By far the most necessary cyanides used in cyanidation space sodium cyanide and also calcium cyanide. The latter product is offered in one impure type analyzing close come 50% NaCN equivalent. The previous is offered in various qualities from 85 come 98% NaCN.
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In to compare the dissolving effects of the cyanides the ammonium, sodium, potassium, magnesium, calcium, strontium and also barium top top gold and silver it was found that the alkaline radical did not influence the dissolving result of the specific cyanide. The cyanogen content was the only far-reaching factor affecting dissolution. Because that example, 1 mol of pure calcium cyanide weighing 92 grm. Would dissolve the very same amount of yellow or silver as 2 mols of pure salt cyanide weighing 98 grm. Since their CN contents is the same in each case, i.e. 52 grm.
Decomposition of Cyanide Solutions
A water equipment of an alkaline cyanide hydrolyzes together follows:
NaCN + H20 HCN + NaOH
The degree to i beg your pardon this hydrolysis proceeds in solutions of advertising cyanides in water depends mostly on the quantity of totally free alkali in the cyanide. If this alkali is appreciable, then the decomposition the cyanide can be negligible. In the absence of appreciable free alkali, hydrolysis can be retarded through the addition of lime. In exercise the addition of lime to a cyanide pulp is virtually universal, not just to stop loss that cyanide through hydrolysis but likewise to neutralize any acidic constituents of the ore which otherwise would certainly liberate hydrocyanic acid. An additional factor affect decomposition the cyanide remedies is the existence of carbon dioxide in the air. The carbonic acid, being stronger than hydrocyanic acid, decomposes alkaline cyanide options as follows:
NaCN + H2COS = HCN + NaHCO3
This reaction, also, have the right to be prevented by the usage of lime or other alkalies. Together alkalies preserve the alkalinity of the solution and react v the carbon dioxide to form innocuous link such as calcium carbonate.
EFFECT that LIME ~ above CYANIDE LOSS
|Lime after 6 Hour||Start|
Lime after ~ 6 Hour
|NaCN loss lb/ton||0.34|
The result of tests displayed in above demonstrate the impact of alkalinity on loss of cyanide through hydrolysis and also by the action of carbon dioxide in the air. Two remedies were prepared, every containing 0.051% NaCN; 0.061% CaO as hydrated lime was included to one, if the other was left in the natural, contempt alkaline state. The solutions, at a temperature of 24 C. Were agitated in large mouthed open up bottles on rolls because that 6 hrs after i m sorry they to be titrated for complimentary cyanide and lime content. The pH of each systems was established before and also after the test. Indigenous the outcomes it may be provided that throughout the test the equipment without lime varied in pH from 10.2 to 9.7; in 6 hours this solution lost 33% that its cyanide, or 0.34 lb. NaCN per ton. In the instance of the various other solution whereby the pH varied from 11.6 to 11.2 the ns of cyanide was nil.
Dissolution of precious Metals
The adhering to reactions have been offered for the resolution of gold in dilute cyanide solutions:
4Au + 8 NaCN + 02 + 2 H20 = 4 NaAu(CN) 2 + 4NaOH. This is well-known as Eisner’s equation.
2Au + 4 NaCN + 2 H20 = 2NaAu(CN)2 + 2 NaOH + H2. This was argued by Janin.
2Au + 4 NaCN + 2 H20 + 02 = 2 NaAu (CN)2 -f- 2 NaOH -f- H202. The hydrogen peroxide created being supplied in the reaction:
2 Au + 4 NaCN + H202 = 2 NaAu(CN)2 + 2 NaOH. These reactions were said by Bodlaender. The overall equation, however, is the same as Eisner’s.
Analogous equations have been offered for the dissolution of metallic silver in cyanide solutions.
Barsky, Swainson and Hedley identified the free energies of development of the complicated gold-cyanide and silver-cyanide ions. Indigenous the data obtained, lock calculated the free-energy changes in the assorted reactions suggested and pointed the end which that these space theoretically pos-sible under ordinary cyanidation conditions.Their results proved that because that Eisner’s equation the reaction will certainly proceed virtually to completion, i.e. Until virtually all the cyanide has been consumed, or every one of the metal dissolved.For Janin’s equation the equilibrium constants are so very unfavorable the the formation of hydrogen might be considered impossible under simple cyanidation conditions.For Bodlaender’s equations the equilibrium constants space favorable, therefore, the reaction proposed room possible. In this link Bodlaender actually uncovered that hydrogen peroxide was formed and he had the ability to account for approximately 70 every cent of the theoretical lot of hydrogen peroxide that must be created according come his equation. In watch of this, it would show up that Bodlaender’s equations to express the true reactions the take place when metallic gold and also silver are liquified in dilute cyanide solutions.
Effect of Cyanide Concentration on rate of Gold and Silver Dissolution
The price of dissolution of gold in cyanide remedies attains a best in happen from concentrated to dilute solutions. His work-related shows the this preferably is reached at a systems concentration the 0.25% NaCN.
According to Christy, for all helpful purposes, remedies weaker 보다 0.001 per cent KCN execute not dissolve gold.
Research discovered that the rate of dissolved of gold increased rapidly with increase in strength of systems up to and also including 0.10 per cent KCN.
White uncovered that maximum price is at around 0.027 per cent KCN, or 0.020 every cent NaCN, as soon as the cyanide systems is saturated v oxygen.
The equipment strength discovered for most rapid dissolution was 0.05 every cent NaCN. .
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The reason of the wide variations in the equipment strengths uncovered by various investigators to offer maximum price of yellow dissolution more than likely lies in the range of methods employed in determining these figures. This variations encompass such factors as the ratio of volume of systems to gold surface. Violence that agitation, price of aeration. If a large volume of cyanide systems is used and a fairly small surface of gold exposed to the cyanide solution and also if agitation is sufficiently intense to eliminate the assets of the reaction native the surface of the yellow as swiftly as they are formed, climate the regulating factor administer the rate of dissolved of the gold should he the oxygen concentration that the equipment in contact with the gold. If wait is used and the tests are run at sea-level the best concentration the oxygen in equipment will he 8 mg every liter. Climate according come the reaction:
4 one + 8 NaCN + 0, + 2 H,0 = 392 32 4NaAu(CN), + 4 NaOH,
there would certainly be no benefit in having an ext than 392 parts by weight of NaCN because that every 32 parts by load of oxygen or 98 parts of NaCN because that every 8 components of oxygen in the cyanide solution. In various other words, the maximum rate of dissolved of yellow under ideal conditions of agitation and aeration need to take place in options containing 0.0098 per cent NaCN. In support of this, Hedley and Kentro found that by making use of 10 sq. Cm. Of gold in 1 liter that cyanide solution and aerating intensely (28 liters every hour), the maximum rate of dissolution take it place in between 0.011 and also 0.051 every cent NaCN; the price of dissolution in the previous solution was 95′ Relative rates of dissolution of Gold, Silver and Their Alloy
Using 100 ml. Quantities of 0.10 every cent NaCN solution, through surface areas of the metals and their alloys measure up 10 sq. Cm., and also with a constant volume that air for aeration and also agitation, it was determined the relative prices of dissolution of gold, silver and two gold-silver alloys. Native these outcomes it may be observed the silver dissolved at about half the rate at which yellow dissolved; the rates of dissolved of the alloys were in between those that gold and silver, virtually in proportion come the composition of the alloys. The quantities of gold and silver dissolved out that the alloys were in virtually the exact same proportion as the percentages of those metals in the alloys.
Relative prices Of dissolution Of Gold, Silver and Gold-Silver Alloys
Analysis of original Metal
|Dissolution price mg/sq. Cm/hr|
Analysis of dissolved Metal