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Ball distribution in vertical agitator ball mills at low (top) or high flow rate. (Image credit: Netzsch) Ball distribution in vertical agitator ball mills at low (top) or high flow rate. (Image credit: Netzsch)
Ball distribution in horizontal agitator ball mill. Ball distribution in horizontal agitator ball mill.
Due to the special design of MasterNibs beater blade mills, the beater blades are easily and quickly to exchange. Due to the special design of MasterNibs beater blade mills, the beater blades are easily and quickly to exchange.
Wear at the front side (left) of a beater blade in a beater blade mill. The right edge has still the initial shape. Wear at the front side (left) of a beater blade in a beater blade mill. The right edge has still the initial shape.
In the horizontal grinding chamber of MasterRefiner agitator ball mills grinding balls and inner tank are wearing out to a lesser extent compared to vertical mills. In the horizontal grinding chamber of
MasterRefiner agitator ball mills grinding balls and inner tank are wearing out to a lesser extent compared to vertical mills.
The iron content increases exponentially with rising energy consumption. The iron content increases exponentially
with rising energy consumption.
01/07/2017 | Technology
NETZSCH-Feinmahltechnik GmbH

Less metal transfer during cocoa grinding process

Cocoa beans, nibs and powder are abrasive products. Therefore, a considerable amount of material from the grinding tools is introduced into the product during refining. By means of the measures described below, less iron is introduced, and the critical introduction of tungsten and cobalt is entirely omitted.

By Dr.-Ing. Gertrud Bauermeister,
Application manager Cocoa, Netzsch Feinmahltechnik GmbH

Cocoa is ground in two stages. The first stage uses state-of-the-art beater blade mills for the grinding of cocoa nibs into cocoa liquor. These are impact mills with fast rotating rotors and slotted screens that act as a grinding track. Only particles, which are smaller than the gap size can leave the grinding chamber and are pumped out via the ring channel. During the grinding of cocoa nibs, the originally rectangle beater blades with a 26 mm thickness are worn round on the leading edge after about 200 hours. All of this worn material is transferred into the cocoa liquor.
In the second stage, the pre-ground cocoa liquor is finely ground. This is carried out in agitator ball mills. Usually mills which are equipped with vertical grinding vessels are being used. Depending on the product flow and in combination with gravity, the grinding balls will compact in the bottom zone of the grinding vessel. Friction between the balls, the grinding arms, and the grinding tank, generates significant wear. This leads to further metal contamination in the cocoa liquor, and consequently, into the chocolate.
Various chocolates between 43 and 73 percent cocoa content were analyzed and a considerable amount of contaminants were detected. Are these amounts in conformity with the regulations? According to article 3 in regulation 1935/2004 material and articles intended to come into contact with food shall be manufactured in compliance with good manufacturing practice so that they do not transfer their constituents to food in quantities which could endanger human health.
For the determination of which quantities could endanger human health, the EDQM (European Directorate for the Quality of Medicines & Health Care) has published several limit values – in mg per kg of food – of various metals and alloy constituents. For iron the Specific Release Limit is 40 mg, for chromium 0.25 mg and for cobalt 0.02 mg. For tungsten, no limit values are currently published.
The quantities detected in the chocolates exceeded the limit values (SRL – Specific Release Limits) significantly. The values for iron were between 51 mg and 160 mg, for chromium 0.6 mg through 3.35 mg, for cobalt 0.16 mg through 0.91 mg and for tungsten 0.14 mg through 1.64 mg. The exceeding factor of 3.5 to 4 for iron is still in the lower range. The limit value for cobalt is exceeded by a factor between 9 and 45 and is the most significant.
The metal content in the final product is related to wear. By using the example of iron content, the correlation of wear and energy consumption is evident. During grinding the iron content increases drastically as energy consumption rises. There are several possibilities to reduce the energy consumption. In the first grinding step, pre-grinding, the gap size of the screen should be as fine as possible. The finer the gap size during pre-grinding, the lower the total energy consumption. The energy savings rate is between 10 and 15 percent. In the second grinding step, the fine grinding, the energy consumption in a Netzsch MasterRefiner horizontal agitator ball mill is significantly lower compared to vertical ball mills. The energy savings is about 40 percent. In contradiction to vertical ball mils, there is no compaction of the grinding balls, which reduces wear considerably.

Finer screens decrease energy consumption

In addition, to reduce the transfer of iron, tungsten and cobalt Netzsch has made certain design considerations. The special design of the MasterNibs beater blade mills allows an easy and fast exchange of the beater blades. Therefore, it is not necessary to use hard metal blades, preventing the transfer of tungsten and cobalt into the cocoa liquor. The use of screens with gap sizes of 100 µm or 150 µm saves between 10 and 15 percent energy consumption, which correlates to lower iron contamination. In MasterRefiner agitator ball mills with horizontal grinding chambers the grinding balls, grinding arms and the inner tanks show significantly less wear compared to vertical ball mills. Consequently 38 percent less iron is transferred into the cocoa liquor.    •