Grinding Technology in Paint and Raw Material Production

Grinding Technology in Paints and Raw Material Production

1. Introduction

In the sector today, bright colors and paints are expected to have longer service lives. This significantly increases the need for quality requirements. Manufacturers spend considerable time researching and selecting the best raw materials and grinding equipment suitable for product composition, viscosity, particle size distribution (before and after dispersion and grinding processes), and other properties. (1) With advancing technology, there is a growing need for materials in fine (<100 μm), very fine (<10 μm), or ultra-fine (<1 μm) size ranges. (2) Since existing materials in the ball mills used in facilities cannot physically grind to very fine sizes, some facilities utilize stirred and vibrating ball mills in intermediate product grinding circuits (3); however, stirred ball mills, jet mills, or orbital mills are being used. These equipment types have inherent advantages and disadvantages. However, in recent years, interest has increased in stirred ball mills due to their lower specific energy consumption compared to drum and vibrating ball mills, as the amount of energy released per unit time and volume in these types of equipment is very high. (4)

2. What is Grinding Technology?

Grinding is the final stage of size reduction and is applied to particle sizes smaller than 25 mm. It is a unit operation with high and inefficient energy consumption. As particle size decreases, particle resistance to fracture increases, which in turn increases energy expenditure. (5) Grinding theories must proceed from the assumption that the material is brittle.

Rittinger Theory

It proposes that the energy consumed for particle size reduction is proportional to the newly created surfaces.

Kick Theory

It proposes that the required energy depends on the reduction ratio (L1/L2).

Bond Theory

It bridges the gap between the two theories above. New grinding technology reduces specific energy requirements while increasing power input and consequently production capacity. (7)

3. Types of Grinding Technology

3.1 Bead Mills Sand was initially proposed as the grinding sphere, known in literature as "sand mill," and in 1957 DuPont Company brought the sand mill to a level suitable for paint manufacturing. In subsequent years, bead mills (pearl mills) came into use as glass or steel beads were employed as grinding spheres.

3.1.1 Operating Principle

Bead mills operate in series and consist of four main components: • A pump equipped with a gearbox, • A stirrer shaft containing grinding disks arranged on it, • A grinding chamber (grinding vessel) with a cooling jacket containing this shaft, • A main drive unit (electric motor or hydraulic drive). Paint components mixed in the vessel undergo pre-dispersion in the dissolver and are fed to the grinding chamber from below via a feed pump. The pump, which can be adjusted continuously through its associated gearbox, is critical for series operation. This is because it affects the dwell time of the material in the vessel. Fast passage creates short grinding time; slow passage creates long grinding time. After desired fineness is achieved, material passage through the outlet is determined with a stopwatch. After this, the pump adjustment is not altered; only the consistency of the fixed flow rate is checked. When adjusting the flow rate, the paint passing through the mill is collected in a separate vessel. Subsequently, an amount of paint equal to the internal volume of the grinding chamber is collected in another vessel. After this, the mill outlet discharge is directed to the product vessel. The previous materials are fed back into the mill inlet and dispersed again. The disks rotating with peripheral speeds of 8-12 m/s through the shaft drive push the mixture (paint to be ground and grinding spheres) rapidly into the chamber from their edges. Shear force and shear gradient emerge in proportion to the magnitude of mechanical energy applied and the total surface of the disks and the distance of the blades from the internal surface of the chamber. A grinding chamber forms between the two disks where turbulent flow develops. Agglomerates collide with the mass approaching them at speeds determined by shear force and shear gradient, and break apart. This mass is the grinding beads. However, the agglomerates themselves also aid grinding. At this point, the fluidity characteristics exhibited by the dispersion medium of the ground paint play an important role. Good grinding is achieved depending on the frequency and intensity of collision.

3.1.2. Types

3.1.2.1. Conventional Bead Mills Open to atmosphere and vertical chambered. A screen basket sits around the upper part of the chamber. The paint to be ground enters from below and is ground, exiting the top of the chamber after completing dispersion while the grinding spheres remain inside. In other words, separation is provided between the paint and grinding spheres. The paint passing through the screen is removed.

3.1.2.2. Closed Bead Mills

With the development of mechanical seal systems, these mills are obtained with the sealing medium mounted instead of the screen basket, where the mill chamber body and the drive section at the head are perfectly sealed. They are used with vertical-horizontal chambers and circular-square cross-sections. They are closed to atmosphere and separation of grinding sphere and paint at the end point of dispersion is performed by a type of closed screen system. It is considered that vertical chamber closed bead mills have certain advantages. As the paint to be dispersed is fed from below by pump and rises upward, the grinding spheres press downward by their own weight, creating additional back pressure. This provides positive contribution to dispersion quality. To achieve the best product quality here, the smallest bead should be used so that the contact surface area between the beads and pigments increases and the gaps between the beads decrease. (9) Thus, efficient grinding is provided with less specific energy, and the desired product quality is achieved. However, certain obstacles are encountered at this point. The first is that the product viscosity must be suitable for this, and the second is the quality of the mixture. The primary function of the screen is to keep the beads within the grinding circuit. Therefore, to prevent screen clogging, the screen opening size must be larger than the maximum particle size. The bead size should preferably be two to three times the screen opening size.

3.2 Stirred Ball Mills

They have been used popularly in many industrial sectors such as paints and chemicals for the past 30 years. These mills have higher energy intensities than conventional ball mills and groundmass separation from the final product is accomplished smoothly in continuous grinding. (10) This mill's operating principle is based on moving the grinding medium charge through a cylindrical chamber using rotation applied to a shaft, with the charge mounted around the shaft via rods, disks, or spirals, and size reduction occurs through friction and wear between the material and balls. (11) Due to the operating principle, the forces used in these mills are different; abrasion and cutting forces are present primarily along with impact. (12) In recent years, the reason stirred ball mills have become an alternative to conventional size reduction equipment is that the amount of energy released per unit volume within the equipment is very high when compared with others, and grinding to fine sizes can be accomplished economically. Here grinding is defined by two parameters: stress intensity and stress frequency. The performance of grinding in stirred ball mills is affected by geometric and process variables. (13) Compared to other grinding devices, stirred ball mills are easier to operate, have shorter grinding times and lower energy consumption. (14) They are equipment with the capacity to grind to micron and sub-micron fineness levels. (15) However, since it is unclear where in the mill and in what manner particles are subjected to fracture, two conditions are required to ensure sufficient size reduction at a specific time interval with the mill. These are: • The number of collisions occurring per unit time inside the mill, • The amount of energy released in each collision, The most important problem is accurately determining the fracture rates and fracture distribution functions depending on particle size.

3.2.1 Operating Principle

Stirred ball mills, first designed in 1920, have become the focus of interest in the past 20 years and continue to be used without change to their basic structure. The cylindrical body has a water jacket to prevent overheating, and inside this body is a high-speed rotating stirrer. (16) With the beginning of the stirrer's rotation, a characteristic flow profile develops within the mill. This profile determines both the medium movement within the mill and the equipment's energy consumption. The movement mechanism of the medium-material mixture is quite complex. In most studies aimed at determining flow profiles and energy distribution, calculations are made assuming laminar flow within the mill and homogeneous Newtonian fluid. Flow profiles are examined using CFD techniques, and based on the determined flow profile, specific energy distribution within the mill is calculated. As a result, the movement of each particle making up the grinding medium within the flow is determined. (17) Flow begins with the rotating disk pushing the fluid toward the mill wall. The fluid reaching here is directed upward. To maintain the continuity of movement, the fluid continues toward the shaft, where the movement began. The flow profiles within the mill produce a typical specific energy distribution primarily determined by the fluid's velocity gradient. Two regions with high energy density can be identified within the mill. (18) The increase in energy density in these two regions achieves high-performance grinding.

Two groups of design and process variables affect grinding:

Design Variables: height/diameter ratio, stirrer type, position and number, distance between disks and rods on the stirrer shaft, dimensions of disks or rods and their positions on the shaft (19). These variables in designs serve to optimize the resulting flow profiles and ball movements. Process Variables: Stirring speed, ball density, size and charge ratio, pulp density and viscosity. The stirring speed appears to refine the particle size distribution of the product obtained. At the same time, the mill's power increases linearly, creating an increase in the amount of specific energy spent per unit volume, thus providing a marked reduction in the grinding time required to reach the desired product particle size. As a result, mill capacity is increased. (20) Studies have determined that the optimum ratio between ball size and particle size is between 7:1 and 20:1 when considering the nip angle in roller crushers. Again, similar studies have shown that when large balls are used, low stirring speed is more effective on grinding, and when fine balls are used, high stirring speed is more effective. Low-density balls are less efficient in grinding compared to high-density balls, particularly when low stirring speed and coarse feed are used. This is because compressive force is as effective as shear force. Ball charge also increases the power drawn by the mill linearly. Pulp density has great importance for fineness. However, when the pulp solids ratio exceeds 75% by weight, particle fracture rates have been found to decrease. This is because fine particles in the pulp cause further increase in pulp viscosity and reduce grinding performance. (21)

3.2.2. Types

Today, there are many commercially designed types with various components changed. There are laboratory-scale mills smaller than 1 liter and equipment with net body volumes reaching 3,000 liters and motors of 1,100 kW. (22) Besides those listed below, other types exist: Cage mills, Cutting Blade Mills, Atox Vertical Mill, Raymond, Polysius, Alpine and Szego Mills. (23)

3.2.2.1 Horizontal Stirred Ball Mill: Isa Mill

Mount Isa Mines, with over 50 years of experience in stirred grinding, developed large-scale, continuous-operation, and robust equipment through joint work with German Netzsch-Feinmahltechnik GmbH. This made possible the mill's use of inexpensive and natural grinding medium and open-circuit operation. The power used in ultra-fine grinding applications also provides transition to regrinding and coarse grinding applications. (24) Some others: Tower mill, Svedala mill, Maxx Mill, Pitt Mill, Drais Mill. The most important advantages of horizontal stirred ball mills are as follows: • High capacity (100 tons/hour), • Very fine product size obtained (d80=7 μm), • Low-cost natural grinding medium use, • High energy efficiency (26).

Tower and Vertical Mills

Low-speed (3 m/s) vertical stirred mills. Designed in 1950 by "Japan Tower Mill Company Ltd"; the name was later changed to "Kubota Tower Mill Corporation". (27) In the mill, stirring is provided with the aid of a spiral mounted on the main shaft in the grinding medium, and it is generally operated wet. During grinding, small particles rise upward while large particles remain in the grinding medium. They rise through the spiral channel and fall downward by the spiral and mill chamber. (28) The maximum particle size fed to the mill is 3 mm and the product size obtained is less than max. 70 μm. Capacities exceed 50 tons/hour. The advantage of these mills is first that steel balls up to 30 mm can be used. They are large stirred mills. In ore preparation, in regrinding, they are frequently applied in closed circuit with small hydrocyclones. If smaller medium is used, it can cause problems such as balls escaping from the top of the mill during grinding.

3.2.2.2 Jet Mills

Grinding is accomplished through impact and wear effects as particles collide with each other and the chamber body by air supplied at very high pressures into the mill chamber. Feed size is typically below 0.5 mm. The grinding chamber is designed to minimize wear. The ground material in the system is carried by fluid motion through an air separator, and after separation into fine particles, returns to the system with coarse particles. The fluid medium uses hot pressurized steam, air, or inert gas. These mills are used in limited areas in industry. Their capacities range from 0.5 to 5,000 kg/hour. (29)

3.2.2.3 Vibrating Mills

Consists of grinding chambers stacked on top of each other. By applying specific vibration, material and balls move within the chamber, and particles are pulverized as a result of crushing and compressing ore. The ball charge ratio is at 60-70% by volume and size ranges from 10-15 mm. The biggest disadvantage is problems with its capacity. Additionally, mechanical assemblies frequently malfunction. Maintenance and repair costs are quite high. (30)

3.2.2.4 Orbital (Planetary) Mills

Developed for grinding to micron and sub-micron sizes. Operates in a high-level gravitational field. With the effect of high centrifugal field, very high energy is released within the mill. Due to these characteristics, it is possible to obtain very fine-sized material in a shorter time. The disadvantage is higher grinding cost and frequent problems with mechanical assemblies. (31)

3.2.2.5 Pendulum Mills

The most well-known pendulum mill type is the Hikom mill. The grinding chamber is suspended and rotated at 600-800 rpm with accelerated motion about its own axis. (32) The particles inside are pulverized by the balls, and the ground product exits the grinding chamber through holes in the chamber. The product then passes through a separator and is separated from the fine portion. Compared to other mill types, it provides energy savings between 31-70%. (33)

3.2.2.5 Ring (Bracelet) Mills

Generally used at laboratory scale. Consists of nested rings. The material located between the rings is crushed by the eccentric motion of the rings on the frames and pulverized in a short time. Due to their low capacities, they find limited application in industrial areas.

3.2.2.6 Nailed Mills

Used for grinding dry or slightly moist material with maximum feed size of 30-40 mm to 100 microns at high capacity. (34) References (1)https://www.turkchem.net/islak-ogutme-ekipmanlarinin-secimine-farkli-birbakis-acisi.html (2)(4)(13) Dikmen, S., Ergün, Ş.L. "Stirred Ball Mills", Mining, Vol. 43, No. 3, p. 4, December 2004 (3)Harbort et al., 1999; Young and Gao, 2000, cited in Dikmen, S., Ergün, Ş.L. "Stirred Ball Mills", Mining, Vol. 43, No. 3, p. 4, December 2004 (6) Türk, Umutcan. "Ball Mills", Fırat University Engineering Faculty Metallurgy and Materials Engineering Department, Master's Thesis, p. 14-15, October 2015, Elazığ (9)https://www.turkchem.net/macromedia-on-ogutme-ve-dispersiyon-ihtiyaclariicin-devrim-niteliginde-cozum.html (7) https://www.turkchem.net/ogutme.html (16) Sepulveda, J.L., 1981, "A Detailed Study on Stirred Ball Mill Grinding", Doctoral Thesis, Department of Metallurgy and Metallurgical Engineering, The University of Utah, cited in Dikmen, S., Ergün, Ş.L. "Stirred Ball Mills", Mining, Vol. 43, No. 4, p. 4, December 2004 (22) Murphy, A. Gao, M. and Vargas, A., 2004, "IsaMill - new Tool for improving Plant Recoveries" www.xstratech.com/downloads/im newtool.pdf, cited in Dikmen, S., Ergün, Ş.L. "Stirred Ball Mills", Mining, Vol. 43, No. 4, p. 4, December 2004 (17) Blecher, L. Kwade, A. and Schwedes, J., 1996, "Motion and Stress Intensity of Grinding Beads in a Stirred Media Mill. Part 1: Energy Density Distribution and Motion of Single Grinding Beads", Powder Technology, 86, cited in Dikmen, S., Ergün, Ş.L. "Stirred Ball Mills", Mining, Vol. 43, No. 4, p. 5, December 2004 (18)(15) Dikmen, S., Ergün, Ş.L. "Stirred Ball Mills", Mining, Vol. 43, No. 4, p. 6-8, December 2004 (19) Tüzün, M.A., 1994, "A Study of Comminution in a Vertical Stirred Ball Mill", Doctoral Thesis, Chemical Engineering Department, University of Natal, cited in Dikmen, S., Ergün, Ş.L. "Stirred Ball Mills", Mining, Vol. 43, No. 4, p. 11, December 2004 (20) Mankosa, M.J., Adel, G.T. and Yoon, R.H., 1986, "Effect of Media Size in Stirred Ball Mill Grinding of Coal", Powder Technology, 49, 75-82, cited in Dikmen, S., Ergün, Ş.L. "Stirred Ball Mills", Mining, Vol. 43, No. 4, p. 12, December 2004 (21) Zheng, J., Harris, C.C and Somasundaran, P., 1997, "The Effect of Additives on Stirred Media Milling of Limestone", Powder Technology, 91, 173-179, cited in Dikmen, S., Ergün, Ş.L. "Stirred Ball Mills", Mining, Vol. 43, No. 4, p. 13, December 2004 (10) Toraman, Ö.Y., Çayırlı, S., "Horizontal Stirred Ball Mill: Isamill", Niğde University Journal of Engineering Sciences, Vol. 4, No. 2, p. 100, Niğde, 2015 (11)(26) Toraman, Ö.Y., Çayırlı, S., "Horizontal Stirred Ball Mill: Isamill", Niğde University Journal of Engineering Sciences, Vol. 4, No. 2, p. 103-104, Niğde, 2015 (5)(14)(12)(24)(25)(32)(34) Hacıfazlıoglu, H., AKÜ Science Journal, No. 01, p. 17-28, Ankara, 2009 (29) Weller, K.R. and Gao, M. 2000. Ultrafine-Grinding, CSIRO Minerals and MIM Process Technologies, Australia, cited in Hacıfazlıoglu, H., AKÜ Science Journal, No. 01, p. 24, Ankara, 2009 (30) Smith, E.A., 1974. Grinding Very Hard and Very Soft Materials Processing 16, November, cited in Hacıfazlıoglu, H., AKÜ Science Journal, No. 01, p. 26, Ankara, 2009 (31) Ding, J., Tsuzuki, T. and Cormick, P.G., 1997. Mechanochemical Synthesis of Ultrafine ZrO2 Powder, Nanostructured Materials, 43, (4), 3-15, cited in Hacıfazlıoglu, H., AKÜ Science Journal, No. 01, p. 26, Ankara, 2009 (33) Hoyer, D.I., Boyes, J.M., 1994. High-Intensity Fine and Ultrafine Grinding in the Hikom Mill. In: Proceedings of the XVth CMMI Congress, vol.2. SAIMM, Johannesburg, pp. 435-441, cited in Hacıfazlıoglu, H., AKÜ Science Journal, No. 01, p. 27, Ankara, 2009 (23) Hohmann, R.H., 1977. Grinding of Wheat and Limestone With the Szego Mill, M.Sc. Thesis, Department of Chemical Engineering, University of Toronto, p. 172, cited in Hacıfazlıoglu, H., AKÜ Science Journal, No. 01, p. 28, Ankara, 2009 (27) Celep, O. Alp, İ., Türk, T. "Stirred Media Mills in Fine Grinding Technology and Their Applications in Ore Preparation", Karadeniz Technical University Mining Engineering Department, Istanbul Earth Sciences Journal, Vol. 21, No. 2, p. 64, Trabzon, 2008 (28) Gao, M. and Holmes, R., 2008, Developments in fine and ultrafine grinding Technologies, XXIII. International Mineral Processing Congress, Istanbul, 30-37, cited in Celep, O. Alp, İ., Türk, T. "Stirred Media Mills in Fine Grinding Technology and Their Applications in Ore Preparation", Karadeniz Technical University Mining Engineering Department, Istanbul Earth Sciences Journal, Vol. 21, No. 2, p. 65, Trabzon, 2008 Ecem Nur Tek Chemical Engineer Mey Kimya Sanayi ve Dış Ticaret Ltd. Şti.