Effect of iron minerals on cyanide leaching
2020-08-19 10:35Introduction：Effect of iron minerals on cyanide leaching Iron minerals in ores can be divided into two categories: iron oxide minerals, such as hematite(Fe 2 O 3 ), magnetite(Fe 3 O 4 ), goethite(Fe 2 O 3 2H 2 O), and siderite(FeCO 3 );The other isiron
Effect of iron minerals on cyanide leaching
Iron minerals in ores can be divided into two categories: iron oxide minerals, such as hematite(Fe2O3), magnetite(Fe3O4), goethite(Fe2O3·2H2O), and siderite(FeCO3); The other is iron sulfide minerals, for example, pyrite(FeS2)and marcasite (FeS2, Crystalline form is different from pyrite) and pyrrhotite (Fe5S6-Fe16S17) etc. Iron oxide minerals have stable chemical properties and do not participate in the leaching process. Iron sulfide minerals have active chemical properties and can react with cyanide solution and the oxidation products of iron sulfide minerals can also react with cyanide solution.Pyrite, marcasite and pyrrhotite are common minerals in gold ores. In the alkaline cyanide solution of air, their reaction ability with cyanide is as follows: pyrrhotite > marcasite > pyrite. The oxidation process of pyrite and marcasite is divided into the following stages:
1. FeS2 is partially decomposed into FeS and S due to weathering or wet grinding;
2. Free S is oxidized to produce H2SO3 and H2SO4, while FeS is oxidized to produce FeSO4;
3. FeSO4 is oxidized to produce Fe2(SO4)3;
4. Fe2(SO4)3 is further oxidized to produce insoluble 2Fe2O3·SO3 ;
5. 2Fe2O3·SO3 is further oxidized and finally Fe(OH)3 is formed.
All of the above oxidation products can react with cyanide and increase the consumption of cyanide:
S+NaCN=NaCNS
FeS2+NaCN=FeS+ NaCNS
FeS+7NaCN+1/2O2+H2O=Na4Fe(CN)6+NaCNS+2NaOH
When the protective alkali is insufficient, the acid formed by oxidation of pyrite will decompose cyanide into hydrocyanic acid:
H2SO3+2NaCN=Na2SO3+2HCN
H2SO4+2NaCN=Na2SO4+2HCN
When alkalinity is present, the reactions of ferrous compounds are as follows:
FeSO4+2NaOH=Fe(OH)2+Na2SO4
FeSO4+6NaCN=Na4Fe(CN)6+ Na2SO4
Fe(OH)2+2NaCN=Fe(CN)2+2NaOH
Fe(CN)2+4NaCN= Na4Fe(CN)6
3 Na4Fe(CN)6+2Fe2(SO4)3=Fe4【Fe(CN)6】3+6Na2SO4
The pyrrhotite is easily decomposed into sulfuric acid , ferrous sulfate , basic ferric sulfate , ferrous carbonate and ferrous hydroxide under the condition of water and air . It is similar to pyrite and marcasite , but the pyrrhotite is characterized by high decomposition speed , maximum amount of generated sulfate and maximum oxygen consumption . In this case , oxidation roasting and washing should be carried out for the oxidized iron sulfide ores before cyanidation and the refractory sulphide iron ore should be leached with alkali solution to make ferrous iron into Fe(OH)3 precipitation.
In order to reduce the harmful effect of oxydate from rapidly oxidized iron sulphide iron ores during leaching, sufficient alkali can be added before cyanidation, and air stirring can be used to convert the harmful cyanide iron salt into insoluble ferric hydroxide precipitation. In the course of this alkali treatment, the treatment time and strength of the alkali treatment are determined by the actual need. In practice, in order to test whether ferrous compounds are completely oxidized to positive iron, the following methods can be used: a small amount of solid sodium cyanide is added to the pulp treated with partial alkali and filtered after shaking for a few minutes. The filtrate was adjusted to acid with hydrochloric acid, and ferric chloride was added. If Prussian blue was formed, there was a ferrocyanide, which indicated that the oxidation of ferrous compounds was not complete.
Pyrite and its oxidized products can form soluble alkali metal sulfides in alkaline cyanide solution. Alkali metal sulphides consume cyanide and oxygen in the solution to form thiocyanates:
Na2S+NaCN+1/2O2+H2O=NaCNS+2NaOH
Therefore, excessive alkali metal sulfides in cyanide solution will consume a large amount of cyanide and oxygen in the solution, which will affect the leaching of gold and silver. Secondly, alkali-metal sulfides precipitate some of the dissolved silver from cyanide solution.
2NaAg(CN)2+Na2S=Ag2S+4NaCN
When lead salt or mercury salt exists in the solution, the soluble alkali metal sulfides react with lead or mercury to form metal sulfides with very low solubility.
Na2S+Pb(AC)2=PbS(solid precipitate)+2NaAC
In order to eliminate the adverse effects of metal sulphides, lead salts are often added in industrial production.
In the process of cyanidation, cyanide is often in contact with the equipment of iron and steel materials, storage tanks, etc., but the reaction to these materials is very weak, because the surface of metal iron is covered by oxide film, which insulates the direct interaction between cyanide and iron.
During grinding, due to the wear of liner and steel ball, a large amount of iron powder enters into the slurry, especially when the cyanide is added to the ball mill, the fresh iron powder will react with the cyanide, thus increasing the consumption of cyanide.
Fe+6NaCN+2H2O=Na4Fe(CN)6+2NaOH+H2
