Quality requirements and grinding process control of heavy calcium carbonate slurry for papermaking

1. Quality requirements for heavy calcium carbonate slurry for papermaking

The heavy calcium carbonate used in the wet end of the papermaking must meet the following requirements: high whiteness, high purity, stable particle size and distribution, low hydrochloric acid insoluble content and abrasion, slow settling performance, and the like. These specifications are closely related to the stability of the paper machine wet end production process and the control of paper quality indicators.

(1) The whiteness of the high whiteness heavy calcium carbonate filler affects the whiteness of the paper product, and affects the control of the hue stability and production cost of some paper dyeing auxiliaries. For the standard control of calcium carbonate whiteness is ≥90%, the standard control of calcium carbonate whiteness of some specialty paper enterprises is increased to ≥92%.

(2) High calcium carbonate content, low hydrochloric acid insoluble matter, low abrasion as heavy calcium carbonate for filler, the texture is relatively soft, the higher the calcium carbonate content, the more the friction is reduced, the damage to the papermaking equipment can be reduced, and the damage can be prolonged. The service life of the production equipment, the market for the control of calcium carbonate content is ≥ 98%.
Hydrochloric acid insolubles and wear indicators are mainly for non-soluble salts and hard impurities contained in calcium carbonate. If the index is too high and long-term use is bound to cause rapid wear of the paper machine and coating equipment, especially for the headbox. The relevant parts of the wire section and the press section are obviously damaged. The control requirement of the hydrochloric acid insoluble matter of the filler grade calcium carbonate used for papermaking is ≤0.35%, and the abrasion control requirement is 矣22g/m-2.

(3) Particle size distribution According to different uses, the requirements for the particle size and distribution of heavy calcium carbonate are also different. For the current grinding calcium carbonate used in the wet part of papermaking, the particle size is controlled to 65, and the particle size is required to be concentrated. Distribution in a certain area, preferably in a normal distribution, in order to facilitate the adjustment of operating parameters during the production process, particle size control range of -2μm content of 65%, -45μm content of 100%. The particle size of the calcium carbonate used as a filler is controlled to improve the retention of the filler under the same paper machine conditions and to provide a uniform and good surface performance for paper printing.

(4) The sedimentation performance mainly measures the speed of the sedimentation rate of the suspended particles of the ground calcium carbonate slurry under steady state. The sedimentation rate is too fast, which is not conducive to the storage during the production process, and will lead to excessive power consumption of the agitation. It also indicates that the ground calcium carbonate may have excessive particle size, poor dispersion or less added dispersion. If there is no sedimentation for a long time, there may be excessive grinding particle size or excessive dispersing agent. Such quality problems may cause abnormal operation of the wet end system of the paper machine. Normally, calcium carbonate with a content of -2μm and 65% (solid content: 17%) -18%) 15min settling velocity is ≤ 1mm.

2. Heavy calcium carbonate grinding process and process parameter control

(1) Brief flow of grinding The grinding process can be adjusted according to the actual production requirements of the slurry quality. The relevant grinding equipment suppliers will provide suitable grinding equipment and processes according to the capabilities of the grinding machine and the subsequent slurry quality indicators.
The immersion procedure is essential in the grinding process to allow it to be gradually swelled for subsequent grinding operations, to provide stable control of the slurry quality, and to reduce frequent fluctuations in the current of the grinding motor.
Equipped with a storage tank to ensure that the concentration of the slurry is stable and can be continuously supplied, reducing the occurrence of breakage problems.
There is a strict start-stop sequence during the operation of the grinder, otherwise it will cause problems such as blockage of the grinder or burning of the grinding motor. The opening of the grinder must be operated one by one according to the flow direction of calcium carbonate. After the slurry flows out from the outlet of the single grinder, the corresponding control switch is activated. Otherwise, the grinding opportunity of the idling causes damage to the grinding medium ball and affects the subsequent grinding quality.
Immediately after the completion of the grinding process, the grinding unit should be flowed with clean water until the concentration of heavy calcium carbonate at the outlet of the mill is very low, and then the grinding power is turned off one by one according to the order of opening to prevent the calcium carbonate agglomeration from affecting the next grinding work.

(2) Quality selection of raw powder The choice of raw calcium carbonate powder is closely related to the quality of the finished grinding slurry. The evaluation index of the finished finished ground calcium carbonate should firstly be from the whiteness of the original powder, the content of calcium carbonate, the content of insoluble matter of hydrochloric acid, etc. The three indicators are comprehensively considered, because these three indicators are not only the acceptance index of the original powder or the quality index of the finished slurry, only the whiteness of the slurry after grinding will be slightly higher than the original powder, and the other two indicators are subsequently ground. No improvement can be made in the process, and other indicators of the finished slurry can be adjusted to meet the required requirements by adjusting the grinding parameters or equipment status. These three indicators also reflect the purity of the calcium carbonate raw powder from the side, and also measure the quality of the source.

(3) Selection of grinding concentration The grinding concentration is crucial for the maximum efficiency of the grinding machine and the control of transportation cost. Too high concentration is not conducive to the dispersion of the slurry, but also affects the normal operation of the grinding machine, which may cause the grinding machine to block. Or the grinding motor burns down, and the amount of dispersant used is greatly increased; if the concentration is too small, the effective power of the grinding machine is not maximized, and the transportation cost is also increased. At present, the domestic control range of ground calcium carbonate concentration is (70±2)%.

(4) The use of dispersant and the addition of calcium carbonate raw powder are very easy to lose water and precipitate or even agglomerate during soaking and grinding, so it is necessary to add a dispersing agent in the production process to increase the dispersing property. Due to the difference in grinding quality requirements of calcium carbonate, the type, type and dosage of dispersing agent used are different. When immersing, an appropriate amount of dispersing agent must be added according to the concentration and the viscosity of the finished slurry. The quality of the dry powder is relatively dispersed. 0.2%-0.3% (dispersant active ingredient is 100%); at the same time, a small amount of 100 times diluted dispersant is added to the slurry outlet of the mill in an amount of 10-20 mg/L to alleviate the particle size reduction. The negative impact of increased viscosity.

(5) Selection of grinding media The grinding ball beads can be roughly classified into the following types: glass beads, corundum beads, high alumina ceramic beads, zirconia beads, and the like. Due to the difference in type and price, it can be selected according to requirements and production cost. Currently, the most used grinding media are high alumina ceramic beads, followed by zirconia beads.
Taking into account the quality of the medium itself and the control of the particle size of the calcium carbonate after grinding and the current of the grinding machine, the amount of the medium is 1/2~3/5 of the grinding volume of the grinding machine, and the types of media used by different grinding units are different, according to For the grinding process, a unit can be added with 2.0~2.5mm, a two-stage unit with 0.7~1.0mm, and a three-stage unit with 0.25~0.5mm. The finer the particle size of the slurry in the grinding unit, the smaller the media level.
After the grinding machine has been running for a period of time (about 3 months), the medium-grade medium that has been worn out for a long time in the second-stage unit can be transferred to the third stage according to the actual situation, and part of the medium in the section is transferred to the second stage, and the grinding is performed in a section. A part of the medium of 2.0~2.5mm is added to the machine to save production cost on the basis of ensuring the grinding quality.

(6) Selection of system piping and pump Calcium carbonate slurry is easy to precipitate, and the pipeline for discharging slurry should be easy to disassemble to deal with the blockage of calcium carbonate precipitation after the end of the pumping process, so the common pipeline is easy to disassemble. For stainless steel or cast iron pipes, the pump should be pumped with mud pump (except for the pump and pipeline of the grinding unit). The inlet of each mud pump should be connected with a clean water pipe to clean it in time when the pumping is over or there is a blockage.
It is better to use a transparent ring-shaped wire-wound hose for the pipe between the grinding units to observe the actual direction and position of the calcium carbonate slurry. The abnormal problems can be dealt with in time to reduce the idling of the grinding unit caused by the blockage of the pipe; The air pump is used for pumping between the grinding units, and the installation location can be flexibly arranged according to the actual grinding situation.

(7) Selection of screening equipment during grinding process There are two vibrating screens in the whole grinding process: the first vibrating screen is located between the foaming tank and the temporary storage tank, and the screen is a 250 mesh nylon net, mainly removing the original The powder and the foreign matter of the large particles in the foaming process; the second vibrating screen is arranged between the slurry outlet of the last grinding machine and the finished slurry storage tank, and the screen adopts a 325 mesh nylon mesh, which mainly removes the grinding process. Small particle impurities (including partially broken small media).
Both screens must be kept clean to prevent blockage of the mesh to form a slurry overflow. At the same time, the slurry outlet of each grinder is also equipped with a corresponding slit screen to prevent the grinding medium from being pulled out during high-speed operation. The slit screen at the outlet part is easily dried due to the loss of calcium carbonate, which must be regular (usually 2h). Wash it.

(8) Preservation of calcium carbonate slurry Calcium carbonate contains bacteria. Although it has been heated at a high temperature, it cannot be killed. Especially in summer, it is very easy to spoil and produce odor, which affects the quality of paper. In order to control the growth of bacteria, it is usually added in the summer to maintain the quality of the slurry. Generally, 400~500mL of fungicide is added to the 40t liquid ground calcium carbonate. Other seasons may or may not be added depending on the actual situation.

(9) Problems in the grinding process and adjustment methods To ensure that the quality of calcium carbonate grinding is stable and qualified, it is necessary to control the following parameters in the production process: calcium carbonate particle size, normal operation current of the grinding unit, and calcium carbonate viscosity. These three indicators interact with each other.
The particle size of the ground calcium carbonate can be measured by the corresponding particle size meter. After the stable grinding operation is controlled, the value of the motor current of each grinding machine or each grinding unit corresponding to the stable range of calcium carbonate particle size can be known. And its control range, and detect the corresponding viscosity.
When the motor current is low and the viscosity of the slurry is at a normal level, the grinding medium ball is worn out, and an appropriate amount of the corresponding type of medium must be added; if the viscosity of the slurry is lower than the normal level, the added dispersing agent may be excessive or The concentration of the foam is low. If the current of the motor is higher than the normal level, there may be problems such as a small amount of dispersant added, a calcium carbonate agglomeration inside the grinder or an excessively high concentration of the slurry.
The relationship between the above parameters must be flexibly controlled according to the actual conditions of the production process. The relationship between the parameters and the method of adjusting the abnormality are shown in the table below.

Table 1 Relationship between parameters and abnormal adjustment

3. Conclusions On the basis of understanding the indicators of the ground calcium carbonate raw powder and the finished grinding slurry, combined with the quality requirements of the ground calcium carbonate, the above-mentioned grinding operation process and parameters are rationally applied to the production. Grinding calcium carbonate products suitable for papermaking are of great benefit.