Zinc Oxide Production Technology
Although zinc oxide has been known since ancient times, its use as a pigment is relatively recent. It was developed as a pigment in place of basic lead carbonate. The lead pigment was superceded because it turned black on atmospheric exposure. Production of zinc oxide by burning zinc in air was developed by Le Clair in France in 1840, hence the French Process. Zinc oxide was the prime white pigment for another 100 years, when titanium dioxide was developed as an alternative with superior opacity to zinc oxide.
In USA, zinc oxide production followed a different path. The American or Direct Process was developed. It was termed direct because it could be made directly from an oxidic ore.
In both pyrometallurgical routes the zinc oxide is removed from the combustion area by an excess of air which provides both pneumatic transport and cooling. The zinc oxide agglomerates which allows the collection of the sub-micron particles by fabric filters.
The hydrometallurgical routes to zinc oxide are primarily based on zinc hydroxide or basic zinc carbonate as intermediates.
Several routes of making French Process have developed over the years. All of the routes use the geometry of the combustion zone and the air flow to control the particle size of the zinc oxide. The constraints of this system allow a surface area range from around 2.5 m2g-1 to 12 m2g-1 to be practically achieved.
Le Clair’s process used horizontal retorts. In the UK, vertical crucible (retort) furnaces were developed in the 1920’s. These were in two banks of four crucibles that were fired centrally by coal. Subsequently the furnaces were modified to operate using heavy fuel oil. With this fuel, single crucible furnaces were designed. Such furnaces are robust, flexible, simple to operate and can produce around 3 tonnes of zinc oxide daily. County Durham in the UK was a centre of this technology. This production method spread to many areas of the world and is still in use in several different forms. It is a semi continuous process, depending on whether the zinc feed is solid or liquid.
In the US muffle furnaces were developed with upper and lower compartments were separated by a thermally conductive arch constructed from silicon carbide brick. The furnace is fired in the upper compartment and the heat transferred to the liquid zinc which is maintained in the lower compartment. It is a continuous process. The zinc vapor is led to an exit point and combusts in a large chimney. These furnaces have an underflow of liquid so jumbo block rather than individual ingot can be charged. These furnaces were further modified in Western US by adapting for feed of zinc alloy. After a campaign the furnace is tapped to remove the less volatile elements such as aluminum and copper. Muffle furnaces are also used on a world wide basis, both for primary and secondary zinc feedstock.
An alternate route, also developed in the US is zinc distillation columns. Whereas electrolytic smelting of zinc produces a pure product, pyrometallurgical routes generate a zinc, lead and cadmium mixture. If the liquid is cooled a product assaying 98.5% zinc results, this is termed Prime Western and is used in hot dip galvanizing. Lead levels are too high to use directly for French Process zinc oxide hence distillation columns were developed in the US. The cadmium and lead columns are in series. Zinc vapor exits from the top of the lead column and is usually oxidized to oxide, rather than condensing to SHG zinc. Columns can also use secondary zinc feedstocks.
Rotary furnaces were developed with the view of minimal fuel usage. The exotherm from the zinc vapor combustion can be used to melt and vaporize the zinc. Rotary furnaces use a coke generator which provides a high temperature carbon monoxide feed. The combustion of zinc vapor within the furnace provides radiant heat to the furnace interior. The furnace must be fed solid zinc on a continuous basis. The disadvantage of this furnace is that the yield of product is lower than alternate routes. It is best suited to recovery of bottom dross from hot dip galvanizing.
In Norway an alternate technique was developed using direct electric heating. It differs from a muffle furnace in that the electrical resistance heating is situated directly above the zinc liquid surface. The zinc vapor is lead up a very long shaft where lead preferentially condenses out and runs back to a collection chamber. This furnace was designed to operate with secondary zinc. It is very good for high lead feedstock, however all the input cadmium reports to the zinc oxide product. From a zinc yield viewpoint it is very effective, the disadvantage being that the cost of electrical heating is very high in many areas of the world.
A direct electric arc route was in use in the US but is no longer practiced.
The Direct or American Process was the favored zinc oxide manufacture method for many years. The term direct pertains to oxidic zinc ores or calcine, oxidized zinc sulfide ore. Until flotation techniques were developed and electrolytic zinc smelting became dominant, this was the direct route to zinc oxide.
In this process the calcine/coke mix is formed into a bed on a grate and fired. Generation of carbon monoxide reduces the oxidic zinc and the zinc is expelled as vapor. The zinc is reoxidised as it comes in contact with lower temperature air, forming zinc oxide particulate. The combination of the reduction reaction and the zinc vapor pressure combine to purify the zinc. An added advantage is that the low levels of lead and sulfur combine to give lead sulfate which is also white.
The process evolved into different mechanical routes. Fixed grate systems were modified to continuously operating grates. Also slow or intermittent rotary systems were developed so that a higher yield of zinc oxide could be achieved.
The combustion occurs within a carbon monoxide/air environment and hence the product has different physical characteristics to French Process. Direct process is preferred for some applications for this reason. However for many applications, the product purity and lack of fine control have put it at a disadvantage with French Process in our present era where quality control has become paramount.
An area where a variant of the process is used extensively is in rotary kilns. The kilns may be cylindrical Waelz kilns or alternatively rotary hearth furnaces but in each case the feedstock is an impure oxidic zinc and a carbon reductant. Electric arc furnace dust from the Steel Industry is a significant feedstock. The material remaining after zinc volatilization is compatible with the Steel Industry. The crude zinc oxide requires further processing before recovery as zinc metal or oxide.
The dominant chemical process zinc oxide is generated as a co-product with sodium hydrosulfite, a powerful bleaching agent. Zinc hydrosulfite is first synthesized in aqueous solution. Neutralization of the zinc salt with sodium hydroxide precipitates zinc oxide.
The zinc oxide is then filtered and dried. The surface area is typically 25 m2g-1. Due to a combination of sulfur impurities and the fineness of the product, this material is lemon yellow in color. The process is operated at a number of facilities world wide.
Most other chemical routes are via hydroxide or basic carbonate routes.
Neutralization of zinc sulfate with sodium carbonate precipitates zinc carbonate. Alternatively basic zinc carbonate can be generated after ammonia/ammonium carbonate leaching of crude zinc oxide materials. In both cases controlled deposition of the carbonate yields zinc oxide. The key to the success of this process is to make a product that is considered to be equivalent to French Process zinc oxide. This requires a significantly larger particle size than is usual for such materials.
There are limited markets for the high surface area zinc oxide produced directly from the carbonate and hydroxide. Such products are sometimes termed ‘Active’.
An alternate route for direct recovery of zinc oxide via ammonium chloride leaching was developed. Particle morphology was very similar to French Process.
Tetrapod zinc oxide has been made by controlled oxidation of zinc powder.
The three-dimensional nature of the crystal are of value in specific applications.
In addition to zinc carbonate there are several other salts which can be thermally decomposed to zinc oxide. A route by which controlled particle size can be achieved is spray pyrolysis of a solution such as zinc acetate.