Zinc Oxide Grades

Introduction
There is a very wide range of commercial grades of zinc oxide. In addition to the different processes there are a number of other factors such as purity, surface area, coatings and individual customer requirements. Another factor is nomenclature, many zinc oxide companies have their own system. The seal system is still used, however the specifications between suppliers, for red seal for example, are not necessarily the same.

French Process

Purity – The purity of a zinc oxide product is determined by a combination of raw material and also the degree of purification that is achieved within the furnace. The French Process achieves a significant purification due to the vapor pressures exerted by impurity metals. Of the metals found in primary and secondary zinc, only a few have significant vapor pressures at the boiling point of zinc. Cadmium has a lower boiling point, essentially all cadmium present in the zinc metal reports to the product. Lead has a low but significant vapor pressure. As the furnace is charged with new metal, over time the lead builds up to an equilibrium level where the lead input and outputs are equal. Other elements in secondary zinc such as iron and aluminum have too low vapor pressure to report to the zinc oxide. Other elements that will report at some level in the zinc oxide are:

• Bismuth – when used as an alternate to lead it would be a component of bottom dross.
   
• Magnesium – due to the high vapor pressure, magnesium from die cast derived raw material will be present in the product.
   
• Manganese – although manganese is present in secondary zinc due to dissolution of the steel in galvanizing, it has a very low vapor pressure. Nevertheless, two of the major uses of zinc, rubber and ceramics, have very tight manganese specifications.

Larvik electric furnaces can make relatively pure zinc oxide from secondaries, provided that the cadmium input is very low. If cadmium content is significant, high product purity can be achieved by the use of distillation. Cadmium and lead distillation columns used in series allow the same zinc oxide purity as product made from SHG zinc.

The use of SHG zinc as a raw material essentially guarantees that the zinc oxide product will meet all major uses. The level of lead is being reduced in general and continuous galvanizing, however, the lead analysis of secondaries such as top dross and bottom dross vary considerably between facilities.

There is a very wide range of specifications for different end uses and geographical locations, how these are met depends on the practices of the individual zinc oxide manufacturer.

Surface Area
The surface area of commercial zinc oxide can be controlled between the limits of around 2.5 m2g-1 to 12 m2g-1, depending on the furnace configuration. Surface area can be measured relatively easily by BET absorption or air permeability. Scanning electron microscopy and surface area measurement  together are an excellent way of characterizing the physical characteristics.
5 m2g-1 has been the standard surface area level for many years. This may have been due to the fact that it corresponds to the particle size for maximum opacity and hence preferred by the Paint Industry. The range of 4 to 6 m2g-1 has become the standard for the rubber industry. Higher surface area zinc oxides are preferred for some applications where transparency rather than opacity is required. The higher surface area zinc oxide is also more reactive.

Coatings
Some rubber specifications call for coated zinc oxide. Propionic acid is most commonly used. A propionic level of around 0.3% is sufficient to give a hydrocarbon chain coating which is compatible with rubber compound and hence is more easily dispersed.

Pelletized Zinc Oxide
One of the best known characteristics of zinc oxide is its extreme stickiness. It is not an easy material to transfer in handling systems. Aggressive handling can also lead to problems with dust. Accordingly, French Process zinc oxide can readily be pelleted. For most applications in rubber and also the Chemical Industry, the pellets are just hard enough for material handling but soft enough to be crushed in the processing. The Ceramic Industry prefers very hard pellets.

Grades
The permutations of the above properties combine to give many grades. Given the number of suppliers and also individual customer requirements, there must be a great deal of overlap. The French Process Zinc Oxide Industry could consider a unified grading system.

Direct Process
Direct Process now accounts for a much lower proportion of total zinc oxide output. Most of the production capacity is in Asia and Europe. Notwithstanding its alternate name, American Process, it is no longer used in North America. The markets served are in paints, where the lower reactivity is advantageous, and also in ceramics. Direct Process zinc oxide has a more acicular structure and larger particle size compared to French Process. Due to the nature of the raw material used, Direct Process zinc oxide typically has a lower purity than French Process.

Rotary kilns are a variant of French Process which gives a product not unlike Direct Process. These kilns are primarily used to recover bottom dross.

Chemical Zinc Oxide
There are a number of Chemical processes for zinc oxide in use, other alternate routes have been operated historically and new ones are being planned.

Hydrosulfite Zinc Oxide
This zinc oxide is produced as a co-product with sodium hydrosulfite, a bleaching agent used primarily in the Paper Industry. It is typically very fine, around 25 m2g-1 and could be termed nano sized. Sulfur is present as zinc sulfide and other sulfur containing compounds with total sulfur content up to 2%. It is lemon yellow in color. The product can be sold to agricultural markets for use in both crops and animal nutrition. It is also suitable for ceramic markets. The product can also be subjected to further processing and upgraded for Rubber Industry applications.

Active Zinc Oxide
Grades which are manufactured from precipitated zinc hydroxide or basic zinc carbonate intermediates are often termed in this way. Mild heat treatment decomposes the materials into zinc oxide. The zinc oxide is relatively reactive, due to the very high surface area which can be above 50 m2g-1. Such products are useful in the area of catalysts and latex rubber.

Further controlled thermal treatment can generate products suitable for other markets.