How to optimise the wet grinding process for paint production?

Wet Grinding Process: how it works and what industrial equipment is used

Wet grinding is an industrial process in which the movement of ceramic microbeads is used to separate solid particles incorporated in a liquid medium and, in some cases, reduce their size. The rotational and linear movement of the microbeads generates impact, friction and shear forces. Through this action, homogeneous distributions of solid particles in the base liquid are obtained in the manufacture of products such as paints, pigmented varnishes, enamels, nail varnishes, phytosanitary products, printing inks, etc.

What is wet grinding?

Wet grinding processes mixtures of solids in liquids, which are essentially fluids or may present variations in viscosity. This process is used in the manufacture of paints, pigmented varnishes, enamels, nail varnishes, phytosanitary products, printing inks, etc.

In this process, agglomerates that were not separated during the pre-dispersion stage (due to electro-physical bonds between certain components) are broken by applying additional energy generated by the movement of ceramic microbeads.

For this wet grinding process, beads or microbeads are used. They are accelerated inside a chamber thanks to the movement of rotating discs or pegs located inside it. The resulting turbulent and chaotic movement produces high shear and impact forces, breaking the bonds between particles, reducing the size of the agglomerates and returning the particles (pigments and fillers) to their initial size. This process achieves a perfect and homogeneous distribution of the particles in the base liquid.

With O+B wet grinding equipment, it is possible to optimise the dispersion and grinding process according to the characteristics of the product to be manufactured and the type of production developed in the factory. In this way, high-quality fineness, transparency, gloss and pigment development can be achieved in manufacturing processes.

What factors affect the wet grinding process?

To make the wet grinding process more efficient, it is important to achieve the smallest possible agglomerate size. Hence the importance of a good pre-dispersion process to separate these agglomerates as much as possible.

Other factors to be considered to achieve a better wet grinding process include:

During pre-dispersion

• The disperser shaft’s speed (between 24 and 28 metres per second).
• The diameter ratio between the vessel and the blades.
• The fineness or size of the agglomerates obtained in the pre-dispersion phase, before the grinding process
• The presence of additives that help during pre-dispersion.

During the grinding phase

• The size of the microbeads.
• The viscosity of the initial liquid resulting from the pre-dispersion process (recommended between 25 and 45 Poises).
• The maximum temperature of the paste at the end of the grinding process, since temperature increases during the wet grinding process.

What types of mills are used in wet grinding equipment and how di they differ?

Wet grinding equipment (mills) are versatile machines that allow regulation of the movement and the forces generated, adapting them to product characteristics and production goals.

Oliver Batlle’s grinding equipment [https://oliverbatlle.com/en/grinding-machines/] is offered in 3 different types: immersion grinding equipment [https://oliverbatlle.com/en/insights-on-basket-milling/](also described in a previous article), continuous grinding equipment and recirculation grinding machines. The latter represents an evolution from continuous mills, as they include a set of mechanisms that increase the product flow inside the grinding chamber and allow the product to pass through it several times. 

Since immersion mills, also known as ‘batch mills’, have already been discussed, we will now analyse continuous mills, such as the Supermill EHP by O+B,  and recirculation mills, such as the Supermill Plus SMP.

Continuous Mills and Recirculation mills

When the wet grinding process is carried out with continuous equipment, the grinding paste is forced through a closed chamber by a pump. This chamber contains the micro beads, the shaft with the blades that move the beads, and a sieve that keeps the beads or micro beads inside the chamber, letting the grinding paste out of the chamber once it has gone through the mill chamber.

In the wet grinding process using continuous equipment, the grinding base or paste is forced to pass through a closed chamber by means of a pump. Inside this chamber are the microbeads, the shaft with blades that move the microbeads, and a separator that retains the beads or microbeads inside the chamber while allowing the grinding base to exit once it has passed through the mill chamber.

In order to control excessive heating of the product, the grinding chamber is equipped with a cooling jacket through which cooling water can circulate at the lowest possible temperature. The use of cooling water between 2 and 8 degrees Celsius is recommended.

In summary, the basic components of continuous mills are:

• cooled grinding chamber, which can be horizontal or vertical;
• shaft with propellants: blades and pegs;
• sieve;
• motor and variable speed transmission by means of frequency converter;
• mechanical seal.

What are the basic components of continuous mills used in wet grinding?

Let us take a closer look at the different components of this type of wet grinding equipment.

Screen or separator

The screen is the component that prevents microbeads from leaving the grinding chamber while allowing the grinding base to pass through. The opening of the screen depends on the size of the beads used in the process. It must be between 2.5 and 3 times smaller than the diameter of the microbeads.

There are 2 types of sieves:

1. 1. Radial screen : composed of an adjustable spacing of 0.4, 0.6 and 0.8 mm.  It offers high wear resistance and has relatively small outlet surface.
      
   2. Johnson screen: A screen with a fixed opening. It offers a large outlet surface, allowing a much higher flow rate than adjustable radial screens. Its wear is greater than that of the radial screen.
      

Blades and pegs

Both O+B’s Supermill Plus SMP and Supermill EHP wet grinding machines are fitted  with blades of the same material, but with different shapes. In the Supermill EHP, a continuous grinding machine, both faces of the blade are flat. In the Supermill Plus SMP, a recirculation grinding machine, the front face of the disc is fitted with a metal insert designed to increase bead turbulence.

Pegs and counterpegs are used to generate turbulences in immersion mills such as the Mill-ennium RS.

Mechanical seal

Continuous grinding equipment are airtight machines. To close the tank while allowing the shaft to rotate, a mechanical seal must be installed. This seal may consist of several parts or of a single cartridge unit that is directly installed.

The quality of the mechanical seal and its correct installation largely determine the quality of the mill. A possible failure could lead to leakage of cooling liquid into the vessel containing the product, causing contamination.

Micro beads or beads

Another component of continuous and recirculation mills are the grinding beads or microbeads used in the wet grinding process. These microbeads impact and rub against the agglomerates in the paste, providing the energy applied in the process.

The greater the difference between the densities of the microbeads and the grinding base, the greater the impact energy generated. In decreasing order of quality, the most commonly used microbeads are:

	Specific density	Apparent density	Ø mm Available	Application in O+B equipment
Yttria-stabilised Zirconia oxide	6.0 kg / l	3.6 kg / l	0.7-0.9 / 1.2-1.4 / 1.6-1.8	Supermill Plus SMP Supermill EHP
Zirconia Silicate	3.8 kg / l	2.3 kg / l	1.4-1.6 / 1.8-2.0	Supermill Plus SMP Supermill EHP

To achieve an optimal wet grinding process, the bead size must be proportional to the size of the agglomerates. The beads must not enter the spaces between large agglomerates, as they would lose their impact capacity. Conversely, if the agglomerates are very small and the beads very large, the opposite effect would occur.

In continuous and recirculation mills, the recommended microbead load in the grinding chamber is between 70% and 85% of its useful volume.

If the particle size resulting from pre-dispersion is standardised and homogenised as much as possible, the wet grinding process will be facilitated. 

With good pre-dispersion, it is much easier to determine the most suitable bead size and quantity for the specific process.

What criteria should be considered when choosing the type of mill best suited to a manufacturing plant?

To determine the type and model of mill that best suits a paint and related products factory, it must be taken into account that the result of the grinding process does not depend solely on the selected mill, but also on the pigments and fillers used.

When high-quality pigments and fillers are used, it may be possible to achieve the desired result with the dispersion process alone.

How to choose the right grinding and dispersion equipment according to production needs?

Oliver + Batlle provides our clients with a team of technicians and consultants who recommend the option that best suits each project when choosing a grinding and dispersion equipment.

Some aspects to be considered beforehand could be:

• Continuous mills have no limitation on the volume to be processed.
• The more energy the mill can apply to the product, the better the results obtained.
• Automation: different levels of automation of the grinding process can be established depending on the needs of each industry.

Why is it advisable to seek specialised  technical advice when selecting wet grinding equipment?

Oliver + Batlle invites you to contact our specialists and ask for further information about our wet grinding equipment.

Share any doubts or experiences in the comments section and follow us on LinkedIn to keep up with our news.