Лако-красочные материалы — производство

The zinc is boiled, and the resulting vapor is oxidized by combustion in air under defined conditions. The crystallographic and physical properties of the ZnO can be controlled by adjustment of the combustion conditions (e. g., flame turbulence and air excess). The chemical composition of the ZnO is solely a function of the composition of the zinc vapor.

Many types of furnace are available to produce vapor of the required purity from various raw materials and obtain a high yield of zinc. Pure zinc (super high grade, SHG; high grade, HG) or, to an increasing extent, metal residues (e. g., scrap zinc, die casting dross, or galvanizer’s top or bottom dross) are used as raw materials. Various liquid — or vapor-phase separation techniques are used for separating Cd, Pb, Fe, and Al from zinc metal before it is oxidized:

1. Muffle Furnaces or Retorts of Graphite or Silicon Carbide. The metal is fed into the furnace either batchwise as a solid or continuously as a liquid. The heat of vaporization is supplied by heating the outside of the retort with a burner. In a muffle furnace the vaporizing section is separated by a silicon carbide arch from the heating chamber. The heat from burning gas or oil in the heating chamber is transferred to the zinc bath from the arch by radiation.

The nonvolatile residues (iron, lead and aluminum in the case of dross from smelting and casting) accumulate in the retort or in the muffle furnace and must be removed at intervals. This is facilitated by tipping the retorts.

2. Fractional Distillation. The vapor, containing Cd, Pb, Fe, Al, and Cu, can be purified by fractional distillation in columns (New Jersey Zinc Co.) with silicon carbide plates. Oxidation takes place at the exit of the column.

3. Furnaces with Two Separate Chambers. The metallic raw material, which can be in large pieces, is fed into the first chamber where it melts. This is connected to the second, electrically heated chamber where distillation takes place in the

absence of air. The first version of this type of furnace was constructed by Lundevall [2.81].

The nonmetallic residues are removed at the surface of the melting chamber. Impurities, such as Fe, Al, and some of the Pb, accumulate in the distillation chamber and are periodically removed in the liquid state. The last traces of lead are then removed by fractional distillation.

4. Smelting Process in a Rotary Kiln. Indirect zinc oxide is also made by smelting in a rotary kiln, starting from the same raw materials. Melting, distillation, and part of the oxidation all take place in the same zone, allowing utilization of a large part of the heat of combustion of the zinc. By controlling the temperature and partial pressures of carbon dioxide and oxygen, the impurity content (e. g. Pb) can be controlled and the shape and size of the ZnO particles can be adjusted.

2.3.3.4