Application of grinding aid in dry production of ultrafine heavy calcium carbonate

The preparation of powders, especially the preparation of ultrafine powders, plays an increasingly important role in the modern materials industry. The energy consumption of materials is very large. According to statistics, in modern industry, about 80% of energy is used for material crushing and grinding. Practice has proved that when the material is pulverized and crushed, the use of appropriate grinding aid can increase the grinding yield and reduce energy consumption. The effect and mechanism of grinding aids in powder pulverization have been extensively studied. However, due to the obvious selectivity of the grinding aids, the types and dosages of grinding aids required for different types of materials. It is different from usage. In this paper, the application status of grinding aids in powder preparation is reviewed through the results of the actual machine evaluation.

1 grinding aid mechanism
Grinding aids are a general term for chemicals added to a mill system during material grinding. Its role is mainly to improve the grinding efficiency, speed up the breaking speed of ore particles, and change the rheological characteristics of the ore slurry due to the dispersion of the agent, and some can also inhibit the corrosion of the steel ball and the liner, and finally reach Reduce energy consumption, steel ball consumption and selective grinding.

During the material comminution process, the material is usually subjected to different kinds of stresses, causing the material to form cracks and expand, and then pulverized. According to Griffith's law, reducing the surface energy of the particles reduces the stress required to break them. From the process of particle rupture, the adsorption of the grinding aid molecules on the new surface can reduce the external stress required for crack propagation and promote the crack propagation. Therefore, from this point, a chemical substance capable of lowering the surface energy of the solid can reduce the strength of the particles, thereby improving the pulverization efficiency or reducing the pulverization energy consumption. Rebinger first studied the effect of liquids on the fracture of solid materials in both organic and inorganic chemical additives.

He believes that liquids will greatly affect the fragmentation, and the addition of surfactants can amplify this effect because the surface energy of the surface of the solid surfactant is reduced after the surfactant molecules are adsorbed, resulting in a weakening of the bonding force. In addition to the strength and hardness of the above particles, the main factors affecting the grinding efficiency and energy consumption from the pulverization process are the pulp mass fraction, viscosity and the dispersion state of the material, the interaction between the material and the grinding media and the mill liner. Wait. These factors all affect the fluidity of the material in the mill. Therefore, improving the fluidity of the material in the mill to a certain extent can improve the pulverization efficiency.

Through a large number of studies, it is believed that the grinding aid improves the flowability of the dry powder or slurry, and significantly improves the speed of the continuous passage of the material through the mill, thus affecting the material pulverization process; in addition, the improvement of the fluidity also affects the particle in the mill. The distribution in the grinding medium and the grinding action of the grinding medium; the presence of the grinding aid prevents the mutual agglomeration or bonding between the particles by maintaining good dispersibility between the particles. In this sense, grinding aids are substances that reduce the viscosity of the slurry and increase the fluidity of the slurry.

2 dry process of heavy calcium carbonate production process and characteristics
Calcium carbonate is generally classified into heavy calcium carbonate and light calcium carbonate. Heavy calcium carbonate, also known as ground calcium carbonate, referred to as heavy calcium, is obtained by physical processing; light calcium carbonate, also known as precipitated calcium carbonate, referred to as light calcium, is obtained by chemical processing. Heavy calcium is widely used in many fields due to its numerous advantages, and the real value of heavy calcium application is its deep processing products, namely ultra-fine, high-purity, surface-modified products. Therefore, the preparation of high value-added ultra-fine, high-purity heavy calcium products has a good industrial application prospect.
The main processes are as follows:
Raw stone → one crushing → secondary crushing → mill (Raymond mill, ball mill, roller mill, etc.) → classification (classifier) ​​→ bagging features:
(1) Low investment and high-quality production of stable products;
(2) The production conditions can be flexibly controlled for different use requirements, and products with different requirements can be produced at a lower cost;
(3) The surface modification treatment of the product can be carried out to improve the performance of the product;
(4) When producing ultra-fine and fine products, the efficiency is low and the energy consumption is large.

3 Experiment and evaluation of grinding 600 mg heavy calcium carbonate
3.1 Raw materials and equipment
Original stone: a large calcite in a mine in Hunan; whiteness is over 92%; equipment: domestic Raymond mill and grading system; grinding aid: H SC G A50, supplied by San Nopco (Korea) Co., Ltd.;
Product use: building coatings and paper fillers.
3.2 Experimental methods
After the secondary crushing of the raw stone, the mill inlet is added with different added amount of the raw liquid.
3.3 Results and discussion
3.3.1 Raymond mill experimental data (see Table 1)

3.3.2 Experimental evaluation
Whiteness: no adverse effects, but also a certain degree of improvement; sieve residue: residue reduction of 4.1%, product coarse particle number reduction; particle size: product particle size is finer, added value; moisture: because the grinding aid is liquid The product, the moisture content increased slightly, within the controllable range; energy consumption: ton dry powder energy consumption decreased by 18.8%; yield: when the grinding aid addition amount was 0.075%, the yield increased by 24.8%.

4 Experiment and evaluation of grinding 1500 mesh heavy calcium carbonate
4.1 Raw materials and equipment
Original stone: a large calcite in a mine in Qingyang, Anhui, with a whiteness of over 93%; equipment: domestic ball mill and imported grading system;
Grinding aid: HS-CGA50, supplied by Sannopco (Korea) Co., Ltd.
4.2 Experimental methods
After the secondary crushing of the raw stone, the mill inlet is added with different added amount of the raw liquid.
4.3 Results and discussion
4.3.1 Experimental data of ball mill (see Table 2)

4.3.2 Experimental evaluation
Whiteness: A slight decrease (down 0.4%). Analysis of the reasons: (1) For the case of ultrafine grinding, the grinding efficiency is improved due to the addition of the grinding aid, and the whiteness may be affected; (2) due to the addition of the grinding aid, there is a grinding medium (steel ball) It must be cleaned. If it is used for a long time, the effect on whiteness may be reduced. Particle size: As the amount of feed increases, the particle size becomes thicker to some extent, but within the required range.
Yield: When the amount of grinding aid added was 0.057%, the output per hour increased by 2.0 tons, and the monthly increase was 57%.

5 Quality and quality evaluation of heavy calcium carbonate before and after grinding aid
5.1 Raw materials and equipment <br> Heavy calcium carbonate: # 1 1500 mesh (HS-CGA50, added amount 0.057%); #2 1500 mesh (diethylene glycol, added amount 0.06%); #3 900 mesh (no addition Grinding Aid); #4 900 mesh (HS-CGA50, added amount 0.094%)
Resin: LDPE extrusion equipment: domestic twin-screw extruder
5.2 Experimental methods
The LDPE and GCC were mixed at a ratio of 1:1, extruded at 150 to 160 ° C using a domestic twin-screw extruder, and extruded through a 120 mesh screen.
5.3 Results and discussion
5.3.1 Evaluation of dispersibility and liquidity (see Table 3)

P1: LDPE extrusion pressure
P2: LDPE: GCC (1:1) mixed extrusion pressure ΔP: P2-P1
Evaluation: The dispersibility and fluidity of GCC to which the grinding aid H SC G A50 was added had better dispersibility and fluidity than that of GCC without addition and addition of diethylene glycol. It can increase production efficiency and GCC can be better dispersed in the resin.
5.3.2 Evaluation of color difference (see Table 4)
5.3.3 Evaluation of color difference (see Table 5)
Evaluation: From the results of the above two color difference evaluations, the finer the G CC particle size and the better the dispersion, the better the appearance of the obtained product.

6 conclusion
6.1 By adding grinding aid (HS-CGA50) during the grinding of heavy calcium carbonate, the production efficiency of heavy calcium carbonate can be improved under the same energy consumption, the productivity can be increased by more than 20%, and the GCC particle size is increased. The more detailed the effect is.
6.2 After adding the grinding aid (HS-CGA50), the calcium carbonate powder has a fluffy feeling after bagging, and the hand feels smooth.
6.3 Compared with no additives added and added, it has better dispersibility and fluidity; it is beneficial to obtain high quality products.

Author: Li Daqiang [Shengnuopuke (Shanghai) Co., Ltd.