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General Chemistry

by: Mitchel Dare DVM

General Chemistry CHEM 101

Mitchel Dare DVM

GPA 3.82

Mamadou Diallo

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Mamadou Diallo
Class Notes
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This 2 page Class Notes was uploaded by Mitchel Dare DVM on Tuesday October 13, 2015. The Class Notes belongs to CHEM 101 at Longwood University taught by Mamadou Diallo in Fall. Since its upload, it has received 23 views. For similar materials see /class/222427/chem-101-longwood-university in Chemistry at Longwood University.


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Date Created: 10/13/15
Gold Cyanide Solution Leaching Gold With Cyanide Since the 1890 s cyanide has been used to recover gold from gold bearing ores And today over 115 years later most of the worlds gold is recovered with cyanide playing a large part in the bene ciation of the yellow precious metal Chemically it is a rather simple reaction 4 Au 8NaCN 02 2 H20 4 NaAuCN2 4 NaOH That presumes that the only elements are the gold Sodium Cyanide and water However as any geologist will tell you no two ores are the same and their chemical composition will vary greatly throughout the ore body These quotextraquot elements in the mineral compounds will often play havoc with a chemical reaction as illustrated above Copper is de nitely worth mentioning since copper minerals will dissolve in cyanide solutions and cause a increased use of cyanide the coppercyanide complexes formed by the dissolution will tend to inhibit the dissolution of gold in the cyanide solution Zinc the element used to precipitate gold from solution if present in the ore will bond with the cyanide to form a zinc cyanide compound Another element that plays with the cyanide chemistry is nickel Nickel however does not interfere with the gold going into solution but rather the precipitation of the gold from the cyanide solution Arsenic and antimony do present a larger problem by reacting with the cyanide and using up all of the excess oxygen leaving little or no oxygen to effect the dissolution of gold Carbonaceous gold ores can have the carbon adsorb the gold onto its surface and as a result will not be recovered from the pregnant solution Leaching gold from sulfide ores is difficult at best Generally the recovery for cyanide leaching of sulfrde or refractory ores is no better than 30 which is not a worthwhile venture The use of alkalies such as calcium oxide will prevent the decomposition of cyanide in solution to form hydrogen cyanide gas It reduces the volume of cyanide required to leach the gold or silver In addition hydrogen cyanide is highly toxic to people So the few dollars spent on adding a cheap calcium oxide to the ore or solution prior to leaching is worth the money spent Most cyanide leaching is carried out at a alkaline pH of between 10 and 11 depending upon lab testing of individual ores and the optimum leachingchemical use rates The cyanide solution strength is also important in leaching gold with the typical range of solution being in the 002 005 NaCN The gold particle size has a tremendous effect on the time required for dissolution in a cyanide solution Generally the finer the gold the quicker it will dissolve A 45 micron particle of gold would dissolve in 1013 hours while a 150 micron particle might take from 20 to 44 hours to dissolve in the same solution Oxygen plays an important role in the leaching of gold in a cyanide solution also It has been proven that the rate of dissolution of gold in cyanide solution is directly proportional to the amount of oxygen present Normal water will have 89 ppm dissolved oxygen present in it If this oxygen is used up by other reactions it may be necessary to aerate the solution inducing oxygen into it to speed up the reaction With cost being always the determining factor except in safety the decision to aerate and speed up the reaction will be made based upon economics and laboratory testing It is not used much anymore because most leaching is heap leaching carried out in the outdoors where drip emitters or sprays distribute the cyanide solution to a large structure of gold ore called a quotheapquot And while the pile of ore is called a heap it is not a haphazard pile of rocks Much thought and design goes into the making of a heap leach to derive the best most economical solution for recovering the gold from the ore Once the gold has been dissolved in the cyanide and the ore body has been reasonably depleted of its gold there are two main processes for recovering the gold from the pregnant cyanide solution One is the MerrillCrowe zinc precipitation process and the other is the adsorption of the gold onto activated carbon The oldest method Merrill Crowe involves rst removing the oxygen from the solution then mixing a ne zinc powder with it 200 mesh and recovering the very ne gold precipitate on a precoat lter since the gold precipitate is very ne ranging from a few microns to 50 or so microns The zinc reacts with the cyanide 2AuCN Zn 2Au ZnCN4392 Other chemicals have been used to leach gold and they include bromine chlorine and thiourea There has also been a lot of experimentation with various biological media for recovering gold from ores but no one has come up with a more cost effective and productive method than leaching with cyanide In some special circumstances some of the other methods may show promise but for a good oxide gold ore CN leaching is usually the best of the leach methods for the yellow precious metal Silver is also leached easily using cyanide however much silver ore is in sul de forms and at higher concentrations several ounces per ton and above so other methods such as gravity concentration and froth otation may be employed Information provided by Charles Kubach Mining and Mineral Processing Engineer Reference Chemistry of Cyanidation American Cyanamid


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