The grinding-aids effect of moisture, triethanolamine (TEA) and ethylene glycol (EG) on grinding performance and product quality of calcite
Abstract
In this study, the effects of liquid additives such as water, triethanolamine (TEA) and ethylene glycol (EG) on the dry fine grinding of calcite were investigated by using a laboratory scale vertical stirred mill. The experiments were carried out by a batch operation, and the change in average particle size (D50) and specific surface area (SSA) (m2 /g) and colour properties (L*, a*, b*, ∆E, WI) of product. The results show that the chemical additives and water promotes the fine grinding of calcite and that the maximum specific surface area of calcite (2.97 m2 /g) obtained with EG additives 0.5%. The average particle size at this point was 3.16 µm. The results also show that the lightness (L) values of the ground calcite products slightly increased from 97.62 to 98.53 with grinding aids (TEA) increased from 0% to 0.5%. The whiteness index (WI) value of the ground calcite products slightly increased from 97.41 to 98.25 with grinding aids (TEA).
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Introduction
Grinding is an important industrial operation that is used for the size reduction of materials, production of large surface area and/or liberation of valuable minerals from their matrices. In addition to mineral processing, it is widely used in the manufacture of cement, pigments, paints, ceramics and pharmaceuticals [1]. However, it is known that it is also one of the unit operation with the lowest energy efficiency [2]. The need for fine particles has increased in the field of preparing raw powders and high value added products in many industries such as mineral, ceramic, pigments, paint and pharmaceutical, therefore, the research in fine grinding has gained more importance. Stirred media mill has been used in recent years for grinding particles to micron and submicron sizes due to their easy operation, simpler construction, higher grinding rate and lower energy consumption compared with other fine grinding machines [3]. On the other hand, it is known that particle aggregation/agglomeration causes poor flowability of dry material to be ground in a mill. Also, grinding media and liner coating results in a poor dry grinding efficiency due to the cushioning effect [4]. Grinding aids (GAs) which can improve the efficiency of the grinding remarkably with a small amount addition should more positively be applied to the grinding operations, especially to dry ultrafine grinding with higher energy consumption [2]. Moreover, the penetration of GAs into a crack within a particle could promote propagation or the crack, resulting in easier breakage of the particle [3]. They are mostly organic compounds and commonly consist of glycols and amines. The high polarity in their chemical functioning of – OH, -NH2, causes the tendency to resist the agglomeration [5].
In most of the studies on GAs, the effect of moisture [6] and grinding aids [7-11] have been discussed to get the fine powders in wet grinding method for calcite/limestone, but there are only a few reports in dry conditions for calcite/limestone [2,12,13]. Moreover, in these studies, product analyses consist of surface area, fineness, particle size distribution, crystalline structure and specific energy consumption. Neverthless, effects of GAs on colour properties of ground products were not found in the literature.
The purpose of this study was to investigate the effects of moisture and chemical additives such as triethanolamine (TEA) and ethylene glycol (EG) on grinding performance (fineness and surface area) and product quality (colour properties) during dry fine grinding as grinding aids of calcite using a laboratory stirred bead mill.
Conclusion
Experimental studies on grinding performance using calcite in a stirred bead mill have been carried out. The effects of grinding aid on the particle size and size distribution (PSD) and surface area (SSA) area were examined. The followings were found out:
1. The median diameter (D50) at the TEA and EG amount of 0.5% was about 0.35 times as small as the particle size without an additive, and about 9 times smaller than that before grinding.
2. The use of 0.5% weight of grinding aid in the mill indicated enough beneficial effect on product size, surface area and colour.
3. The lightness (L) values of the ground calcite products slightly increased from 97.62 to 98.53 and 98.39 with grinding aids TEA and EG, respectively.
4. The total colour difference (∆E) increases with increasing grinding aid, apparently reaching a maximum at 4.49 for TEA at a rate of 0.5%. This means that there is a high colour difference between the feed and micronized calcite products with grinding aid, which is considered to be a natural result of the grinding process and depends on the chemical additive type.
5. The whiteness index (WI) value of the ground calcite products which is one of the colour properties slightly increased from 97.41 to 98.25 with grinding aids (TEA) at a rate of 0.5%.