Talc is one of the high-quality non-metallic minerals in China, and white talc, in particular, is very famous in the world. Due to the influence of resource distribution and price, it has a limited application in papermaking coating pigment. The special rheological properties of talc make the use of paint and coating talc more difficult. Thus, the development of the paint and coating talc suitable for domestic coated paper means a lot for the talc production enterprises and coated paper mills in China.
Talc can be used as a special functional pigment in the production of coated paper since it improves the quality of coated paper, especially improving the printing performance and printing operation performance. For talc product manufacturers, by selecting a suitable type of talc with proper grinding process and equipment, and with the addition of the new efficient and economical dispersants and wetting agents, it is possible to produce the talc powder product, of which the particle size distribution and viscosity meet the requirements of the papermaking coating pigment.
1. The selection of talc
There are different types of talc, and its different mineral compositions and crystal morphologies lead to different physical and chemical properties, which greatly affect talc’s application. These properties can’t be changed by mineral processing. So, it is an important procedure to select the proper talc mineral in the production of talc coating pigment.
The talc as a coating pigment in the European paper industry is mainly chlorite and the talc-associated mineral. Since the properties of chlorite are similar to pure talc, the talc for papermaking pigment is taken as pure talc in Europe. High calcium talc, different from other talcs due to its high whiteness and good dispersion rheological properties, has different effects on the performances of papermaking coating and coated paper. Talc with high impurities such as quartz is very abrasive, which limits its application in high-speed paper machines and coaters. So, it is essential to analyze and evaluate the physical and chemical properties of talc products before the selection of pigment to determine the proper type of talc suitable for papermaking.
2. Superfine grinding of talc
How to effectively solve the problem of dry peeling and control the particle size distribution and particle crystal morphology of talc powder stably and accurately is an important step in the production of talc products for paper coating.
Normally, talc products for papermaking coatings are produced by dry superfine processing. By beneficiation, primary crushing, superfine grinding, classification, and other processes, the average particle size of talc powder product can reach D50 of 2~3μm, and it is possible for the product to be mixed, packaged, and delivered as per customers’ needs. It can also be mixed at the site of use.
For talc ore with poor quality or low talc content, wet grinding and flotation are sometimes used for beneficiation and processing.
Dry grinding combined with wet grinding is used for producing a superfine talc coating, of which 90 percent or more of the particles measure 2μm. That is, after dry grinding, D60~D70 of the talc power can reach 2μm, and then wet superfine grinding is processed to the required particle size with the addition of wetting agents and grinding aids.
Please be noted that due to the special properties of talc such as hydrophobicity, the solid content of talc slurry changes greatly during wet superfine grinding, and the deterioration of rheological properties may be more serious, which will lead to high energy consumption and production cost.
3. Compound modification and granulation of talc powder
The low bulk density of talc powder after superfine grinding will increase the cost of packaging and transportation. Talc’s special hydrophobic property makes it have poor operating performance in dispersing in coated paper mills, and the generated dust will pollute the working environment.
In foreign countries, especially in Europe, compression and granulation are usually used to solve these problems, and also there is special production equipment. Granular talc products for paper coating can be produced by compression, granulation, drying, and other processes. The modified granulated talc product has the advantages of low transportation cost and good operation dispersion performance.