1, the mechanism of cyanide soluble gold
The cyanidation method is a method in which gold is leached from ore by cyanide and gold in the solution is separated. The basic chemical reaction formula is:
4AU+8NaCN+O2+2H2O→4Na AU(CN)2+4NaOH
It includes the absorption and dissolution of oxygen, the diffusion of its components to the gold surface, adsorption, electrochemical reaction and other steps. The diffusion of O2 and CN- plays a crucial role in the leaching rate of gold.
2, leaching agent
The cyanide which can be used for gold dissolution is: KCN, NaCN, NH4CN, Ca(CN) 2 When cyanide is selected, the solubility, chemical stability, consumption and price of cyanide to gold should be considered comprehensively. Most of China's gold mines use NaCN.
3. Protection of alkali
In addition to mechanical reasons, cyanide loss has chemical reasons: one is the loss and harm caused by the hydrolysis of cyanide to form HCN gas; the other is the acid formed by the oxidation of carbon dioxide and sulfide in the solution (H2SO3, H2SO4) and cyanide HCN gas generating action thereof; Third, when pyrite oxidation, in addition to generating H2 SO4, but also generate some ferrous sulfate (Fe SO4), and generated with a cyanide Fe (CN) 6, and when the alkali solution and oxygen When Fe SO4 can be oxidized to Fe2(SO4)3, and then reacted with alkali to form Fe(OH)3 precipitate, Fe(OH)3 does not react with cyanide. Therefore, the addition of alkali acts to protect cyanide. The base is called a protective base. Lime is usually used as a protective base in production.
4. Main factors affecting the dissolution rate of gold
4.1, cyanide and oxygen concentration
The concentration of cyanide and the concentration of dissolved oxygen in the solution are the two main factors determining the rate of gold dissolution. Gold dissolves rapidly in the dilute cyanide solution because oxygen has a higher solubility in the dilute cyanide solution and a faster diffusion rate, thus ensuring the minimum oxygen concentration required for gold dissolution.
The cyanide consumption of different ores is different because the ore contains different amounts of impurities that consume cyanide. Conventional cyanide concentrations are generally between 0.03% and 0.10%.
4.2, temperature
The dissolution rate of gold in cyanide solution is related to temperature. Generally, the temperature is high and the dissolution rate is fast. Under the condition of no special process requirements, the slurry temperature can be maintained at 150C~250C to meet the requirements of leaching.
4.3. The size and shape of the gold particles
The dissolution rate of gold is directly proportional to the surface area exposed by the gold particles, so the grinding size of the cyanidation operation is finer than that of the flotation.
4.4, pulp concentration and slime
The slurry concentration and slime content directly affect the diffusion rate of the solvent and the contact of the solvent with the gold particles.
4.5, leaching time
During the whole leaching process, as the leaching time is extended, the leaching rate of gold is gradually increased, but the leaching speed is also decreasing, and the leaching rate is gradually approaching a certain limit value.
4.6, the influence of impurity ions
In the cyanide solution, most of the associated minerals can dissolve to varying degrees, which affects the leaching of gold. Among them, the influence of metal minerals is more serious, some will accelerate the dissolution of gold, and some will block the dissolution of gold, thus complicating the cyanidation process. These substances, which can affect the cyanidation process, are called impurities, and most of them are harmful to the cyanidation of gold.
4.6.1, speed increase effect
An appropriate amount of lead, bismuth and thallium salts present, advantageously dissolution of gold, the leaching rate of gold can be increased, since these metals with gold by substitution, changing the characteristics of the solid surface, thereby promoting the dissolution and diffusion of gold process.
4.6.2, blocking effect
In cyanide solutions, certain impurities have an adverse effect on the dissolution of gold. First, the consumption of oxygen in the solution, such as pyrrhotite, arsenopyrite , stibnite, etc. in the dissolution of alkaline cyanide, can cause a large consumption of dissolved oxygen; second, the consumption of free cyanide in the solution. A metal mineral that is symbiotic with gold, which dissolves in a cyanide solution, often forms a complex of cyanide, usually dissolving a metal ion, which consumes several sodium cyanide molecules. When the sulfide in the ore is decomposed, the released sulfur ions react with cyanide to form a thiosulfate which does not contribute to the dissolution of gold; the third is to form a film on the gold surface. In the cyanidation process, impurities can form various films on the surface of the gold particles that hinder the contact of gold and cyanide solution, reducing the dissolution rate of gold, such as sulfide film, calcium peroxide film, oxide film, insoluble cyanide film. The impact of flotation drugs.
5. Effect of associated minerals on cyanidation process
When using cyanidation of gold silver, because the composition of the mineral ore complex, complex chemical reactions may occur between the agent and minerals, minerals and minerals, minerals and oxygen, the leaching of gold and silver will have different effects, most of which are Harmful. Some reflect the consumption of cyanide and oxygen in the solution, and some of the reaction product forms a film covering the surface of gold and silver, hindering the leaching of gold and silver. Therefore, in general, gold-bearing ores with complex compositions may increase cyanide consumption during cyanidation, or reduce the leaching efficiency of gold and silver, and sometimes even prevent gold and silver from leaching. It must be pre-processed.
In associated minerals, metal minerals and their sulfides, oxides, hydroxides and various salts interact with cyanide and oxygen, while non-metallic minerals do not react with cyanide.
5.1, copper minerals
Common copper minerals in gold-bearing ores are: chalcopyrite, porphyrite, chalcopyrite, azurite, malachite and natural copper. Most copper minerals are more easily dissolved by cyanide and consume a lot of cyanide. In general, the temperature of the cyanide slurry and the concentration of free cyanide in the cyanide solution should be minimized in industrial production.
5.2, iron ore
Iron ore is often the most associated mineral in gold ore. Different iron minerals play different roles in cyanide. Among them, oxidized minerals have little effect on cyanidation leaching, while pyrite, pyrite, pyrrhotite is the most common, most pyrite has little effect on cyanide leaching, and most of the pyrite and Pyrrhotite is easy to oxidize and decompose and consumes free oxygen, thus affecting cyanide leaching. Therefore, for the iron oxide ore with faster oxidation, in order to reduce the damage, sufficient alkali can be added before cyanidation, and the gas is stirred and stirred. The iron salt which is detrimental to cyanide is converted into an insoluble iron hydroxide precipitate. When such an alkali treatment is carried out, the treatment time and strength are determined as needed.
Pyrite and its oxidized product can form soluble alkali metal sulfide in alkaline cyanide solution, and then consume cyanide and oxygen to form thiocyanate. In the production, in order to eliminate the adverse effects of metal sulfide, Lead salts are often added.
During the grinding, the iron powder enters the slurry due to the abrasion of the liner and the steel ball. Especially when the cyanide is added before the grinding, the fresh iron powder also reacts with the cyanide, thereby increasing the consumption of cyanide.
5.3, zinc minerals
Metal zinc is easily dissolved in cyanide solution, and the dissolution rate is relatively fast. Compounds such as zinc complex salts, thiocyanates, sulfates and intermediate sulfides which are produced when sphalerite is dissolved in cyanide solution consume a large amount. Oxygen and cyanide.
5.4, ​​lead minerals
A small amount of lead is good for gold leaching, and excessive lead will have an adverse effect, which will reduce the leaching rate of gold, and also increase the cost of leaching solution replacement and the grade of gold mud.
5.5, mercury minerals
5.6, bismuth minerals
The stibnite is the most common antimony mineral in the gold ore. It is easy to react with the alkaline cyanide solution. Due to the large consumption of oxygen and the accumulation of various antimony salts, the gold is difficult to cyanide leaching.
5.7, arsenic minerals
Among gold ores, arsenic minerals are commonly found in realgar (Aa2S3), orpiment (As2S2) and arsenopyrite (FeAsS). Arsenic-containing ores are extremely detrimental to the oxidation process and even prevent the cyanidation process from proceeding.
5.8, selenium minerals, antimony minerals, silver minerals
6. Preparation and pretreatment of cyanide ore before leaching
Before the gold-bearing ore enters the cyanide leaching, it needs a series of necessary preparation and pre-treatment operations: first, to provide suitable raw materials for leaching; second, to reduce the mineral processing agent entering the leaching; third, to reduce cyanide The amount of harmful impurities in the ore or the influence of these harmful impurities.
6.1, grinding
Purpose of grinding:
First, the gold particles in the ore are dissociated, and the gold particles wrapped in other minerals are fully exposed;
Second, reducing the particle size of the gold particles and increasing their specific surface;
Third, expose the gold to a fresh surface and eliminate the adverse effects of certain films on its surface;
Fourth, during grinding, the slurry is subjected to strong aeration and agitation during the process. When the slurry is alkaline, certain minerals are subjected to a certain "alkali leaching" effect, which is beneficial to eliminate the adverse effects of certain impurities on gold leaching.
6.2, drug removal
The flotation agent will have a certain hindrance effect on the leaching of gold. The larger the dosage, the more obvious the effect. Therefore, for the cyanide leaching of flotation concentrate, the drug removal operation is necessary. The de-dosing operation usually adopts concentration, filtration (pressure filtration), and is actually a process of dehydration, which is favorable for meeting the requirements of leaching concentration.
6.3, alkaline soaking
Before the cyanide leaching, the cyanide ore is pre-exposed under alkaline conditions and subjected to relatively strong aeration and agitation treatment, called alkali leaching, to eliminate the harmful effects of certain impurities on gold leaching in cyanide solution.
If the alkali leached pulp is formed in the solution which is harmful to cyanide production, it should be dehydrated and washed after alkali leaching to reduce harmful components from entering cyanide leaching.
6.4, roasting
Some gold-bearing ores, because the minerals associated with harmful cyanide or gold particles are encapsulated in other minerals. If directly cyanide leaching, the leaching rate of gold is very low, and a large amount of cyanide and oxygen are consumed, so it is calcined. After the treatment, the cyanidation method can often obtain better technical and economic effects.
6.5, biological oxidation
6.6, pressurized oxidation
6.7, other oxidation
7. Leaching equipment and leaching operation
7.1, leaching equipment
7.2, the operation of the leaching operation
7.2.1 Strictly control the concentration of sodium cyanide
Suitable sodium cyanide concentrations are determined by experimentation and production practice. In the case of ensuring that the leaching rate is not lowered, the concentration of sodium cyanide should be selected.
The control of sodium cyanide concentration in the leaching operation should follow the following principle: Under the premise of ensuring the dissolution efficiency of gold, the concentration of sodium cyanide should be appropriately reduced so that the concentration of sodium cyanide in each series leaching tank is the same, or the front is lower than the rear leaching tank. The smaller the fluctuation range of sodium cyanide concentration controlled by each leaching tank, the better. Cyanide is preferably added in multiple tanks.
7.2.2, minimize production fluctuations
It is necessary to maintain the leaching concentration or to reduce the fluctuation range of the slurry concentration. Poor control of concentration will affect drug consumption, leaching time, aeration conditions, and leaching specifications.
7.2.3, the time for the leaching tank to stop running should not be too long
7.2.4, ensure that the air is inflated and evenly dispersed in the slurry
7.2.5, ensure the mixing capacity, check the impeller condition
7.2.6. Check the lime concentration and other dosages
8. Management of cyanide
Cyanide is a highly toxic substance that must be strictly managed. A complete management system should be established for its transportation, storage and use.
8.1, cyanide operation
Since cyanide is a highly toxic substance, the staff of the cyanide plant must pass special safety training so that every worker who comes into contact with cyanide should understand the nature of cyanide and understand the people and animals. Hazards and prevention, first aid measures, understanding the relationship between cyanide and environmental protection, so as not to violate safety regulations in daily operations, causing bad consequences.
Cyanide can be poisoned by a complete skin system. Therefore, neither liquid nor solid can be directly contacted, and it should not be allowed to enter the wound. When preparing cyanide solution daily, it is necessary to wear a labor protection product, and Turn on the ventilation. After the work is completed, it should leave the site immediately, and the cyanide preparation tank should be sealed. It is strictly forbidden to bring food or tableware into the scene to avoid pollution. After the operation, wash your hands and utensils with detoxifying liquid or water.
8.2, cyanide poisoning and first aid
The toxicity of cyanide and its compounds is mainly derived from cyanide (CN-), which can enter human body through three ways: human respiratory toxicity: 1, respiratory system; 2, digestive system; 3, complete skin system. When cyanide enters the human body, it generates hydrogen cyanide and inhibits cytochrome oxidase, so that it can not absorb dissolved oxygen in the blood (so that the oxidation function in the body stops). Paralysis of the nerve function, and finally lead to rapid and complete hypoxia in the body tissue and suffocation and death.
After cyanide poisoning, the symptoms can be roughly divided into three stages:
First, mild symptom stage: At this time, there are nausea, vomiting, and constipation. There is bitter almond smell in the mouth, breathing slightly, head congestion, dizziness, this slight poisoning, as long as it is sent to fresh air. , can quickly restore health;
Second, the stage of difficulty breathing; the characteristics of poisoning include tinnitus, tremor, general malaise, weakness, difficulty in breathing, prominent eye, convulsions, paralysis, etc.;
Third, paralysis stage (also known as asphyxia stage): urinary incontinence, conditioned reflex disappeared, intestinal venting, high angle bow reflex to death.
Cyanide is an acute poisoning drug. If human oral administration of 0.1 g of sodium cyanide, 0.12 g of potassium cyanide or 0.05 g of hydrocyanic acid, it will instantly die. However, cyanide is a temporary poison. No matter whether it is a solid or a liquid, it does not cause poisoning as long as it does not enter the human body. Even if it is inhaled, it will be discharged quickly if it is moved to a place with fresh air, because the toxicity of cyanide is in the human body. No savings. In the case of acute poisoning, the following first-aid measures can be taken according to their severity: slight poisoning will be immediately moved to fresh air; if the poisoning is heavier, the following first steps (1) and (2) will be completed within two and a half minutes. Carry out other steps:
(1) Immediately evacuate the site and go to a fresh place in the air;
(2) inhaling pentoxide pentoxide;
(3), inject 1% methylene blue, add 25~50% glucose 20~30 ml;
(4), injection 30% sodium thiosulfate 30~40 ml;
(5), injection of cardiotonic agent, stimulant;
(6) Perform artificial respiration;
(7) After 30~40 minutes, repeat steps (2)~(5), the dosage is one-half of the original dosage;
(8) After restoring consciousness, give the patient a gastric lavage.
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