molienda en húmedo, wet grinding

The wet grinding process occurs when a product is transformed from solid to liquid, regardless of its viscosity: paints, inks, enamels, nail varnish, among others. In this process, agglomerates that were not separated in the dispersion phase (due to the electro-physical bonding of some components) are broken by applying extra energy.

This wet grinding process uses beads or micro beads, which are accelerated inside a chamber thanks to the movement of the blades and pegs located within the chamber. The resulting turbulent and chaotic movement produces a high shearing and force power, which breaks the bonds between particles, thus reducing the size of the agglomerates and bringing the particles (pigments and fillers) to their initial size. This process achieves a perfect and homogeneous distribution of the particles in the base liquid.

O+B’s wet grinding equipment makes it possible to optimise the dispersion and grinding process according to product characteristics and the specific production type taking place in the factory. Thus, high-quality fineness, transparency, gloss and pigment development is achieved in any ink, paint, pigment, varnish and adhesives manufacturing process, among others.

What factors affect the wet grinding process?

The smallest possible agglomerate size is key in achieving a more efficient wet grinding process. Hence, the importance of a good pre-dispersion process to achieve maximum separation of these agglomerates.

In addition, other factors to take into account to achieve a better wet grinding process are:

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 micro beads.
  • The viscosity of the final paste resulting from the dispersion phase (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.

Wet grinding equipment (mills) are versatile machinery that regulates the motion and force so that it can be adapted to product characteristics and production goals.

Oliver Batlle’s grinding equipment is offered in 3 different types: immersion grinding equipment (described in a previous article), continuous grinding equipment and recirculation grinding machinery. The latter represents an evolution from continuous mills, as it features a circulation system that changes the direction of paint flow so that the product can go through the grinding chamber more times. The recirculation system prevents pressure and temperature issues, ensuring that the micro beads break the agglomerates several times.

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

Continuous 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.

The grinding chamber is also equipped with a cooling jacket, in order to control excessive heating of the product.

molienda en húmedo molino continuo

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 transmission: frequency converter;
  • mechanical seal.

continuous mill

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

Sieve

The sieve is the component that prevents the micro beads from exiting the grinding chamber and the grinding paste to pass through. The span of the sieve, i.e. its opening or micron size, depends on the size of the beads used in the process. This should be 2.5 to 3 times smaller than the size of the micro beads.

There are 2 types of sieves:

    1. Radial screen : composed of an adjustable spacing of 0.4, 0.6 and 0.8 mm. It is highly resistant to wear and the output surface is relatively small.
      tamiz radial
    2. Johnson sieve: it is a sieve that always has the same spacing. It offers a large outlet surface, and much higher flow rate than adjustable sieves. It suffers greater wear than the flap grinding disc.
      Tamiz Johnson

Blades and pegs

Both O+B’s Supermill Plus SMP and Supermill EHP wet grinding machines are equipped with blades of the same material, but different in shape. The Supermill EHP, a continuous grinding machine, has a blade with two flat faces, while the front face of the Supermill Plus SMP’s blade, a recirculating grinding machine, is fitted with a metal block designed to increase the turbulence of the beads.

blades

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

Pitones y Contrapitones

Mechanical seals

Continuous grinding equipment are watertight machines. To close the vat without stopping the shaft, a mechanical seal needs to be installed. This seal can be comprised of several parts or it can be a single part or cartridge that is directly installed.

The quality of the mechanical seal and its correct installation determine to a great extent the quality of the mill. A possible breakage would result in a coolant leakage into the vat where the product is located, causing contamination of the product.

Micro beads or beads

Another component of continuous and recirculating mills are the beads or micro beads that intervene in the wet grinding process. These grinding beads are the ones that impact and rub with the agglomerates of the paste optimising the process.

The greater the difference between the densities of the micro beads and the grinding paste, the greater the force energy. The most commonly used micro beads, in decreasing order of quality, are the following:

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 optimal wet grinding process, the size of the beads must be proportional to the size of the agglomerates. In this way, the beads will not go into the potential spaces between the large agglomerates, since they would lose the ability to impact against them. If the agglomerates were too small and the beads too large, the opposite effect would occur (the agglomerates could get into the spaces between the beads).

In this type of continuous and recirculating mills, the recommended micro bead loading in the grinding chamber is of 70 to 85% of its useful volume.

If the particle size resulting from the pre-dispersion is adjusted and homogenised as much as possible, determining the most suitable bead size for the process in question will be much more accurate.

beads

What criteria should be taken into account to choose the type of mill that will best suit a manufacturing plant?

In order to determine the type and model of the mill that is best suited to a paint and related products factory, one must bear in mind that the result of the grinding process does not only depend on the grinder chosen, but also on the pigments and fillers used.

When using improved pigments and fillers, it may be the case that only the dispersion process achieves the desired result.

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 volume limitation to process.
  • The more energy the mill can apply to the product, the better the results.
  • Automation: different levels of automation of the grinding process can be established depending on the needs of each industry.

request information mixing equipment

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

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acabado de pinturas, paint finishing

The paint finishing stage in the manufacturing process is, together with the dyeing stage, part of the final mixing phase. Both are performed just before filling.

Following the dispersion and grinding phase, discussed in a previous article, a product finishing or completion stage is required. Mixing systems are used in this new stage, by means of mixing turbines, to obtain a turbulent motion of the product. The end goal of the paint finishing stage is to achieve a homogeneous mixture from several liquids, and thus create a product that complies with the established properties that should characterise the final product.

At Oliver + Batlle, we provide mixing equipment capable of combining both the dispersion and finishing stages of paints and related products. We also have highly efficient finishing tanks recommended for mixing, dilution, addition of liquids, tinting, and completion of products without the incorporation of solids.

tanques de acabado

The mixing performed at the end of the manufacturing process includes several stages such as:

  • product finishing: mixing of the dispersed product with liquid end components by means of stirring;
  • dyeing: mixing of the finished product with dyes or colouring pastes;
  • storage: mixing of the finished product to keep it in suspension before filling.

What should be taken into account when choosing a mixer for the paint finishing stage?

When selecting the mixer model that is best suited to a particular manufacturing process, several parameters must be taken into account. These include the established manufacturing process, the volume of product being manufactured, its viscosity, and its rheology or thixotropy. This last concept includes the characteristic plasticity, elasticity, and minimum viscosity the final product must have.

Taking these parameters into account, the type of turbine, shaft speed and motor power can be defined in order to select the paint mixer that best optimises the process and perfectly matches the needs and requirements.

As for the finishing tank, it is crucial for the tank construction to be very sturdy while allowing for easy integration into an automated manufacturing system.

tanques de acabado en fabrica de pintur

What type of turbines can be used in the paint and related product finishing process?

As previously mentioned, Oliver + Batlle‘s industrial agitators perform both dispersion and mixing processes for paints and related products. As highlighted in the article on dispersion, this equipment has a series of elements in common, such as: the transmission geared motor, the shaft with its turbine, the shaft with the blade, and the frequency converter.

finishing equipment elements

During the mixing process, it is also important to take into account the type of shaft blade that best suits the end product’s characteristics.

Different types of turbines can be used, depending on the product’s final viscosity. Each type also requires a specific speed in order to achieve an optimal mixing process.

Different types of turbines for paint finishing machinery, their characteristics and the most common applications shall be dealt with next.

Sabre type turbine

turbina sabre

The sabre type turbine facilitates a smooth motion and axial flow in the paint finishing tank. This type of turbine is commonly used for finishing all paint types that incorporate synthetic resins, emulsions, solvents, or slurries.

In addition to achieving high product homogenisation, sabre turbines are very easy to clean.

Double-flow sabre type turbine

turbina sabre de doble flujo

The double-flow sabre type turbine produces a smooth motion and axial flow, downward at the centre and upward at the end.

It is usually used in the finishing stage of medium viscosity paints that incorporate synthetic resins, emulsions, solvents, or slurries.

Like the sabre type turbine, it facilitates high product homogenisation and is easy to clean.

IBC folding arm sabre turbine

turbina sabre de brazos abatibles IBC

The IBC folding arm sabre turbine produces a smooth motion and axial flow suitable for homogenising medium and low viscosity paints. It is also used for finishing low viscosity paints that incorporate synthetic resins, emulsions, and solvents.

However, it is relatively difficult to clean the IBC folding arm sabre turbine.

Inclined-blade turbine

turbina alabes inclinados

The inclined-blade turbine is suitable for finishing medium viscosity paints that incorporate synthetic resins, solvents, or slurries. It provides a smooth flow and axial movement facilitating high product homogenisation and easy cleaning.

Anchor or mini-anchor turbines

ancor turbine

Anchor and mini-anchor turbines are used in the finishing of medium viscosity paints that incorporate synthetic resins, solvents, or slurries. They are suitable for the manufacture of products that settle.

This type of turbine produces smooth motion and tangential and axial extraction flows.

They also achieve high homogenisation, although they are not so easy to clean.

ther types of paint finishing turbines

otras turbinas para el acabado de pinturas

There are other types of turbines that Oliver + Batlle also provides for the finishing of medium or high viscosity paints. These turbines produce varied flows depending on their geometry and the characteristics of the product for which they are used.

They are recommended for single product vessels, as cleaning times are longer.

At Oliver + Batlle, we provide our clients with a team of specialists who will advise them on the acquisition of disperser equipment and paint and related product finishing that best suits their production needs.

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tipos de residuos en la fabricación de pinturas

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envasado de pinturas, paint filling

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When it comes to choosing equipment for filling paint or related products, one must bear in mind that there are many models that can be adapted, to a greater or lesser extent, to the specific needs of each client and product. Read more