In this paper, the factors affecting the grinding efficiency of pigment paste are discussed, and the effects of the process parameters of grinding equipment, the type and size of grinding medium, different grinding process, the rotational speed of grinding equipment and the flow rate of pigment paste on the grinding efficiency are emphatically studied. In this paper, the optimal process parameters of large flow sand mill for grinding solvent-based paint paste are given: The grinding effect and grinding efficiency can be improved when the grinding speed of the rotating shaft is 700 ~ 800r/min, the first grinding flow is 6.5L/min, the second grinding flow is 5L/min, and the filling ratio is 70% to fill φ 1.2 ~ 1.4mm pure zirconia beads.
Liquid paint, usually composed of paint, solvent, pigment (filler) and a small amount of additives. Essential paint is a solid pigment dispersed in the liquid coating of a uniform, stable dispersion. As the main equipment of producing paint, grinding and dispersing equipment plays an important role. High-performance coatings such as car coating on grinding fineness requirement enhances unceasingly, and coating production enterprises on the improvement of production efficiency, energy consumption and environmental protection, how to choose high efficiency grinding of dispersion equipment, improved grinding process, optimizing grinding media type and so on, to optimize particle size distribution, improve the dispersion efficiency, saving energy and reducing consumption, such as purpose, It is a new problem faced by coating production enterprises.
Ⅰ. The Experiment
1. The Experimental Materials
- Hydroxy acrylic acid resin A, Industrial Products, Nanjing;
- Rutile titanium dioxide, Industrial Products, Sichuan;
- Phthalocyanine Blue, Industrial products, Nantong;
- Medium pigment carbon black, Industrial Products, Shaoyang;
- Propylene glycol methyl ether acetate, S-100A solvent, Industrial products, Jiangsu;
- Dispersant, industrial products, Tongling.
2. The Laboratory Equipment
- Horizontal sand mill A(Chongqing);
- Horizontal sand mill B(large flow horizontal sand mill, Zhangjiagang);
- Scraper fineness meter (Guangzhou).
The sand mill used in this test is the horizontal disc type sand mill. The medium distribution in the grinding chamber of horizontal disc sand mill is uniform, the grinding efficiency is high, and it is easy to start with load. The structure diagram of horizontal sand mill A is shown in Figure 1, and the structure diagram of horizontal sand mill B is shown in Figure 2.
3. Test Method
In this test, hydroxyl acrylic resin A was used as the base material, and three pigments, rutile titanium dioxide, phthalocyanine blue and medium pigment carbon black, were used for grinding and dispersive test. The specific formula of color paste is shown in Table 1. A certain amount of color paste was prepared according to the formula, and it was pre-dispersed on the dispersion equipment for 15min and then sent to the sand mill for grinding. Fineness was detected at intervals (carbon black and phthalocyanine blue were immersed in the mixture of solvent and dispersant, and resin was added for pre-dispersion after 24h).
Ⅱ. Result And Discussion
1. Influence of Parameters of Sand mill on Dispersion Efficiency
450kg 50% titanium white slurry, 250kg 10% phthalocyanine blue slurry and 250kg 10% carbon black slurry were prepared and cyclically ground on horizontal sand mills A and B, respectively. The fineness was measured by scraper fineness meter at intervals to investigate the influence of different sand mill parameters and separation mode types on dispersion efficiency. The comparison of sand mill parameters is shown in Table 2. The experimental results are shown in Figure 3 ~ Figure 5.
It can be seen from Table 2 that compared with A, horizontal disc sand mill B has A small volume and A large energy density (power/volume). Pure zirconia beads with higher density and hardness can be used to improve the grinding effect. Moreover, the loss of smaller volume in color conversion is relatively small, which reduces the consumption cost of color paste grinding.
The separator structure of THE B-type sand mill is a rotor dynamic separation cylinder structure (FIG. 6), which can use the grinding medium with smaller particle size without blocking the screen. The change of medium separation mode is an important technical innovation in the development of sand mill. The gap ring flow area used by type A traditional sand mill is small. When using small size grinding medium, because the screen is static structure, it is easy to be blocked. While the normal size of the grinding medium due to smaller wear, drum gap ring due to long-term wear, resulting in large gap size, also prone to grinding medium stuck in the gap ring phenomenon, resulting in reduced flow. B type horizontal sand mill is used dynamic centrifugal separation system, separation of the rotor turns the medium and the centrifugal force makes the medium to be left to the rotor, the rotor center mainly paste, screen mesh separation arrangement in the center of the rotor, paste can smoothly through the screen mesh aperture and outflow, not easy the clogging and wear and tear, can guarantee the stability of the printing ink separation.
2. The Effect of Grinding Media on The Grinding Efficiency of Color Paste
(1) The Effect of Grinding Media Type on Grinding Efficiency
Grinding media can be divided into glass beads, ceramic beads (including zirconium silicate beads, zirconium dioxide beads), steel beads, etc. Chemical composition and manufacturing technology of grinding media differences decided the different crystal structure, which in turn determines the different grinding medium compressive strength and wear resistance, different content determines the density of the grinding media, and the kinetics formula P = mv, the impulse of grinding medium P is proportional to the mass of the grinding media, the greater the density of grinding media, the greater the kinetic energy, The more efficient the grinding is.
450kg titanium dioxide slurry was prepared and filled with glass beads (φ 2 ~ 3mm) and zirconium silicate beads (φ 2.2 ~ 2.4mm) at 65% volume filling ratio in A type A sand mill. The grinding was carried out under rated power and the fineness was measured at regular intervals. The test results were shown in Figure 7.
Figure 7 shows that the dispersion efficiency of zirconium silicate beads is slightly higher than that of glass beads, mainly because zirconium silicate beads have higher packing density and hardness than glass beads, and have higher kinetic energy, which improves the grinding efficiency. But also found in the actual production process, compared with zirconium silicate beads, glass beads wear rate is higher, more likely to appear broken bead phenomenon, stuck in the gap of medium separation, blocking the discharge device, affect the grinding efficiency.
(2) The Impact of The Size of Grinding Media on The Grinding Efficiency
The size of the grinding medium determines the number of contact points between the grinding medium and the color paste. The more contact points of the grinding medium with small particle size under the same volume, the higher the theoretical grinding efficiency; But on the other hand, when the pigment particle size is large in the initial grinding stage, the impulse of the grinding medium with small particle size is small, and the better grinding effect cannot be achieved. In the late grinding period, the particle size distribution of the lapping medium with large particle size is worse than that of the lapping medium with small particle size due to the lack of contact points and energy density.
In combination with the calculation of the filling amount of grinding media in the sand mill and the author’s practical application experience, the packing density of φ 2.2 ~ 2.4mm zirconium silicate beads, φ 1.2 ~ 1.4mm pure zirconia beads and the mixing beads of the above two grinding media in the mass ratio of 1∶1 were determined respectively in the horizontal disc sand mill B by 70% volume filling ratio. The results are shown in Table 4.
250kg phthalocyanine blue paste was prepared and cyclically ground in the above three different grinding media. Fineness was measured at regular intervals, as shown in Figure 8. According to the curves in the figure, when φ 2.2 ~ 2.4mm zirconium silicate beads were used alone, the dispersion efficiency of the first 30min was higher. When φ 1.2 ~ 1.4mm zirconium oxide beads were used alone, the final fineness of the product was low. When the mixing method of large and small beads was used, the efficiency did not improve compared with that of φ 1.2 ~ 1.4mm grinding medium, but decreased.
3. Impact of Grinding Process on Grinding Efficiency
Different varieties of color paste, due to the type of pigment (inorganic pigment, organic pigment, carbon black), fineness requirements, viscosity of color paste and other parameters, the corresponding grinding process is not the same. It is of great significance to choose suitable grinding process of color paste to improve the grinding efficiency and grinding effect of color paste, and to improve the stability of product quality.
250kg carbon black pulp was prepared according to the formula, and three different grinding processes were adopted for grinding, and the fineness was determined as follows:
- Sand laying A single machine with two cylinders and three channels, fineness 17.5μm;
- Sand laying B single machine, double cylinder, three channels, fineness 12.5μm;
- Sand A + sand B two, fineness to 10μm.
The results show that the mode of A type sand mill with one line and B type sand mill with two lines, combined with the characteristics of A type sand mill with high efficiency in the early stage and B type sand mill with high efficiency in the later stage and low fineness in the end, improved the dispersion efficiency. After using the process for a period of time in actual production, the grinding efficiency of carbon black and organic pigment paste is increased by more than 15%, and the paste has better storage stability.
4. Influence of Slurry Flow on Grinding Efficiency
Horizontal sand mill B(large flow horizontal sand mill) was used for the test. The φ 1.2-1.4mm pure zirconia beads were filled with 70% filling ratio, and the grinding efficiency was compared under the same rotating shaft speed (800r/min) and the same cooling water condition. The experimental results are shown in Table 5.
The table 5 shows that under the same rotation speed, choose a big traffic, paste in the cylinder body of the residence time is short, in the early stages of the inorganic pigment dispersion and organic pigment dispersion, fineness were similar, late and dispersed organic pigments, appropriate to reduce the flow, increase the residence time in the cylinder body, can guarantee by grinding fineness requirement could be achieved with twice, Reduce the number of grinding tracks. Therefore, the flow rate of the first color paste is 6.5L/min, the flow rate of the second color paste is 6.5L/min for inorganic pigment, organic pigment and carbon black color paste.
1. The grinding efficiency and effect of pigment paste is related to the parameters of grinding and dispersing equipment, the type and size of grinding medium and the grinding process; The dynamic centrifugal separation system of the medium separation structure of the grinding equipment has advantages in the stability of medium separation compared with the traditional drum-type screen ring. The grinding process of filling horizontal sand mill A(φ 2.2 ~ 2.4mm zirconium silicate bead filling) and series horizontal sand mill B(φ 1.2 ~ 1.4mm pure zirconia bead filling) was adopted to improve the grinding effect and stability of hydroxyl acrylic resin color paste, and the grinding efficiency was increased by more than 15%.
2. The large flow horizontal sand mill has the design characteristics of high energy density and dynamic medium separation structure, and achieves better grinding effect and higher grinding efficiency. When 6.5L/min is used as the first grinding flow rate, 5L/min is used as the second grinding flow rate, and φ 1.2-1.4mm pure zirconia beads are filled with 70% filling ratio, the grinding effect is better, and the grinding efficiency of color paste can be improved.