Research and practice have shown that the addition of chemicals during wet or dry grinding will have some effect on the grinding process. Chemical substances (solid, liquid or gaseous chemicals) that increase the efficiency of pulverization or reduce energy consumption during fine pulverization are called grinding aids. Because of the high energy consumption of ultrafine pulverization and the low energy utilization rate, the research of grinding aids has important theoretical and practical significance.
First, research on the type of grinding aids shows that many substances have a grinding effect. From the chemical structure point of view, the grinding aid should have selectivity and good dispersion, can adjust the viscosity of the slurry, has strong anti-Ca 2+ , Mg 2+ ability, and is less affected by pH value. That is to say, the molecular structure of the grinding aid should be compatible with the complex physical and chemical environment of the grinding system. In the specific application, it is necessary to carry out the necessary screening of the type of grinding aid according to the nature of the material to be ground.
This section mainly introduces surfactant-based wet grinding aids. The common types are shown in Table 1.
Grinding agent name | Grinding object | Consumption /% | effect |
Sulfonated polystyrene and its derivatives (relative molecular mass 5000~20000) | Copper, iron ore | 0.02 | The viscosity of the slurry is reduced by 30%~50%, and the product is increased by 3%~5% for less than 325 purposes. |
Maleic anhydride and styrene copolymer | Ore and coal | 0.01~0.04 | Slurry viscosity decreased by 23% |
Sulfonated polyethyl methacrylate | Ore and coal | - | - |
Polyacrylate | limestone | 0.1~0.6 | Fineness improvement |
Lignin derivatives (sulfonic acid, carboxylic acid, etc.) | Cement raw meal | 0.02~0.1 | The viscosity of the slurry is 1000mPa·s |
Polysaccharide (CMC carboxylated starch, sodium alginate, etc.) | - | - | Improve grinding efficiency and uniform product size |
Dihydrocarbyl disulfide sodium phosphate | Iron-bearing quartzite | - | Improve hematite efficiency and reduce quartz efficiency |
C 4-6 sodium carboxylate | Slag | 0.035~0.05 | Improve grinding efficiency |
Oleic acid | Hematite | - | Reduce pulp viscosity and increase grinding concentration |
Fatty acid calcium salt | limestone | 0.1~1 | 14 times more new surface |
Sodium naphthenate | Limestone | 1 | Newborn surface increased 18 times |
Sodium thiocycloalkanate | Limestone | 1 | - |
Dodecyl ammonium chloride | Limestone | - | Alkaline improves mine efficiency |
The effect of oleic acid on hematite grinding at different pH conditions has been studied, as shown in Table 3. It is apparent from the table that the pH is improved by 8.7 when the grinding effect is improved. This may be because the acid-soap complex formed most in this pH range. Obviously, the effect of surfactants on grinding is closely related to the formation of surface active ion-molecular complexes in solution.
Pharmacy name | Consumption /% | Less than 500 mesh yield increase /% | Pharmacy name | Consumption /% | Less than 500 mesh yield increase /% |
Triethanolamine | 0 0.025 0.050 0.100 0.150 | 0 +41 +60 +42 -33 | Sodium oleate | 0 0.066 0.111 0.155 0.222 | 0 -25 -twenty two +52 +34 |
TF-279 | 0 0.05 0.15 0.20 0.25 0.50 | 0 +43 +45 +65 +71 +52 | Sodium sulfate | 0 0.0125 0.0375 0.0625 0.0875 0.125 0.150 | 0 +5 +25 +68 +135 +66 +54 |
Oleic acid concentration / (mol / L) | Less than 400 mesh yield change /% | |||
3 × 10 -7 3 × 10 -5 | pH=3.0 +1.0 +2.3 | pH=6.6 -2.8 +7.8 | pH=8.7 +9.0 +9.0 | pH=11.0 -1.0 +4.5 |
Where the mouth of the lead zinc ore mineral processing plant, Guangxi Research Institute metallurgical grinding test conducted amine type surfactant was added, the results indicate that the effect of grinding with a hydrophobic carbon chain varies, and too short carbon chains It will reduce the grinding aid effect, and the grinding aid effect of guanamine is better. Its optimum dosage is 175 g/t. The grinding time per unit time is increased by 12% or the mill capacity or mill energy consumption is reduced by 10.2%, which has a good grinding effect.
Second, the mechanism of action of grinding aids
Because the role of grinding aids is complex and subject to many factors in the grinding process, its mechanism of action is still in the research stage, and there is no unified view, and it is still in various assumptions. The proponents of these hypotheses each proposed their own arguments to prove their own assumptions. At present, these assumptions cannot be unified, but they can explain some phenomena. There are mainly Rehbinder's adsorption reduction surface energy hypothesis, Westwood's adsorption for near-surface dislocation migration, R. Klimpel's rheological theory with reduced pulp viscosity and Wu Mingzhu's comprehensive hypothesis.
The author tends to the comprehensive hypothesis proposed by Wu Mingzhu, and believes that the mechanism of the grinding aid in the grinding process is mainly dominated by two university schools, namely, the theory of adsorption reduction and hardness and the rheological theory dominated by the reduction of slurry viscosity. Although these two theories have their own limitations, they represent different grinding stages. Since the factors affecting the pulverization efficiency are very complicated, in addition to the way of stress application, there are strength properties of the material, surface properties, viscosity of the slurry, flocculation and dispersion of the particles, and the like. Therefore, the above two theories can be regarded as uniform. Under certain conditions, one mechanism of action may be dominant, and the other may be secondary, but from the perspective of the entire pulverization process, the two mechanisms of action At the same time, the role of grinding aids is undoubtedly a superposition of these effects. In the case of rough grinding, the adsorption-reducing hardness theory plays a leading role, and it is preferable to use a lower-hardness type agent; when fine grinding, the rheological theory predominates, and it is preferable to use a viscosity-reducing agent. The preferred grinding aid should be an agent that both reduces hardness and disperses. Tests have shown that the use of a reducing agent in combination with a dispersing agent, if used properly, will result in a good grinding aid.
Therefore, the main role of grinding aids in the grinding process is:
1 affects the strength (hardness) of the material. Rehbinder believes that all solids can be seen as a unique colloidal structure cut by a defect network composed of hypermicroscopic cracks. These unique ultramicroscopic cracks have an average spacing of 0.01 to 0.1 μm. When the solid is deformed by force, the new surface gradually develops on the basis of them, and the damage occurs at the place with the most defects. If unloaded, the microcracks that have expanded will re-heal under the action of molecular forces. When a grinding aid is added, due to the structural characteristics of the amphiphilic surfactant, the surfactant penetrates onto the surface of the microcrack, causing dislocation migration of the surface layer lattice, resulting in defects of dots or lines, which not only reduces the surface energy of the particles, but also prevents new additions. The combination of cracks, and penetration into the interstices and cracks of the particles, increases the deformation, thereby reducing the strength and hardness of the solid.
2 Change the rheological properties of the slurry in the mill. At this time, the grinding aid acts as a dispersing agent. Since the grinding aid adsorbs on the solid particles, the wetting of the dispersion medium is improved, and a layer of dispersant molecules coated on the particles adsorbs the film, which mutually repels the particles, reduces shear friction, and reduces the viscosity of the slurry. To improve the fluidity of the slurry and prevent agglomeration between the particles and the particles, between the particles and the grinding medium and the liner. R. Klimpel believes that grinding aids primarily control the viscosity and flow of the pulp. The pseudoplastic pulp has the highest grinding rate, the expanded slurry is the second, and the pseudoplasticity with the yield value is the lowest. In order to make the grinding machine have the highest processing capacity, the concentration of the slurry in the mill should be controlled in the pseudoplastic zone during operation, but because the range of the zone is very narrow, it is difficult to control in production practice. The use of certain grinding aids can change the position of the pseudoplastic zone to a certain extent and expand the range of the zone in order to increase the grinding rate, thereby achieving the purpose of improving the grinding efficiency. Fuerstenau et al. believe that at high pulp concentrations, the slurry is difficult to flow. If there is no additive, the viscous slurry covers the mill wall and the steel ball and sticks the steel ball to the mill. When the mill rotates, the steel ball is brought up. If the pulp is sufficiently strong, the steel ball will be difficult to separate from the wall for centrifugal operation. At this time, the grinding machine is like a flywheel, and its traction power (torque) is also reduced. . Therefore, it is considered that there is a possibility that there is a critical viscosity above which the grinding medium is subjected to centrifugal motion. This critical value depends on the grinding operation conditions, such as the rotational speed, size of the mill, the size of the steel ball, the viscosity of the slurry, and the like. When the slurry concentration is low, the viscosity is small and the additive does not work. The critical viscosity can be estimated by force analysis of the grinding media. Certain additives can control the viscosity of the slurry below the critical value, causing most of the steel balls to slop and drop and increase the fine grain yield.
Therefore, the amount of grinding aid is critical, too much or too little can not achieve the desired effect. In the research on the grinding process of limestone clay mine, Zeng Fan also found that sodium hexametaphosphate is in the optimal dosage range, which can play the role of dispersing fine mud, reducing pulp viscosity and increasing fluidity. [next]
Third, factors affecting the application of grinding aids
The fact that certain chemicals are added to the grinding operation to improve the grinding effect has gradually been confirmed by people. Because of the large consumption of energy and materials by grinding, the research of grinding aids and grinding equipment and grinding process Like the research, it is valued by people. In the research and application of grinding aids, the following points should be noted:
1. Relationship with other operations In terms of grinding, no matter what kind of medicine, anyone who can improve the grinding efficiency can be used as a grinding aid as long as it is economically approved. However, considering the entire beneficiation circuit, the impact of the grinding aid on subsequent operations must be carefully studied.
Generally speaking, the grinding aid does not adversely affect the sorting efficiency of re-election and magnetic separation. For flotation, the grinding aid may have a good effect on the mineral surface or with other flotation agents, and may also have a bad effect. It may also have an adverse effect on dewatering operations. Anything that has a large adverse effect on the operation after grinding is difficult to overcome and cannot be used as a grinding aid.
At the same time, it should be noted that although some drugs do not have the ability to directly improve the grinding efficiency, they can improve the classification process and achieve the result of improving the grinding efficiency. In a broad sense, this is also a grinding aid.
Grinding aids can only be used when the increased grinding efficiency can compensate the cost of the agent, and there are still some time to use. Some synthetic grinding aids are expensive, which often limits its application, but it can be found from the research of grinding aid mechanism. Many drugs may play a role in grinding, so it is meaningful to note the impact of existing flotation agents on grinding. For example, studies on the effects of collectors such as amines, oleic acid, and some dispersants on iron ore wet grinding are one of the examples of attempting to increase the grinding efficiency by adding a general flotation agent to a grinding machine.
2. Environmental impact Grinding is carried out under complex physical and chemical conditions. Grinding aids can affect various parameters of the pulp (such as zeta potential, pH value, surface tension, viscosity...), some parameters of the slurry. It will inevitably affect the grinding effect of the agent. A number of tests have indicated that the agents have different grinding effects at different pH values. Especially for ionic agents, the optimal dissociation is related to the pH value, so comprehensive consideration should be given when selecting agents.
3. The charge of the grinding aid should pay special attention when adding the grinding aid. It should try not to add the grinding aid opposite to the mineral charge. Even if the amount is small, it is harmful to the grinding. This is because the adsorption reduces the zeta potential of the mineral particles, thereby causing the flocculation to be strengthened. The test results show that the higher the degree of flocculation, the lower the grinding rate.
4. The effect of surfactant foaming due to the adsorption of surfactant on the particles to make the particles hydrophobic, as a result of the particles adsorbed to the stable bubbles generated by the surfactant, if a sufficient amount of bubbles adhere to the particles, the particles can Floating up, so that it escapes the role of the grinding medium, the result indirectly reduces the grinding efficiency.
5. The role of water The most common medium effect in the grinding process is the presence of water. The wetting of water and ore particles increases the penetration of water into the porous regions of the particles, thereby promoting the fragmentation of the ore particles. P. Somsundaran et al. believe that the action of water may be mainly due to the reversible reaction between unsaturated surface bonds and water molecules. The following reaction between silica, siloxane, and water may affect the cleavage of the oxygen bond between the two silicon atoms, thereby promoting fragmentation:
—Si—O—SI—+H 2 O→SiOH•OHSi
In general, wet grinding is more efficient than dry grinding. HEL-Shall et al. suggested that the pre-grinding ore should be in contact with water for a period of time to improve the grinding effect.
6. Selectivity of grinding aid Some additives may have different grinding effects on different components in the ore. For example, sodium tripolyphosphate has been found to be significantly better than quartz in the grinding of hematite under alkaline conditions.
Therefore, if the grinding aid has a large difference in the grinding efficiency of different minerals, it is possible to use this difference to promote a certain mineral sorting efficiency and grinding efficiency further. In the study of grinding aids, it is not possible to simply use screening to check the grinding effect, and other means are needed to obtain the correct conclusion.
7. Differences between large and small grinding machines Industrial grinding conditions are very different from those of laboratories. Therefore, there are many industrial grinding tests in foreign countries that cannot repeat laboratory results. The main reason is that industrial-scale grinding machines are often “in a state of dynamic instabilityâ€, and various factors of change are easy to offset the effects of additives, which is a noteworthy tendency. Therefore, it is necessary to overcome the cause and overcome it in order to be properly used and to exert the true effect of the medicament.
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DTMADE product is designed to keep up with the derivation of the market, innovation and quality stablization always become the first goal we pursue.
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