AN IMPROVED PROCESS FOR MANUFACTURING ZINC OXIDE
Year: 1878

page_text: LETTERS PATENT to Edward Andrew Parnell, of Swansea, in the County of
Glamorgan, for the Invention of "AN IMPROVED PROCESS FOR MANUFACTURING
ZINC OXIDE."
Sealed the 8th October 1878, and dated the 16th August 1878.
COMPLETE SPECIFICATION filed by the said Edward Andrew Parnell at the
Office of the Commissioners of Patents on the 16th August 1878.
EDWARD ANDREW PARNELL, of Swansea, in the County of Glamorgan.
IMPROVED PROCESS FOR MANUFACTURING ZINC OXIDE."
AN
The material which I employ as the source of zinc in my improved process for
making zinc oxide is sulphate of zinc, which I convert into zinc oxide by moderate
heating in contact with carbonaceous reducing, that is to say, deoxidizing agents.
When the zinc oxide is required in a white state for use as a paint the zinc
sulphate has to be carefully freed from those metallic oxides, the presence of which
would communicate colour to the zinc oxide, particularly oxides of iron, manganese,
and copper. The elimination of these oxides I effect by the methods known to
chemists, and usually practised for the purification of zinc sulphate when prepared
from zinc ores by calcination and lixiviation, with or without the addition of
sulphuric acid to the calcined zinc ore. I next mix the purified sulphate of zinc
with powdered wood charcoal in the proportion of one part of charcoal to about
twelve parts of dry or actual zinc sulphate, or twenty-two parts of the crystallized
sulphate. The admixture of these materials is most conveniently effected by adding
the charcoal to the melted crystals of zinc sulphate, the mixture being heated and
stirred until it solidifies.
Sawdust and other carbonaceous materials may be used in this operation, but I
give the preference to wood charcoal.
I next heat the mixture of zinc sulphate and charcoal to dull redness, taking care
to avoid free access of atmospheric oxygen. The zinc sulphate is thereby decom-

page_text: posed, sulphurous acid gas is disengaged, together with carbonic oxide and carbonic
acid gases, and the zinc remains in the state of oxide.
It is desirable that in this operation the heat should not exceed dull redness.
At a higher temperature the mechanical condition of the zinc oxide is not so fine.
There is also a risk at a high temperature of the production of sulphide of zinc
and metallic zinc, which is not the case when the heat does not exceed dull
redness.
The heating of the mixture of zinc sulphate and charcoal may be conveniently
performed in earthen retorts, similar to coal gas retorts, or else in a close muffled
furnace. No stirring is necessary. The operation is complete when sulphurous
acid gas is no longer disengaged. The remaining zinc oxide is cooled slowly if it
is found to contain any undecomposed zinc sulphate; this is removed by washing
with water. When dried the oxide is ready for use.
In preparing zinc oxide for the purpose of manufacturing spelter and for other
applications where purity and whiteness are not essential I employ the crude
solution of zinc sulphate obtained by the lixiviation of calcined zinc ores, with or
without the addition of sulphuric acid, and instead of wood charcoal I employ
small coal as the reducing agent in the proportion of one part of coal to about ten
parts of dry zinc sulphate or eighteen parts of crystallized sulphate. This mixture
is heated moderately as above described in a close muffled furnace or an earthen
retort. An open reverberatory furnace may however be made use of for this
operation, provided care is taken to avoid excess of atmospheric oxygen. In such
case a little extra coal should be added to the zinc oxide occasionally, so long as
that addition is found to cause disengagement of sulphurous acid.
Instead of mixing the carbonaceous deoxidizing material in a solid state with
the zinc sulphate it may also be employed in the state of gas or vapour. In this
manner coal gas, gas from Siemens's "producer," and vapours derived from volatile
compounds of carbon and hydrogen may be applied as the reducing agent. In
such operations the zinc sulphate should be introduced into the retort or close
furnace in a dry and pulverulent state.
Having thus described the nature of my Invention, and the manner in which
the same is to be performed, I desire it to be understood that I claim, the manu-
facture of zinc oxide from zinc sulphate by heating the latter in contact with
charcoal or other carbonaceous material, substantially as described.
In witness whereof, I, the said Edward Andrew Parnell, have hereunto set
my hand and seal, this Seventh day of August, in the year of our Lord
One thousand eight hundred and seventy-eight.
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