El efecto mágico de la cal en el tratamiento de minerales

Introduction: The Unsung Hero of Mining Operations

Lime, chemically known as calcium oxide (CaO), is one of the most versatile and cost-effective reagents in tratamiento de minerales. Derived from limestone (CaCO₃) through calcination at 900–1200°C, lime serves as a pH regulator, flotation modifier, inhibitor, and even a protective agent in cyanide leaching. Its ability to transform into calcium hydroxide (Ca(OH)₂) upon hydration makes it indispensable across multiple stages of mining—from flotation and cyanidation to tailings management and environmental control.

This article explores lime’s crucial roles in:

• Flotation operations – pH adjustment, pyrite suppression, and froth stabilization
• Cyanidation processes – protective alkali, pre-leach treatment, and zinc replacement
• Mercury amalgamation – pH optimization and sulfide inhibition
• Environmental applications – acid neutralization and HCN scrubbing

 

1. Lime in Flotation: Enhancing Selectivity and Recovery

Lime can be used in the flotation process to improve the pH of the pulp; adjust the role of thiocarbonyl trap and some other inhibitors (such as cyanide) activity; can be used to inhibit the flotation of iron sulfide minerals, natural gold particles; precipitation of heavy metal ions in the pulp harmful to flotation; the role of the mud has cohesion into larger clusters.

 

Adjusting Pulp pH for Optimal Mineral Separation

Lime is cheap and easy to obtain and strong alkaline. In the flotation process of sulfide minerals, when the need to raise the pH value of the pulp or to be in alkaline or weakly alkaline media conditions, it is usually used as a pH adjuster. Generally speaking, its function is as follows:

(1) Adjusting the concentration of heavy metal ions in the pulp, forming insoluble 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. Flotation common easily formed insoluble hydroxides are: Al(OH)₃Cu(OH)₂Fe(OH)₂Fe(OH)₃Pb(OH)₂Zn(OH)₂etc.

(2) Adjust the ionic concentration of the trapping agent. Trapping agents in the water are in molecular or ionic states, and the media pH is closely related. Adjusting the pH value can adjust the proportion of the trapping agent in the water in the molecular or ionic state, in fact, adjust the degree of dissociation of the trapping agent.

(3) Adjust the role between the trapping agent and minerals. The role of the trapping agent ion and the mineral surface, and the pH value of the pulp, have a close relationship. The trapping agent anion and OH^– can compete on the surface of the mineral; the higher the pH value, the greater the concentration of OH^– ions, and the greater the role of the trapping agent anion can be rejected.

(4) Adjust the concentration of inhibitors. Some inhibitors are strong bases and weak acids composed of salt, such as the commonly used inhibitor water glass (also known as sodium silicate, its chemical composition of NaO₂SiO₂) that is, which can be hydrolyzed in water so that the slurry is alkaline, the pH value of high and low directly affects its degree of hydrolysis. When pH < 9, silicic acid (H₂SiO₃) son dominantes; cuando pH = 9~13, HSiO^-3 is dominant; when pH > 13, SiO^-3 es dominante.

(5) Adjust the foaming ability of the foaming agent. pH value has a certain influence on the foaming ability of the foaming agent. For example, the foaming ability of pine alcohol oil (2# oil) increases with the increase of medium pH.

(6) Adjust the dispersion and agglomeration of mineral sludge. The pH adjusting agent used in practice is often the dispersant or agglomerator of the slurry, which plays the role of dispersing the slurry or making the slurry agglomerate. For example, Ca²⁺ en la cal puede debilitar la propiedad eléctrica negativa de la superficie del cuarzo, reducir la repulsión electrostática y facilitar la adsorción de floculantes iónicos.

Para flotación de oro, in the treatment of simple metal sulfide ores (such as most of the ores in Shandong), which contain little harmful substances to cyanide, lime is generally used to control the flotation pH = 7~9.

 

Inhibiting Iron Sulfides & Natural Gold Particles

When flotation of various non-ferrous metal sulfide minerals with yellow drug type trapping agent (such as flotation of copper, lead, zinc, and other sulfide minerals from the ore), as the ore usually contains a certain amount of iron sulfide minerals, such as pyrite, albite, magnetic pyrite or sulfur arsenopyrite, etc., with lime suppression of iron sulfide minerals is one of the main methods commonly used in production practice.

In the treatment of valuable heavy metal content of gold-bearing polymetallic sulfide mineral ores (such as copper, copper-zinc, lead-zinc, tellurium, arsenic, and other gold-bearing polymetallic sulfide ores), as these ores contain harmful to the next step of cyanidation and floatable minerals, to eliminate the adverse factors, the actual production process, usually using a combination of mixed flotation and separation flotation flotation process, and lime is often used to adjust the pH Value, generally control the mixed flotation pH = 7 ~ 8, so that most of the useful sulfide minerals float, and then use the control pH = 10 ~ 12 of the separation flotation process, inhibit the selection of pyrite, to achieve the purpose of separation.

Furthermore, natural gold grains are easily inhibited by Ca^2+. When there is a certain amount of lime in the pulp, Ca²⁺ reacciona con el CO₂ en el aire que entra en la pulpa para producir CaCO₃ precipitación, que inhibe la flotación de los granos de oro natural.

In flotation operations, Lime is usually used in the molino de bolas feed belt with lime powder or in the flotation before the chemical tanque de mezcla with lime milk or lime powder and other forms.

The role of lime in the flotation process is most clearly demonstrated by its effect on the properties of the flotation foam. When the amount of lime is appropriate, the foam formed is relatively stable and has the right viscosity; however, if too much lime is used, the foam becomes overly stable and viscous, and may even cause “runaway” phenomena, making the production process difficult to operate and control. At the same time, due to the agglomeration of slime—which often lacks selectivity—large amounts of slime are frequently entrained in the froth concentrate, thereby affecting the quality of the concentrate. Therefore, in gold ore flotation production, the addition of lime must be strictly controlled, and a thorough understanding of its mechanism of action is essential.

 

Other Roles and Applications

Dado que la cal es un electrolito inorgánico, el Ca disociado²⁺ ions adsorbed on the surface of the ore mud, reduce or neutralize the negative charge on the surface of the mud, so that the fine particles coalesce into larger agglomerates under the action of van der Waals force, so in the actual process of production, many mines use it to join the flotation concentrate thickener to accelerate the settling speed of the pulp, optimize the concentration and filtration operations, to prevent the concentrate from running muddy, the phenomenon of muddy concentrate can occur.

 

 

2. Lime in Cyanidation: Maximizing Gold Recovery & Reducing Costs

En el proceso de cianuración, lime can be used as stripping agent, used in the gold concentrate cyanidation plant before leaching thickener, to remove harmful substances in the cyanidation process, to prevent the occurrence of gold concentrate running muddy phenomenon, to reduce unnecessary losses; can be used as a pre-alkali leaching agent, used in leaching operations before the elimination of leaching reaction unfavorable factors; can be used as a protective alkali, used in the leaching process; can be used as an adjusting agent, used in the replacement of zinc powder process, etc.

Role and Application Before Cyanide Leaching

The role of lime before cyanide leaching is summarized in three aspects. On the one hand, for gold concentrate cyanidation plants, it is commonly used as a stripping agent in the pre-leach mill classification or thickener. By adjusting the pH value of the pulp, i.e., the acidity and alkalinity, the anionic properties of flotation chemicals (such as trapping agents, foaming agents) are changed, and they are rendered ineffective by competition to achieve the purpose of shedding from the mineral surface. On the other hand, since it is a weak inorganic coagulant, it is used in the pre-leach thickener to eliminate the charge on the mineral surface, compress the double electric layer, make the fine mineral particles in the pulp form clumps, accelerate the precipitation, prevent the thickener from running muddy, and reduce unnecessary metal loss. In addition, through the pre-leaching alkaline leaching (lime CaO leaching) treatment, adjust the concentration of anions and cations in the pulp, improve the leaching reaction conditions, eliminate unfavorable factors, etc.

During the production process, some cyanidation plants add lime powder to the ball mill classification system; some add lime milk or lime powder to the sand pump box of the thickener before entering the leaching process; some cyanidation plants add one or two alkali leaching tanks separately for preleaching treatment. Generally, the alkalinity of the alkaline leaching process before cyanidation is controlled between 4~8/10000 (in terms of CaO).

During the production process, adding too much lime will accelerate the speed of ore settling and concentration, which is not conducive to normal operation (such as thickener operation), and at the same time, it will generate CaCO₃ precipitates blocking the pipeline; on the other hand, adding insufficient amount will not only affect the effect of drug removal but also fail to achieve the purpose of pre-alkali leaching. In short, the unsuitable amount of addition is not conducive to the normal operation of subsequent cyanidation. Therefore, many cyanidation plants do not add lime in the leaching and replacement operation under special circumstances.

 

Role and Application of Leaching & Washing Operations

To maintain the stability of the cyanide solution and minimize chemical losses of cyanide, an appropriate amount of alkali must be added to the solution to maintain a certain level of alkalinity (referred to as “protective alkali”). Given its inherent properties, lime is typically selected for this purpose. In the cyanide leaching and washing processes, the addition of lime creates favorable conditions for gold leaching and washing; in summary, its role is primarily reflected in the following aspects:

(1) Prevent sodium cyanide ( NaCN) hydrolysis, reduce the concentration of HCN in solution, and prevent the volatilization of HCN gas.

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

(3) Under certain conditions, it can prevent the consumption of sodium cyanide by impurities. Such as the leaching consumption of soluble copper, iron, zinc, and other metal sulfides, such as the combination of sulfur ions and cyanogen (CN^–) ions in sulfide slurry.

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

(5) In the washing process, it can accelerate the precipitation of ore particles, which is beneficial to the washing operation. In the process of cyanidation production, especially for the ore containing a lot of sulfide components, it is usually necessary to control the concentration of lime in leaching operations. For gold concentrate cyanidation plants, the general control of the leaching process is the CaO concentration between 2~5/million.

In actual production, the amount of lime should be strictly controlled. When the amount of lime is too much, maybe due to its flocculation effect to increase the viscosity of the pulp, thereby increasing the solvent diffusion resistance, so that the impurity content of the solution increased accordingly, the formation of calcium peroxide film on the surface of the gold particles, impede the role of gold and sodium cyanide and oxygen, reduce the leaching effect; when the amount of lime is not enough, on the one hand, will increase the consumption of sodium cyanide, affecting the leaching index; on the other hand, increase the When the amount of lime is not enough, it will increase the consumption of sodium cyanide and affect the leaching index.

Hay muchas minas que no prestan atención al control de la cal durante la producción de cianuración, con las consiguientes pérdidas. Por ejemplo, la tasa de lixiviación del oro disminuye de 5% a 10% debido a una adición excesiva de cal, y el consumo de cianuro sódico aumenta exponencialmente debido a una adición insuficiente de cal.

 

Role and Application of Zinc Power Replacement

In the process of zinc replacement in cyanide plants, maintaining the alkalinity (CaO amount) of the replaced gold-containing liquid, i.e., precious liquid, is also important. Proper lime concentration can enhance the clarity of the precious liquid, improve the ionic composition of the precious liquid, change the replacement reaction sequence, affect the replacement reaction speed and zinc consumption, and thus affect the gold replacement rate. In the replacement operation, the role of lime is summarized in the following aspects:

(1) Enhance the clarity of the precious liquid, improve the quality of gold slime. It is not only beneficial to the replacement operation, but also beneficial to the subsequent gold slime smelting operation.

(2) Mediante diferentes controles de pH, evitar la generación de Zn(OH)₂ and cover the zinc surface to prevent the precipitation of gold. At the same time, it can avoid the generation of hydrogen in alkaline solution and reduce the consumption of zinc powder.

(3) Change the impurity ions and their reaction state and order in the replacement reaction process, to extend the life of the replacement “gold cabinet” (filter press), improve the replacement rate, etc.

It is generally believed that when the CaO concentration is low, impurities in the precious metal solution primarily exist as active ions, and the replacement process is then dominated by chemical reactions. During hot seasons, the activity of impurity ions (such as Cu²⁺) increases, accelerating the displacement reaction; the resulting reaction products clog the filter cloth, leading to a decrease in the processing capacity of the “gold cabinet” (displacement tank) and a shortened service life. When the CaO concentration is high, impurities in the precious liquid primarily exist in the form of compounds (such as hydroxides) and flocculants (such as silica mud), and the displacement process is then primarily based on physical changes. In other words, as the precious liquid flows through the filter cloth of the filter press, these compounds and flocs form a “film” on the surface of the filter cloth, thereby reducing its permeability. This, in turn, leads to a decrease in the filter press’s processing capacity and shortens the service life of the “gold cabinet.” When the CaO concentration is appropriate, impurities in the precious liquid coexist in the form of active ions and compounds. At this stage, the replacement process involves both chemical and physical changes, with the chemical reactions following the law of the activity series of elements.

For the gold concentrate zinc powder replacement cyanidation plant, in the actual production control, CaO concentration in the precious liquid is between 3 ~ 8 / million, which can meet the life of the replacement “gold cabinet”, but also to ensure the consumption of zinc powder and gold replacement rate, to ensure the quality of gold sludge products. In the production process, if the amount of lime is too much, in the solution of suspended SiO₂ particles and excessive Pb (AC) 2 presence, will produce colloidal calcium silicate and calcium sublead precipitation on the surface of zinc, deteriorating the precipitation effect of gold. On the contrary, if the amount of lime is small, the turbidity of your liquid is large, affecting the quality of gold slime, and at the same time, due to the small alkalinity, zinc is easy to react with to generate hydrogen and increase the consumption of zinc powder. Therefore, it is necessary to strengthen the control of lime in the zinc powder replacement operation.

 

3. Lime in Mercury Amalgamation & Environmental Control

(1) Influence the effect of mercury amalgamation by adjusting the pH value. The pH of the slurry has a great influence on the effect of mercury amalgamation. In the acidic medium, the base metal attached to the surface of the mercury, if its surface is clean, can promote the mercury to the gold wettability, but in the acidic medium, it can not make the slurry cohesive, because of the slurry pollution of gold particles, and hinder the mercury from achieving the gold wettability. Therefore, lime is usually used to increase the pH value of the slurry, so that the slurry coalesces and consumes the unfavorable factors that prevent the mercury from wetting the gold due to the slurry contamination of the gold particles. Normally, the pH is 8. Ph 0~8. 5 is suitable for mercury amalgamation operation.

(2) Inhibit the activity of sulfide in the slurry, to prevent the mercury plate from becoming “sick”. In the external mercury amalgamation operation, sometimes sulfur or sulfide and mercury can cause mercury pulverization, black spots on the mercury plate, so that the mercury plate loses the ability to capture gold. This phenomenon is especially serious when the ore contains arsenic sulfide, antimony sulfide, and bismuth sulfide. Once this phenomenon occurs, production can be solved by increasing the amount of lime, increasing the pH value of the pulp, and inhibiting the activity of sulfides.

(3) Prevent metal sulfides from adhering to the mercury plate, deteriorating the mercury amalgamation operation. When dealing with ore containing gold polymetallic sulphides, metal sulphides will often occur attached to the mercury plate, deteriorating the mercury amalgamation process. In order to eliminate this phenomenon, production is often used to increase the amount of lime; sometimes, the pH value must reach 12 or more to solve it.

(4) Elimination of internal amalgamation operations of magnetic mercury paste. Mercury amalgamation in non-alkaline media sometimes produces magnetic mercury paste, so that the iron minerals mix into the mercury paste. Mercury amalgamation is mostly carried out in alkaline media. Generally, lime is used to adjust the alkalinity of the slurry, the amount of which is 2% to 4% of the charge volume.

 

 

4. Other Applications

As lime absorbs water to produce slaked lime, Ca( OH) ₂, with a strong alkaline effect, lime is used in gold processing plants and the following applications:

(1) As an absorber of HCN gas in the treatment of cyanide effluent acidification and recovery method to replace sodium hydroxide (NaOH). But its use has great limitations; there is no precedent for domestic use. It is reported that the Canadian Flin Flon beneficiation plant uses lime milk, which is atomized by a special device, so that the HCN gas reacts with the absorbent Ca(OH)₂ para producir cianuro de calcio para su reutilización.

(2) Neutralize the acidity of tailing slurry or effluent to prevent the corrosion of equipment and piping, etc.

 

Conclusion: Balancing Lime’s Benefits & Risks

Lime is invaluable in mineral processing, yet its misuse can hinder recovery. Key takeaways:

• Flotación: Use lime to suppress pyrite, but beware of gold surface passivation.
• Cyanidation: Maintain strict alkalinity (2–8 g/L CaO) to avoid leaching inefficiencies.
• Safety & Environment: Deploy lime for HCN control and tailing stabilization.

By optimizing lime dosage and application methods, miners can unlock greater metallurgical efficiency while minimizing costs and environmental risks.