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The use of grinding aids in the rotary ball mill

Worldwide cement production currently accounts for approximately 1.6 billion tons/year and the grinding process consumes almost 2% of the electricity produced worldwide. The clinker grinding stage consumes approximately one third of the energy needed to produce 1 ton of cement. This refers to an average specific energy consumption of 57 kWh/t and a specific carbon dioxide emission intensity for electricity generation of 9.1 kg CO2 per ton. Furthermore, approximately 60-70% of the total electrical energy used in a cement plant is used for the grinding of raw materials, coal, and clinker. As a result, a small gain in grinding efficiency can have not only a large impact on the operating cost of a plant, but also a reduction in CO2 emission.

Approximately 95% of the feed to the cement grinding circuit is clinker and the remainder is “additives” including grinding aids (GA). Cement quality is measured by surface area (Blaine index). It should be noted that the surface area of ​​cement powder depends on the size distribution of cement particles (smaller particles have a larger surface area).

The efficiency of most rotary ball mills is very low. Cement particles can coat the grinding media, can seal the shielding, and can agglomerate and form small impact-absorbing plates. The action of the grinding media inside a rotary ball mill not only crushes the existing clinker particles, but also compresses them, leading to the formation of electrostatic surface charges of opposite polarity. The cement particles then agglomerate because of the attraction forces acting on them. Consequently, the agglomeration of cement particles reduces the efficiency of the ball mill. This phenomenon is characterized by an increase in energy consumption while keeping the Blaine constant. The degree of agglomeration depends on:

  • The specific characteristics of the materials to be ground
  • The operating parameters of the ball mill
  • The efficiency and distribution of the grinding media
  • The fineness of the cement particles
  • The internal operating conditions of the ball mill.

The phenomenon of agglomeration remains one of the priorities of cement manufacturers, hence the importance of grinding aids. The latter make it possible to partially neutralize the surface charges that have developed during grinding. Additives such as water, organic liquids and some inorganic electrolytes are used to reduce the surface free energy of the material to be ground to improve grinding efficiency. Although the main use of grinding aids is to reduce the agglomeration of cement particles, they are use also to help completely or partially eliminate the “coating” effect on the media, improve separator efficiency, decrease packing problems in storage silos and bulk delivery trucks, improve cement quality and material handling, and increase production capacity.

What makes the application of grinding aids even more desirable are their significant effects on the mechanical properties of cement, whose particle size distribution becomes narrower and shifted towards shorter diameters. The greater the surface area of ​​the hydraulically active components and therefore the greater the Blaine fineness, the faster the cement hardens. However, the Blaine value only gives an indication and not an absolute value, as it does not adequately reflect the fine fraction, which is an important parameter for the grinding process and for the properties of the binder produced. When grinding cement clinker using grinding aids, a narrower particle size range is generated, as the percentage of very fine particles, which only influence the setting time, is reduced. Therefore, the strengths at equal Blaine values ​​are higher than when grinding without grinding aids. In closed-circuit grinding plants it was also observed that cements ground with heavy circulating loads often contain lower amounts of ultrafine and coarse particles. To a certain extent, the aids force the rotary ball mill in the clinker process to work with a higher circulating load.

In the grinding process, a variety of grinding aids are used. There are aliphatic amines such as triethylenetetramine (TETA), tetraethylenepentamine (TEPA) and amino alcohols such as diethanolamine (DEA), triethanolamine (TEA) and triisopropanolamine (TIPA). Glycol compounds are represented as ethylene glycol (EG), diethylene glycol (DEG). In addition, there are more complex compounds such as aminoethyl ethanolamine (AEEA) and hydroxyethyl diethylenetriamine (HEDETA). Phenol and phenol derivatives are also used as grinding aids, while other compounds such as amine acetate, higher polyamines and their hydroxyethyl derivatives are used, but these are not defined in the data sheets for grinding aids.

In general, the concentration range of added grinding aids is 50 to 500 ppm. After the grinding process, the additives may no longer be in their original chemical form. In addition, the composition of the grinding aid may not consist of mixtures of pure compounds, but of more complex raw materials.

Source: https://www.sciencedirect.com/