Application of Large Superfine Grinding Mill in Production of Submicron Heavy Calcium Carbonate

1 Introduction

In the development of crushing equipment, the ball mill family occupies a very important historical position. It is widely used and its equipment is very mature. It is still an important equipment until now. However, with the continuous development of modern industrial production, the requirements for pulverization are getting higher and higher. The traditional ball mill can no longer meet its specific requirements. Therefore, people are trying to find more efficient and energy-saving superfine pulverizing equipment. In this context, the invention of the agitator mill and the process of continuous improvement have begun. In 1928, Klein and Szegvari proposed the concept of agitating grinding and completing the wet grinding and pulverization. In 1948, the high-speed agitating ball mill was introduced. In 1952, the Japanese river end triumphantly invented the tower mill. Until the development of new materials in the 1960s, various mixing mills began to appear. Nowadays, in the development of papermaking, plastics, coatings and other industries, the fineness of the pigment required is extremely fine. For example, the heavy calcium coating grade requires -2um>=90%-95%, d50<=0.5-0.7um, and the output is large, requiring From 2 to 50,000 t / a or even 100,000 t / a. Therefore, the research on grinding equipment is very active. Since the end of the 1980s, from the 80L stripping machine, large-scale superfine grinding mills of 100L, 300L and 500L have appeared, and the quality of equipment has been continuously improved. Small sand mills have been used from the paint and pigment industries. Development of large mixing mills for metal and non-ferrous metal applications.

The Changsha Research Institute of Mining and Metallurgy started the development of vertical spiral mixing mills (or vertical mills or tower mills) in the late 1970s. In the 1980s, JM-230, JM- was developed in the laboratory and pilot. 260 and JM-460 vertical spiral mixer, based on this, extensive working parameters and fine grinding test of materials were carried out. In December 1989, JM-500 dry vertical spiral mixing mill passed the Hunan Provincial Science and Technology Commission. The identification of the organization, in 1990, the JM-1000 wet tower mill passed the appraisal organized by the former Ministry of Metallurgical Industry and the State Administration of Gold Administration. In 1998, the KJLM-400 ultra-fine agitating mill passed the appraisal organized by the Jiangsu Provincial Science and Technology Commission. And has won four national patents.
Since the application of vertical spiral mixing mill in Shandong Rushan Gold Mine Concentrator in 1990, the Changsha Research Institute of Mining and Metallurgy has continuously improved its structure and improved wear-resistant materials, and has been engaged in long-term grinding equipment development and beneficiation. The advantages of the powder process, the design of a series of vertical spiral mixing mill with its own intellectual property rights, products are constantly updated, has been applied in the gold, magnetic, manganese, chemical, rare earth and inorganic non-metal industries more than 260 pairs set. Recently, the 3600L large-scale superfine grinding mill was developed. It was successfully applied in the solar paper industry Gaoxu Chemical Company and Guangdong Dongguan Liweida (Taiwan-funded) company, and achieved a breakthrough in the large-scale production of heavy calcium carbonate wet grinding equipment. It will promote the quality improvement of wet grinding of heavy calcium and the reduction of production cost.

2. Equipment structure and working principle

　
The structure of the large ultra-fine agitating mill consists of a cylinder, a combined agitator, a transmission and a frame. The cylinder is filled with a certain grinding medium (zirconium beads, porcelain beads or glass beads). The transmission drives the agitator shaft to rotate at a medium speed, and the optimized agitator structure forcibly drives the medium ball and the material to perform a multi-dimensional cyclic rotating motion in the cylinder body, and the material utilizes friction under the squeezing force generated by the grinding medium and the agitator rotation. The impact collision is effectively ground. The slurry is pumped into the grinding chamber through the feeding port, and is grinded by strong grinding, shearing and impact collision between the mediums. The finished pulp is discharged from the upper screen through the upper screen and discharged into the slurry tank.

3. Requirements for paper coating heavy calcium

Paper-coated heavy calcium carbonate should meet the following requirements in addition to the physical properties and chemical composition of its ore.

(1) Particle size and particle size distribution requirements: the primer grade heavy calcium carbonate (GCC)-2um is greater than 60%, and the surface coating grade heavy calcium carbonate has a particle size requirement of -2um greater than 90%. It is generally required that the coated heavy calcium whiteness is greater than 90% or higher.

(2) Whiteness: Whiteness is the advantage of natural GCC. Different mineral sources have different whiteness. From limestone to calcite to marble, the whiteness of GCC ranges from 86% to 96%. It is generally required that the coated heavy calcium whiteness is greater than 90% or higher.

(3) Rheology: The muddy GCC has good rheology, the surface coating GCC solid content is 75%, the fineness -2um is more than 95%, and the viscosity requirement is less than 300mPa.s.

(4) Abrasion: Wear of wet GCC. The wear value of vehicle speed greater than 800m/min is usually measured by the Philcon Abrasion Tester. Generally, the filler is less than 25mg and the coating material is less than 5mg.

Domestic standards for wet-grinding calcium carbonate are being developed in China, and manufacturers can refer to implementation and exceed standards.

4. Design and process of large mill

4.1 Mill design

Because of the many factors affecting the grinding process of the ultra-fine agitating mill, such as the nature of the material to be ground and the state of the material to be ground, the desired product fineness and the mechanical structure, it is not possible to theoretically design. The method we have adopted is to carry out the necessary theoretical analysis, economically determine the best large-scale superfine vertical mixing mill by combining the experimental laboratory grinding experiment and the experience gained from the mill already running. The basic structure and parameters are then mechanically designed.

Simplified: The simplified height affects the surface pressure of the medium, and the surface pressure of the medium gradually increases with the height of the cylinder. When the height exceeds a certain height, the change is minimal. According to the situation of foreign equipment and our experience, the diameter ratio of the cylinder is 1:5-1:6. The cylinder liner is made of wear-resistant stainless steel, polymer material lining or corundum lining. The special industry can be designed according to specific requirements. The pump feed port, slurry overflow port, volleyball port and cleaning port should be properly arranged on the cylinder. Considering the heat, the cylinder is designed as a jacket, which proves that the cooling effect is good.

Agitator: The agitator is a key component of the equipment. The assembled disc structure and blade structure are used to better solve the startup and wear problems. On the basis of a large number of experimental studies, the structural parameters have been correctly determined, which is not simple and mechanically counterfeited. The principle is that the gap between the outer diameter of the agitator and the inner wall of the cylinder should satisfy the non-clamping ball, and the full utilization can be transmitted to the cloud holding ball and the wall wear is low. The gap size is also related to the rotational speed, media size, and agitator configuration. The selection of the number of blades mainly considers the energy transmission frequency, the medium filling amount, the power input condition, the grinding residence time, and also considers the force of the stirring body and the stability of the equipment.

Transmission: The combination of the motor directly drives the planetary gear reducer to better reach the working range of the agitator speed.

4.2 Mill process

The basic process steps of the wet grinding calcium carbonate production line are:

(1) Add the ore, water, and additives to the mixing tank for a certain period of time to make it uniform according to the proposed grinding conditions (ie, the ratio of raw ore, water, and auxiliary);
(2) using a slurry pump or a gear pump to drive the uniformly stirred raw slurry in the mixing tank into the buffer tank;
(3) using a diaphragm pump to pump the raw slurry stirred in the buffer tank from the bottom of the mill into the mill for grinding;
(4) With the gradual increase of the pulp in the mill, the grinding product (that is, the finely ground slurry) overflows from the upper overflow port of the mill, passes through the vibrating screen machine, and flows into the slurry tank along the pipeline. Multi-stage vibrating screens and grading equipment are installed as necessary to ensure maximum particle size and particle size distribution.

The above process controls the residence time of the slurry in the mill by controlling the flow rate of the overflow, thereby controlling the fineness of the product and the size of the production, and belongs to the open continuous production process.

For -2um greater than 60%-90% of the product, a section of grinding can be used. For -2um greater than 95%-98% of the product or higher, it is recommended to use two-stage grinding.

5. Application of mill in wet grinding heavy calcium industry

The 3600L large ultra-fine grinding mill has been successfully applied to a large papermaking enterprise in Shandong. Product fineness -2um content greater than 68% of the processing capacity of 4.6t / h (solid content of 72%, according to the slurry), energy consumption of less than 40-50kW.h / t. Product fineness -2um content greater than 90% of the processing capacity of 2.1t / h (solid content of 72%, according to the slurry), energy consumption of less than 80-90kW.h / t.

The heavy calcium slurry produced by the industrial large-scale agitating mill reaches the particle size of a foreign company, and the particle size d90 is less than 2um. It has been used as a coating pigment in the paper industry to meet its industrial requirements.

Since the operation of the large superfine grinding mill in June 2004, the equipment has stable performance, low noise, low energy consumption and easy operation. Its successful development will surely drive the technical progress of wet-grinding heavy calcium manufacturers, improve the quality of wet heavy calcium products, greatly reduce energy consumption and reduce production costs. Taking 100,000 t/a submicron heavy calcium slurry as an example, the electricity cost per ton is 20 yuan, and the annual electricity cost is 2 million yuan. The benefit is considerable.