What kind of dispersing agent is used for wet grinding of calcium carbonate?

At present, large-scale papermaking calcium carbonate production enterprises at home and abroad mostly use wet grinding to prepare heavy calcium carbonate, but high concentration of calcium carbonate is very easy to flocculate, especially when the particle size reaches submicron level, the surface of calcium carbonate particles will be produced. At many active sites, the surface adsorption energy increases, the attraction between calcium carbonate particles increases sharply, and flocculation is very likely to occur. Whether or not a stable suspension can form an important indicator of grinding calcium carbonate emulsion is therefore The choice and use of calcium dispersants is critical.

▲ papermaking calcium carbonate wet grinding site

From a macro perspective, the role of calcium carbonate grinding and dispersing agents mainly has the following three aspects:

(1) improving the rheological properties of the calcium carbonate suspension, reducing the viscosity of the slurry , and improving the dispersion stability of the calcium carbonate particles;

(2) Increasing the impact of the grinding medium on the calcium carbonate particles and improving the grinding effect, improving the grinding efficiency and reducing the grinding cost;

(3) Improve the stability of the finished slurry and prevent the flocculation of calcium carbonate particles.

There are three main types of dispersants commonly used in calcium carbonate grinding:

• Inorganic, such as sodium hexametaphosphate, sodium disilicate, etc.;

• Organic substances such as alkylbenzene sulfonates;

• Polymers such as polycarboxylates, polyacrylic acid derivatives, and the like.

At present, polymer dispersants are commonly used in ultrafine grinding of calcium carbonate.

▲calcium carbonate slurry

(1) Inorganic dispersants

The mechanism of action of the inorganic dispersant is to increase the electrostatic repulsion of the surface by increasing the charge on the surface of the calcium carbonate particles, so that the particles are difficult to further gather and flocculate during the thermal motion and the Brownian motion. Part of the inorganic dispersant also enhances the degree of wetting of the surface of the particles.

There are mainly polyphosphates (such as sodium hexametaphosphate), silicates, carbonates, etc., among which sodium hexametaphosphate is most commonly used. However, it should be noted that inorganic dispersants have an adverse effect on electrical conductivity and dielectric constant, and thus the application of inorganic dispersants is extremely limited, especially in ultrafine grinding.

(2) Organic small molecule dispersant

Organic small molecule dispersants are mainly various surfactants and can be classified into three types: anionic, nonionic and cationic.

The ionic dispersant mainly acts on the electrostatic repulsion of the surface charge. The nonionic dispersant mainly improves the stability of the suspension by steric hindrance. In addition, such dispersant also has a very good effect on improving the surface wetting property of calcium carbonate particles. Great role.

Commonly used organic small molecule dispersants are mainly alkyl polyether or alkylaryl sulfate, alkyl pyridinium chloride, alkylphenol polyvinyl ether and the like.

(3) Polymer dispersant

The relative molecular mass of polymeric dispersants is generally between 1000 and 100,000, and the functional groups generally comprise two parts:

A part of the anchoring group is mainly adsorbed or bound to the surface of the particle by ionic bond, covalent bond, hydrogen bond and van der Waals force in the form of single or multi-point anchoring. The common anchoring group is -NR 3+ , -R 2 , -COOH, -COO - , -SO 3 H, -SO ₃ ^- , -PO ₄ ^2-, polyamines, polyols and polyethers;

The other part is a polymer chain that can be solvated by a dispersion medium , used to construct sterically hindered groups, to generate steric hindrance, to prevent flocculation of calcium carbonate particles, and a common sterically hindered group to construct a polymer chain having a polyester, Polyether, polyolefin, and polyacrylate.

The polymer dispersing agent for calcium carbonate grinding and dispersing mainly includes polycarboxylate, polyacrylic acid derivative, maleic anhydride copolymer, condensed naphthalenesulfonate and nonionic water-soluble polymer (polyvinylpyrrolidone, poly Ether derivatives and the like, among which the most used are sodium polyacrylate dispersants having a molecular weight of 2000-3000.

Polymer dispersants have become the mainstream of ultra-fine calcium carbonate grinding and dispersing agents, with the following advantages:

• In the production process, it is easy to effectively control the relative molecular mass. The dispersing agent with high relative molecular mass has an adsorption film thickness of tens of nanometers on the surface of calcium carbonate, and the dispersion stability range can be quite long when the calcium carbonate particles are far apart. That is, the action occurs, so that better dispersion stability can be obtained;

• The dispersing ability is strong and stable, and it is not sensitive to ions, pH and temperature in the dispersion system, and the stability of the action is good;

• It can significantly reduce the viscosity of the dispersion system, improve its rheology, and save grinding energy.

Although the advantages of polymer dispersants are obvious, there are still some disadvantages:

• The color generally appears yellow or the color becomes dark after the modified particles are sintered;

• Dispersion and flocculation can be mutually converted. If the molecular chain of the polymer is too long, it will cause desorption or folding of the surface of the calcium carbonate particles, causing entanglement between the particles to form flocculation. Therefore, the relative molecular mass of the polymer must be controlled within the optimum range.

The most widely used polyacrylate dispersant in calcium carbonate grinding, but the current domestic products are only suitable for grinding grade 65 calcium carbonate. When used in the grinding of 98 grade calcium carbonate, there will be larger particle size, more dosage, and higher calcium viscosity. High and sticky back problems.

At present, the polymer dispersants such as NBZ-3, DA-50, WH-1 and PD-5 developed in China are not ideal, and there is still a big gap compared with similar foreign products .

Therefore, improving the synthesis process of the dispersant or effectively combining the dispersing agents of different action mechanisms, the advantages are complementary, and the high-quality dispersant with better performance and easier adjustment and control is inevitably one of the main research directions in the future.