Zinc

ZINC

Zinc (chemical symbol Zn) is a bluish white metal that is brittle and has low strength. It is often referred to as a white metal and is widely used as a protective coating over steel.

Properties

Zinc has low strength and is brittle. Although it can be damaged by alkalis and acids, it resists corrosion by water and forms a protective oxide when exposed to air. It is also subject to creep. The tensile strength can be greatly increased by cold-working and alloying.

Working Characteristics

Since zinc is a soft material, it can be hot- and cold- rolled, drawn, extruded, cast and machined. It can be joined by welding, soldering and various mechanical fasteners.

Production of Zinc

Zinc is extracted from zinc blende (sphalerite) ore. The mined ore is crushed and ground and the ore particles are separated from the rock. Element such as copper, lead and iron sulfides tend to be in the ore and are separated by a flotation process. This involves separating the element in the finely grained ore by floating them on a liquid. The floating capacity of the various elements varies. The lead and copper sulfates will float off the top of the liquid. Chemicals are added to cause the zinc sulfide to float and be taken off. This concentrated zinc material is dried and ready to be refined into the metal zinc.

The electrolytis process involves roasting the zinc concentrate and removing soluable parts whit a weak sulfuric acid solution. The solution is filtered to remove some of the other metals. Finally, this solution is moved to electrolytic tanks where cathodes of pure aluminum and anodes of lead or lead-silver are lowered into the tank and electric current is passed between them through the solution. Pure zinc is attracted to and plates the cathodes. The layer is removed, melted and poured in slabs for processing into various applications.

The vertical furnace method involves mixing coking coal briquettes and the dried zinc concentrate into the top of a vertical furnace where they are heated until the zinc concentrate vaporizes. The vapor is removed and into a solid. The solid zinc slabs are ready for use in producing various products.

Uses of Zinc

The major use for zinc is to form a protective coating over steel. This is referred to as galvanizing. Galvanizing involves placing the steel to be coated into a bath of molten zinc, which bonds to the surface. It is important that the coating be free of imperfections such as pinholes that would permit moisture to reach the steel and cause it to rust. Both galvanized sheet and strip material are available.

Since zinc and zinc alloys have low melting temperatures, they are easy to cast and used for some types of hardware and plumbing items. They are usually die cast and finished by polishing or plating with chromium, brass or other materials.

Zinc also finds use as an alloying element in brasses. Various zinc compounds find use in the production of paper, plastics, ceramics, rubber, abrasives, paint and other products. Zinc is also used for specialized products in which corrosion resistance is important, such as anchors, flashing, screws, nails, expansion joints, and corner beads. Solid zinc strip material used to produce a wide range of products, such as low-voltage buss bars, cavity wall ties, electric cable binders, electric motor covers, grading screens, and roofing and fascia material.

Zinc is high on the galvanic table of electrolytic potentials. This means it can be used to coat a material lower on the table to protect the material if galvanic action does occur. The zinc will be sacrificed, thus protecting the coated metal.

Zinc Alloys

Zinc alloys used for die casting consist of about 95 percent zinc and 4 percent aluminum and magnesium. Some copper may be present.