ion in the leachate is increased, which is beneficial to the formation of soluble ions in Au+ and Ag+, thereby increasing the leaching rate of gold and silver. According to the experimental results, it is preferable to leaching gold and silver with a 5% NH 4 CNS solution.
3 The influence of the amount of pyrolusite. It is seen from experiments that pyrolusite has a significant effect on increasing the leaching rate of gold. As the amount of pyrolusite increases, the leaching rate of gold increases gradually, but has little effect on the leaching rate of silver. The suitable amount of pyrolusite is about 5% of the gold content.
4 The effect of leaching time. The rate of thiocyanate leaching gold and silver is relatively fast. With the increase of leaching time, the leaching rate of gold gradually increased; but after 2h, the leaching rate of gold increased little, and the leaching rate of silver did not increase after 3h.
5 temperature effects. From the experimental results, it is known that temperature has an effect on the leaching rate of gold. Under the same conditions, the temperature rises, and the gold leaching rate can be increased. At 323 K, stirring for 1 h, the gold leaching rate reached 97%. When thiocyanate is used for industrial leaching of gold, it is not necessary to heat separately.
6 Change in acid concentration during leaching. The relative acid concentration of different leaching time and the leaching rate of gold and silver are shown in Table 2. As seen from Table 2, during the leaching process, the acid concentration gradually decreased, and the gold leaching rate gradually increased. This is due to the presence of a small amount of carbonate minerals in the ore (CO 2 is emitted when acid is added), which consumes a certain amount of acid. In addition, MnO 2 acts as an oxidant and some acid is consumed during the reaction. After 5 hours of leaching, the acid concentration no longer decreased, and the leaching rate of gold no longer increased significantly.
Leaching time / h | 0 | 0.5 | 1.0 | 2.0 | 3.0 | 5.0 | 7.0 |
c(H + )/(mol·L -1 ) Au leaching rate /% Ag leaching rate /% | 1.0 0 0 | 0.254 81.96 75.55 | 0.204 90.73 76.25 | 0.151 91.40 76.32 | 0.122 95.28 79.23 | 0.075 98.15 81.04 | 0.075 99.07 82.57 |
4) Comparison with other methods of leaching gold
   In order to compare the effect of thiocyanate method and other methods for leaching gold and silver, a certain amount of ore powder is taken, different leaching agents are added separately, and gold and silver are stirred and leached under the same conditions. After stirring for a certain period of time, the mixture was filtered, and the gold and silver contents in the residue were analyzed to calculate the leaching rate of gold and silver. The experimental results are shown in Table 3.
Dip gold method | Cyanidation | Sodium thiosulfate method | Thiourea method | Thiocyanate method |
Leaching agent | NaCN10g·L -1 Ca(OH) 2 5g·L -1 pH≥10 | Na 2 S 2 O 3 100gL -1 CuSO 4 ·5H 2 O15g·L -1 pH~9 | Thiourea 15g·L -1 FeCl 3 3.5g·L -1 Na 2 SO 3 10g·L -1 H 2 SO 4 0.675mol·L -1 | NH 4 CNS50g·L -1 Pyrolusite 5g·L -1 H 2 SO 4 0.5mol·L -1 |
Stirring time / h Au leaching rate /% Ag leaching rate /% | 10 68.02 87.15 | 3 57.84 67.29 | 1 21.68 21.07 | 1 86.68 82.5 |
Stirring time / h Au leaching rate /% Ag leaching rate /% | 17 92.41 84.17 | 6 59.70 69.20 | 3 26.73 24.23 | 3 92.24 84.58 |
Stirring time / h Au leaching rate /% Ag leaching rate /% | twenty four 94.96 65.76 | 10 63.23 73.12 | 5 26.85 24.65 | 7 94.97 84.50 |
  It can be seen from Table 3 that for the golden iron ore concentrates of the above studies, the leaching rate of gold and silver by the thiocyanate method is close to that of the cyanidation method, but the leaching of the sodium thiosulfate method and the thiourea method. The rate is high. The leaching rate of gold and silver is similar to that of thiocyanate and sulfur, which is faster than cyanide. Thiocyanate, like thiourea and sodium thiosulfate, is extremely toxic, while sodium cyanide is a highly toxic substance.
   The thiocyanate method leaching gold has the advantages of high leaching rate, fast leaching speed, low toxicity and little environmental pollution. It is a promising non-cyanide leaching gold method, but industrialization needs further research.
5) Leaching gold from the copper-bearing golden iron ore calcine
   Under acidic conditions, using MnO, as the oxidant, SCN - as a complexing agent, for extracting gold from gold-bearing pyrite flotation concentrate in. Although the leaching rate is high ( 93.0% ), the reaction rate is fast ( 4h ), and it is not polluted, but the SCN - consumption is relatively large and the cost is high. because:
MnO 2 + 2SCN - + 4H + —→ Mn 2+ +( SCN ) 2 + 2H 2 0
Consuming SCN - increases consumption. So choosing an inexpensive oxidant is a must. Fe 3+ is proposed as an oxidant in this case.
1 Thermodynamic analysis of acid containing Fe ( III ) thiocyanate to dissolve gold. In the SCN - solution containing Fe 3+ , the following relationship exists:
Fe 3+ + e - â†â†’  Fe 2+
Fe 3+ + 6SCN - â†â†’  Fe ( SCN ) 6 3-[next]
2 study of leaching gold process conditions. Raw ore pretreatment: from Tongbao copper-bearing golden iron ore concentrate in Henan Province, the main chemical composition is: Au 52. 78 g / t , Ag 143. 6 g / t , Cu 6.45% , Fe 37. 35% , S 39.3 %, SiO 2 8.6% . The ore sample was subjected to thermal analysis and X -ray diffraction analysis, and a pretreatment process of sulphurization and sulphuric acid immersion copper at 610 ~ 650 °C was selected.
   With 5% dilute sulfuric acid, the liquid-solid ratio was 2 : 1 , stirred for 60 min, and the filtrate was filtered to recover CuS0 4 ·5H 2 0. The slag was used to leach gold. Among them, the copper recovery rate is over 98% .
   Selection of process conditions for leaching gold: The oxidant of the process is Fe 3+ , which is formed by the following reaction of Fe 2 0 3 in the calcine:
Fe 2 0 3 + 6H + ==== 2Fe 3+ + 3H 2 0
The production of Fe 3+ has met the requirements of the leaching reaction. The effect of leaching time on gold leaching rate and consumption of NH 4 SCN were investigated under the selected liquid-solid ratio of 2:1 stirring speed. The experimental results show that the leaching rate does not increase after 4 hours of leaching time. At this time, the consumption of NH 4 SCN is 1.015 kg/t calcine, the concentration of NH 4 SCN is 3% , and the leaching rate of gold with NH 4 SCN concentration. The increase of the increase, the leaching rate of gold after 5% is no longer increased, and the gold leaching rate increases with the increase of temperature, but the amplitude is not large, so it is not necessary to enhance the leaching by heating. When investigating the effect of pH on gold leaching rate, it was found that pH had a great influence on the leaching rate of gold. When the pH is increased from 1 to 3 , the leaching rate of gold drops sharply. After pH > 4 , gold is not substantially leached, and the consumption of NH 4 SCN is basically unchanged. Therefore, pH is the use of calcine Fe 3+ Fe key factor in this process as the oxidant 203 to produce acid soluble, pH should be selected in <1.
   Determination of the optimum leaching gold condition: The optimum conditions for leaching gold are 5% concentration of NH 4 SCN solution, temperature control at room temperature (about 30 ° C ), pH = 1 , stirring reaction for 4 h, gold leaching rate according to this condition From 93% to 94% , the consumption of NH 4 SCN is 1.00 ~ 1. 03 kg / t ore.
   The research proves that the process of sulphating roasting of high copper pyrite, using Fe 3+ as oxidant after removing copper, and SCN - as a complexing agent to leaching gold is not only feasible but also economical.
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