Лако-красочные материалы — производство

The direct process is noted for its simplicity, low cost, and excellent thermal ef­ficiency. It consists of an initial high-temperature reduction (1000-1200 °C) of a zinc-containing material (as oxide), the reducing agent being coal. Reduction takes place according to Boudouard’s equations:

ZnO+ C ^Zn+ CO ZnO + CO ^Zn + CO2 C + O2 ^CO2 CO2 + C ^2CO

The zinc vapor and the CO gas are then oxidized to zinc oxide and carbon dioxide above the reaction bed or at the furnace exit. Various zinc-containing materials are used, e. g. zinc concentrates (mainly in China), metallization residues, skimmings from casting furnaces, oxidic residues from the indirect process and above all zinc ashes from hot dip galvanization. The ash from galvanization must first be treated to remove chloride and lead by heating at ca. 1000 °C in rotary kilns.

Only rotary kilns are now used in the EU for the direct process; the use of static furnaces has been discontinued. The zinc content of raw materials is between 60 and 75%. There are two types of rotary kiln:

1. A long (ca. 30 m), fairly narrow (2.5 m diameter) kiln, heated by gas or oil. The raw material (a mixture of zinc-containing material and coal) is charged continuously either countercurrent or cocurrent to the combustion gases. The residues, which still contain some zinc and unburnt coal, leave the furnace continuously at the end opposite to the feed end. The excess coal is sieved out and recycled. The combustion gases, containing zinc vapor, ZnO, and CO, pass into a chamber where oxidation is completed and large particles ofimpu — rities settle out. The gases are then cooled in a heat exchanger or by dilution with air. The zinc oxide is collected in bag filters.

2. A rotary kiln that is shorter (5 m) and has a larger diameter (ca. 3 m).Charging is continuous, but the dezincified residues are removed batchwise.

In both cases, operating conditions are controlled to obtain a high yield and to give the required particle shape and size. Provided no contamination is introduced, chemical purity is determined solely by the composition of the raw materials used.

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