Lime in Gold Ore Mining Process

Lime is a low-cost item that is widely used in gold beneficiation plants. The following is a systematic description of lime properties, its role in flotation, cyanidation, amalgamation and other mining operations, for your reference.

1 Lime properties

Lime (CaO), also known as quick lime, which is obtained by calcining limestone (CaCO3) at 900~1200 °C.
CaCO3→CaO + CO2↑

lime

Lime has strong water absorption, and reacts with water to become calcium hydroxide (Ca(OH)2), commonly known as slaked lime or slaked lime. It has little solubility in aqueous solution, but it can further ionize calcium ions (Ca2+) and hydroxide ions (OH-) in aqueous solution, making the solution more alkaline.

2 Lime in gold flotation

Lime can be used to increase the pH value of the slurry during the flotation process; regulate the activity of thio compound collectors and other inhibitors (such as cyanide).

2.1 Adjust slurry PH value

Lime is cheap and easy to obtain and has strong alkalinity. In the process of flotation of sulfide minerals, it is usually used as a pH adjuster to increase the pH of the pulp.

(1) Adjust the concentration of heavy metal ions in the slurry to form poorly soluble compounds. This is an important adjustment method to eliminate some harmful ions. For example, the addition of OH-ions can cause many metal cations to form insoluble hydroxides. Commonly soluble hydroxides that are difficult to form in flotation are Al(OH)3, Cu(OH)2, Fe(OH)2, Fe(OH)3, Pb(OH)2, Zn(OH)2, etc.

(2) Adjust the ion concentration of the collector. The presence of a collector in a molecular or ionic state in water is closely related to the pH of the medium. Adjusting the pH adjusts the proportion of the collector in the molecular or ionic state of the water, and adjusts the degree of dissociation of the collector.

(3) Adjust the interaction between the collector and the mineral. The effect between the collector ion and the mineral surface is closely related to the pH of the slurry. The collector anion and OH- can compete on the mineral surface. The higher the pH, the greater the OH- ion concentration. It can repel the action of the collector anion.

(4) Adjust the concentration of the inhibitor. Some inhibitors are salts composed of strong bases and weak acids. For example, the commonly used inhibitor sodium silicate( NaO2SiO2) is that it can be hydrolyzed in water to make the pulp alkaline, pH value. The level directly affects its degree of hydrolysis. When pH < 9, the silicate (H2SiO3) molecule dominates; when pH = 9~13, HSiO-3 dominates; when pH > 13, SiO-3 dominates.

(5) Adjust the foaming ability of the foaming agent. The pH affects the foaming ability of the foaming agent. For example, the foaming capacity of pine oil (2# oil) increases as the pH of the medium increases.

(6) Adjust the dispersion and agglomeration of the slime. The pH adjusting agent used in practice is often a dispersing agent or agglomerating agent for the slime, which acts to disperse the pulp or agglomerate the pulp. For example, Ca2+ in lime can weaken the negative polarity of the quartz surface, reduce the electrostatic repulsion, and facilitate the adsorption of the ionic flocculant.

Gold flotation, in the treatment of simple metal sulfide ore with less cyanide harmful substances in the ore, generally control the flotation pH = 7~9 with lime.

2.2 Inhibiting the flotation of iron sulfide minerals

When flotation of various non-ferrous metal sulfide minerals (such as copper, lead, zinc, etc.) with xanthate collectors, the ore usually contains a certain amount of iron sulfide minerals, such as pyrite, marcasite, pyrrhotite, arsenopyrite, etc., inhibiting iron sulfide minerals with lime helps the target minerals to float.

In the treatment of gold-bearing polymetallic sulfide mineral ores with high content of valuable heavy metals (such as gold-containing polymetallic sulphide ores containing copper, copper, zinc, lead, zinc, antimony, arsenic, etc.), since these ores contain To eliminate harmful factors, in order to eliminate unfavorable factors, in the actual production process, the flotation process combining mixed flotation and separation flotation is usually used, and lime is commonly used to adjust the pH value, generally controlling the mixing flotation pH=7~ 8. Allow most of the useful sulfide minerals to float up, and then use the separation flotation process to control pH = 10~12 to inhibit the separation of pyrite and achieve the purpose of separation.

Furthermore, natural gold particles are susceptible to inhibition by Ca2+. When a certain amount of lime is present in the slurry, Ca2+ reacts with CO2 in the air entering the slurry to form a CaCO3 precipitate, which inhibits the flotation of natural gold particles.

The addition of lime in the flotation operation is usually carried out by adding the lime powder to the ore belt of the ball mill or adding lime milk or lime powder to the stirring tank before the flotation.

The role of lime in the flotation process is most apparently the effect on the properties of the flotation foam. When the amount of lime is appropriate, the formed foam is relatively stable and has a suitable viscosity; when the amount is large, the foam is too stable, the foam is sticky, and even causes a “running” phenomenon, making the production process difficult to operate and control. At the same time, due to the aggravation of the cohesive slime, and this cohesion often lacks selectivity, the foam concentrate often entrains a large amount of ore fine mud, which affects the quality of the concentrate. Therefore, the amount of lime added should be strictly controlled in the flotation production of gold mines, and it should be fully recognized.

2.3 Other applications

Lime is an inorganic electrolyte, the dissociated Ca2+ ions are adsorbed on the surface of the slime, reducing or neutralizing the negative charge on the surface of the slime, causing the fine particles to agglomerate into larger agglomerates under the action of van der Waals force, thus in the actual production process. Many mines use it to add to the flotation concentrate concentration thickener, accelerate the slurry sedimentation speed, optimize the concentration and filtration operation, and prevent the occurrence of concentrate running.

3 Lime in cyanidation process

In the cyanidation process, lime can be used as a de-pharmaceutic agent for the gold concentrate cyanide plant before the immersion thickener to remove harmful substances, prevent the occurrence of gold concentrate run-off phenomenon and reduce unnecessary losses; As a pre-alkali dipping agent, it is used to eliminate the unfavorable factors of the leaching reaction before leaching; it can be used as a protective base for leaching; it can be used as a regulator, used in the process of zinc powder replacement, etc.

3.1 Before cyanide leaching

The role of lime before cyanide leaching comes down to three aspects. On the one hand, for the gold concentrate cyanide plant, it is commonly used as a de-agent for the pre-dip grinding classification or thickener. By adjusting the pH value of the pulp, that is, the pH, the anion properties of the flotation reagents (such as collectors and foaming agents) are changed, and they are defeated by competition to achieve the purpose of falling off from the mineral surface. On the other hand, since it is a weak inorganic coagulant, it is used in a pre-dip thickener to eliminate the charge on the surface of the mineral, compress the electric double layer, and form fine aggregates in the slurry to form a clot and accelerate precipitation. Prevent thickeners from running and reduce unnecessary metal loss. Furthermore, by pre-dip alkali leaching (lime CaO leaching) treatment, the concentration of anion and cation in the slurry is adjusted, the leaching reaction conditions are improved, and unfavorable factors are eliminated.

During the production process, some cyanide plants use the lime powder in the ball mill grading system; some add lime milk or lime powder to the sand pump box of the thickener before entering the leaching; some cyanide plants add one or two alkalis separately. The dip tank is pre-impregnated. The alkaline leaching process before the cyanidation process controls the alkalinity between 4 and 8/10,000 (calculated as CaO).

During the production process, excessive addition of lime will accelerate the sedimentation and concentration rate of the ore particles, which is not conducive to normal operation (such as thickener operation), and at the same time, CaCO3 precipitates will be blocked to block the pipeline; on the contrary, the amount of addition is insufficient, which affects both The effect of the drug does not reach the purpose of pre-alkali soaking. In short, the amount of addition is not suitable, which is not conducive to the normal cyanide operation. Therefore, many cyanide plants do not add lime in the leaching and replacement operations without special circumstances.

3.2 Lime application of leaching and washing operations

To maintain the stability of the cyanide solution and reduce the chemical loss of cyanide, an appropriate amount of alkali must be added to the cyanide solution to maintain certain alkalinity (referred to as a protective base). Lime is usually utilized as this because of the nature of the lime itself. In the cyanide leaching and washing process, due to the addition of lime, favorable conditions for the leaching and washing environment of gold are created, and the functions are summarized as follows:

(1) preventing the hydrolysis of sodium cyanide (of NaCN) to reduce the concentration of a solution of HCN, prevent volatilization of HCN gas;

(2) Under certain conditions, the consumption of sodium cyanide by impurities can be prevented. Such as soluble copper, iron, zinc, and other metal sulfides

The leaching consumption of the material; and the combination of sulfur ions and cyanide (CN-) ions in the sulfide slurry;

(3) Neutralize the acid formed by the oxidation of CO2 dissolved in water and sulfide ore, and prevent sodium cyanide from being decomposed by these acids.

(4) The precipitation of the ore pellets can be accelerated during the washing process, which is beneficial to the washing operation. In the cyanide production process, especially for ores containing more sulfide components, it is usually necessary to control the concentration of lime in the leaching operation. For the gold concentrate cyanide plant, the CaO concentration of the general control leaching process is between 2 and 5/10,000.

In actual production, the amount of lime should be strictly controlled. An excessive amount of lime, which may be due to flocculation so that the viscosity of the slurry is increased to increase the diffusion resistance of the solvent, so that a corresponding increase in the content of impurities in the solution, the surface of the gold particles forming the thin film of calcium peroxide, sodium cyanide and gold and hinder the role of oxygen , the effect of reducing leaching; when there is an insufficient amount of lime, sodium cyanide consumption will increase on the one hand, the influence leaching index; on the other hand, increasing the pregnant solution turbidity, causing the washing machine to run thickener tank, affect the subsequent zinc replacement operations smoothly Wait.

In the process of cyanide production, the control of lime is not paid attention to, and many mines cause losses. The ore in Inner since excessive addition of lime the gold leaching rate by 5% to 10%; a Shandong ore Again, a certain period due to insufficient amount of addition of lime, sodium cyanide consumption multiplied by 4. 82kg / t Increase to 9. 20kg / t and so on.

3.3 Lime in zinc powder replacement

In the zinc powder replacement process of the cyanide plant, it is also important to maintain the alkalinity (CaO amount) of the gold-containing liquid, ie, the precious liquid. The appropriate lime concentration can enhance the clarity of the noble liquid and improve the ion composition of the noble liquid. , changing the order of the displacement reaction, affecting the rate of displacement reaction and the consumption of zinc powder, thereby affecting the replacement rate of gold. In the replacement operation, the role of lime comes down to the following aspects:

(1) Enhance the clarity of the precious liquid and improve the quality of the gold mud. It is beneficial to the replacement operation and is beneficial to the subsequent gold smelting operation;

(2) By controlling the pH value, avoiding the formation of Zn(OH)2 and covering the zinc surface hinders the precipitation of gold. At the same time, hydrogen can be avoided in the alkaline solution to reduce the consumption of zinc powder;

(3) Change the impurity ions and their reaction state and sequence during the displacement reaction, prolong the service life of the replacement “golden cabinet” (filter press), and improve the replacement rate.

It is generally believed that when the concentration of CaO is low, impurities in the noble liquid are mainly formed by active ions, and the replacement process is mainly chemical reaction. During the high-temperature season, the activity of impurity ions (such as Cu2+ ions) is enhanced, the displacement reaction is accelerated, and the product clogs the filter cloth, resulting in a decrease in the processing capacity of the “golden cabinet” and a decrease in life. When the concentration of CaO is high, the impurities in the noble liquid are mainly in the form of compounds (such as hydroxides) and flocs (such as silica mud), and the replacement process is mainly physical change. That is, when the precious liquid passes through the filter cloth of the filter press, the compound, the floc or the like forms a “film” on the surface of the filter cloth, which weakens the water permeability of the filter cloth, causes the processing capacity of the filter press to decrease, and the life of the “golden cabinet” decreases. When the CaO concentration is appropriate, the impurities in the noble solution coexist in the form of active ions and compounds. At this time, the chemical and physical changes coexist in the replacement process, and the chemical changes at this time follow the order of elemental activity.

In the production process, due to the inattention to the control of the CaO concentration in the precious liquid, the life of the “golden cabinet” is reduced, which increases the cost and enhances the labor intensity of the workers. In this respect, a mine in Shandong has a painful lesson. The mine has been released 46 times in August and September of 1989, and it has been released three times in 24 hours.

Generally speaking, for the gold concentrate zinc powder replacement cyanide plant, the CaO concentration in the noble liquid is controlled between 3 and 8/10,000 in actual production, which can meet the service life of the replacement “golden cabinet” and guarantee Zinc powder consumption and gold replacement rate ensure the quality of gold mud products. In the production process, if the amount of lime is too much, in the presence of suspended SiO2 particles and excess Pb(AC)2, colloidal calcium silicate and calcium lead chlorate will form on the zinc surface, which will deteriorate the gold. Precipitation effect. On the other hand, if the amount of lime is small, the turbidity of the noble liquid is large, which affects the quality of the gold mud. At the same time, due to the small alkalinity, zinc easily reacts to generate hydrogen to increase the consumption of zinc powder. Therefore, it is necessary to strengthen the control of lime in the zinc powder replacement operation.

4 Lime in amalgamation

(1) The effect of amalgamation is affected by adjusting the pH value. The pH of the slurry has a great influence on the effect of the mercury mixing operation. In an acidic medium, the surface of the base metal attached to the surface of mercury is clean and promotes the wettability of mercury to gold. However, in an acidic medium, the slime cannot be agglomerated. On the contrary, the contamination of gold by the slime hinders the mercury to gold. Wetting. Therefore, lime is usually used to increase the pH value of the slurry, to agglomerate the slime, and to consume the unfavorable factors that hinder the wetting of gold by the contamination of the gold particles by the slime. Usually, the mercury mixing operation pH= 8. 0~ 8. 5 is appropriate.

(2) Inhibit the activity of sulfides in the slurry and prevent the mercury plate from “getting sick”. In the case of external amalgamation, sometimes sulfur or sulfide and mercury can cause mercury to pulverize, and black spots are formed on the mercury plate, which makes the mercury plate lose the ability to capture gold. This phenomenon especially includes arsenic sulfide, barium sulfide and ore in the ore. It is especially serious when strontium sulfide is used. Once this phenomenon occurs, the production of lime can be increased by increasing the amount of lime, increasing the pH of the slurry and inhibiting the activity of the sulfide.

(3) Prevent metal sulfide from adhering to the mercury plate and deteriorate the amalgamation operation. When the ore is treated with gold-containing polymetallic sulphides, metal sulfides often adhere to the mercury plate, which deteriorates the phenomenon of amalgamation. To eliminate this phenomenon, the amount of lime is often used in production, and sometimes the pH must be 12 or more. It can be solved.

(4) Elimination of magnetic amalgam in internal amalgamation operations. In the case of internal amalgamation in a non-alkaline medium, a magnetic amalgam is sometimes generated to cause the iron mineral to be mixed into the amalgam, so that the internal amalgam is mostly carried out in an alkaline medium. Lime is generally used to adjust the alkalinity of the slurry, which is used in an amount of 2% to 4% of the charge.

5 Other applications

Since lime absorbs mature lime Ca( OH) 2 and has a strong alkaline effect, lime has the following applications in the gold selection plant:

(1) Substituting sodium hydroxide (NaOH) as an absorbent for HCN gas in the acidification recovery process of cyanide wastewater. However, its use has great limitations, and there is no precedent in the country. According to reports: The Flinfloon Concentrator in Canada uses lime milk, which is atomized by a special device to react HCN gas with the absorbent Ca(OH)2 to form calcium cyanide for reuse.

(2) Neutralization of tailings slurry or sewage acidity to prevent corrosion of equipment, pipelines, etc.

6 Conclusion

Based on a large amount of data, this paper summarizes the role and control of lime in flotation, cyanidation, amalgamation and other operations in gold mines based on the system of years of production experience. Due to the limited level, there are inevitably many shortcomings, and many criticisms and exchanges are expected. Its purpose is to inspire and attract experience.

⇒ More info
1. The Ways of Increasing Gold Recovery Rate
2. Gold mining equipment

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