The Magic Effect of Lime in Mineral Processing

Lime is widely used in mineral processing areas, like beneficiation processing, flotation processing, and leach processing. Also, lime can be used to raises the pH value, thus precipitating metals diluted in the water into tailing areas. And limestone is often used in the construction of tailing areas and as a part of the cap after closing the mine. Furthermore, lime is an important product used in underground mining.

Properties of Lime

Lime is also called quicklime or white ash, the active ingredient is CaO, which is obtained by calcination of limestone CaCO3 at 900~1200℃. Here is the reaction formula:

CaCO₃→CaO + CO₂↑

Lime has strong water absorption, and becomes calcium hydroxide (Ca(OH)₂) after interacting with water, commonly known as slaked lime or slaked lime. Its solubility in aqueous solution is very small, but dissolved in aqueous solution can further ionized calcium ions ( Ca²⁺ ) and hydroxide ions ( OH^– ), so that the solution is strongly alkaline, the reaction formula is:

CaO + H₂O = Ca(OH)₂ + heat

Ca(OH)₂= Ca²⁺ + 2OH^–

Slaked lime reacts with CO₂ to form CaCO₃ precipitate, where the reaction equation is:

Ca(OH)₂+ CO₂→ CaCO₃↓ + H₂O

This property is the basic reason for the calcium caking in the bottom of the machinery and equipment or in the transport pipes when using lime on site.

The role and application of lime in flotation

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.

Adjust the pH value of the pulp

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 need 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 make many metal cations to form insoluble hydroxides. Flotation common easily formed insoluble hydroxides are: Al(OH)₃, Cu(OH)₂, Fe(OH)₂, Fe(OH)₃, Pb(OH)₂, Zn(OH)₂, etc.

(2) Adjust the ionic concentration of the trapping agent. Trapping agents in the water are 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 between the trapping agent ion and the mineral surface and the pH value of the pulp has a close relationship between 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, the more 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 commonly used inhibitor water glass (also known as sodium silicate, its chemical composition of NaO₂SiO₂) that is, it 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) molecules are dominant; when pH = 9~13, HSiO^-3 is dominant; when pH > 13, SiO^-3 is dominant.

(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⁺ in lime can weaken the negative electric property of the quartz surface, reduce the electrostatic repulsion, and facilitate the adsorption of ionic flocculants.

For gold flotation, 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.

Suppressing the flotation of iron sulfide minerals and natural gold grains

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, in order 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 Ca²⁺, when there is a certain amount of lime in the pulp, Ca²⁺ reacts with the CO₂ in the air entering the pulp to produce CaCO₃ precipitation, which inhibits the flotation of natural gold grains.

Lime in flotation operations, usually used in the ball mill feed belt with lime powder or in the flotation before the chemical mixing tank with lime milk or lime powder and other forms.

The role of lime in the flotation process, the most obvious appearance is the impact on the nature of the flotation froth. When the amount of lime is appropriate, the foam formed is more stable, with the appropriate viscosity; when the amount is larger, it will lead to too stable foam, foam sticky, and even cause the “run tank” phenomenon, making the production process difficult to operate and control. At the same time, because of the agglomeration of the mud phenomenon, and this agglomeration and often lack of selectivity, so that the froth concentrate is often trapped in a large number of ore mud, affecting the quality of the concentrate. Therefore, the gold mine flotation production should strictly control the amount of lime added, to have a full understanding of it.

Other roles and applications

Because lime is an inorganic electrolyte, the dissociated Ca²⁺ 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.

The role of lime in cyanidation and its application

In the cyanidation process, 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.

The role and application of leaching and washing operations

In order 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 (called protective alkali). Due to its nature, lime is usually used for this purpose. In the cyanide leaching, washing process due to the addition of lime, the leaching of gold, washing environment to create favorable conditions, in summary, its role is as follows:

(1) to prevent sodium cyanide ( NaCN) hydrolysis, reduce the concentration of HCN in solution, 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 sulfide

(2) 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;

(3) 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.

(4) 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 CaO concentration is 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.

There are many mines that do not pay attention to the control of lime during cyanidation production, resulting in losses. For example, the gold leaching rate decreases by 5% to 10% due to excessive lime addition, and the consumption of sodium cyanide increases exponentially due to insufficient lime addition.

The role and application of zinc powder replacement in the operation

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) Through different pH control, avoid the generation of 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, the impurities in your liquid mainly exist as active ions, and the replacement process is dominated by chemical reactions. In high temperature seasons, impurity ions (such as Cu²⁺ ions, etc.) become more reactive, the replacement reaction is accelerated, and the products block the filter cloth, resulting in the “gold cabinet” processing capacity is reduced, and the life time is reduced. When the concentration of CaO is high, the impurities in the precious liquid mainly exist in the form of compounds (e.g. hydroxide) and flocculants (e.g. silica sludge), and the replacement process is mainly based on physical changes. In other words, when the precious liquid passes through the filter cloth of the filter press, the compounds and flocculants form a “thin film” on the surface of the filter cloth, which reduces the water permeability of the filter cloth, resulting in a decrease in the processing capacity of the filter press and a decrease in the life of the “gold cabinet”. When the concentration of CaO is suitable, the impurities in your liquid coexist in the form of active ions and compounds. At this time, the replacement process is chemical and physical changes coexist, when the chemical changes follow the law of the sequence of elemental activities.

In the production process, due to the lack of attention to the precious liquid CaO concentration control caused “gold cabinet” life to be reduced, both increasing the cost, but also enhancing the labor intensity of workers.

Generally speaking, for the gold concentrate zinc powder replacement cyanidation plant, in the actual production control CaO concentration in the precious liquid 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 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.

The role and application of lime in mercury amalgamation

(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 its surface clean, can promote the mercury to the gold wettability, but in the acidic medium, can not make the slurry cohesion, but because of the slurry pollution of gold particles and hinder the mercury to 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=8. 0~8. 5 is suitable for mercury amalgamation operation.

(2) inhibit the activity of sulfide in the slurry, to prevent the mercury plate “sick”. In the external mercury amalgamation operation, sometimes sulfur or sulfide and mercury can make mercury pulverization, black spots on the mercury plate, so that the mercury plate lose the ability to capture gold, this phenomenon is especially serious when the ore contains arsenic sulfide, antimony sulfide and bismuth sulfide. Once this phenomenon, 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, will often occur metal sulphides attached to the mercury plate, deteriorating the mercury amalgamation process phenomenon, 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.

(4) Elimination of internal amalgamation operations magnetic mercury paste. Mercury amalgamation in non-alkaline media, sometimes produces magnetic mercury paste, so that the iron minerals mix into the mercury paste, so the 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.

Other applications

As lime absorbs water to produce slaked lime Ca( OH) ₂, with strong alkaline effect, lime in gold processing plant 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 to produce calcium cyanide for reuse.

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