Of these, zinc dust is the only ready product which is, as color or
reducing agent, employed in analytical and technical processes. Its value,
when serving the latter purpose, is determined by the percentage of finely
divided metallic zinc and cadmium contained therein; of equal reducing
power is cadmium, generally associating zinc; injurious, and therefore
uneffective, are zinc oxide and oxides of other metals, also metallic
lead.
Flue dust, condensed in chambers of zinc furnaces with Kleemann's
receivers, is employed with zinc ores in the extraction of zinc, and in
small quantities as substitute for zinc white; its commercial value is
similarly estimated as that of zinc ores.
The various modifications of zinciferous flue ashes from blast furnaces
are an object for continual demand, being both a valuable material for the
production of zinc and, in its superior qualities, a desirable pigment. In
the regeneration of zinc the presence of foreign substances is of some
concern; detrimental are lead, sulphur, and sulphuric acid in form of
lead, zinc, and lime sulphate.
The chemico-technical analysis of these products has until recently been
confined to the volumetric determination of zinc by means of sodium
sulphide (Schaffner's method). But as a remnant of sulphur, as sulphuric
acid, in roasted blende causes a material loss during distillation, and
otherwise being induced to produce a zinc free of lead, the estimation of
sulphur, sulphuric acid, and lead became necessary. These impurities are
determined by well-known methods; sulphur is oxidized and precipitated
with barium chloride, lead by sulphuric acid and alcohol. The examination
of zinc dust, when used for the regeneration of metal, determines the
quantity of zinc resident therein, and employed as reducing agent, the
quantity of metal which causes the generation of hydrogen. Cadmium,
showing the same deportment, must also be considered as well as lead and
arsenic.
A most complete and rapidly working method for the examination of
zinciferous products has originated with the application of neutral
ammonium carbonate as solvent. A solution of this preparation is made,
according to H. Rose, by dissolving 230 grm. commercial ammon carbonate in
180 c.c. ammoniacal liquor of 0.92 s.g., and, by addition of water,
augmenting it to one liter.
This solution dissolves the metallic components, their oxides, and basic
zinc sulphate, and transfers cadmium and lead oxide, also lead, magnesium,
and lime sulphate, into insoluble carbonates. Iron and manganese, when
present as protoxide, are dissolved; of iron sesquioxide but traces, and
of cadmium oxide _in statu nascendi_ a small portion enter into solution.
The solution of ammonium carbonate contains in each 10 c.c. 1 grm.
ammonia, which dissolves 1.5 grm. zinc.
The sample for examination is moistened with water and mixed with an
adequate volume of the solvent, is digested at 50-60° C. until complete
decomposition is effected. The heating of the liquid prevents the solution
of iron, manganese, and cadmium. The content, sediment and liquid, is
thrown on a filter and washed with hot water to which a small quantity of
the solvent has been added. When the solution contains iron and manganese,
it is separated by decantation from the sediment and oxidized with bromine
(according to the method of Nic-Wolff) until a flocculent precipitate of
iron sesquioxide and manganese dioxide becomes visible; it is united with
the original residue and filtered.
The filtrate is diluted till it appears cloudy, boiled to expel ammonia,
tested with sodium sulphide upon the presence of zinc, and, when freed of
all zinc, decanted. The precipitate of zinc carbonate is filtered,
exhausted with water, transferred into zinc oxide by ignition, and
weighed. The gravimetric method can be substituted by the volumetric by
introducing a solution of sodium sulphide of known strength into the
ammoniacal filtrate. On dividing the filtered liquid into various equal
portions other substances, arsenic and sulphuric acid, can be determined
from the same sample. For this purpose the filtrate is concentrated;
divided into two equal portions, one of which is acidified and treated
with hydrogen sulphide for the determination of arsenic, the other is
acidified and used for the estimation of sulphuric acid by means of barium
chloride. The original residue is dissolved in muriatic or acetic acid and
filtered. The lead of the filtered liquid is thrown down by sulphuric
acid, and alcohol, and cadmium, after dissipation of alcohol into gas,
precipitated by hydrogen sulphide. Iron, manganese, alumina, and other
substances present in the solution are determined by known methods.
It is manifest that the determination of substances--zinc, lead, and
sulphuric acid--which are of importance in technical analysis of zinc ash,
can be executed by this method within a comparatively short time. The
application of ammonium carbonate as solvent has the advantage, over the
application of ammonia, that it is a far better solvent, that it
decomposes insoluble basic sulphates, and that the remaining carbonates
are readily dissolved by acids.
The decomposition of zinc dust is accompanied by a lively evolution of
gas; it is therefore necessary to continue the digestion of the sample
till no more hydrogen is given off. Zinc dust contains both metals and
their oxides, and methods which, from the volume of hydrogen generated,
determine indirectly the percentage of metallic zinc do not give the real
composition of the zinc dust. For the determination of the metallic
components the material is digested with a solution of copper sulphate,
which dissolves zinc and cadmium; the liquid is filtered, acidified, and
decomposed with hydrogen sulphide, or treated with a solution of ammonium
carbonate. The use of cupric chloride is not advisable, as it corrodes
lead, and gives rise to the formation of soluble chloride of lead, which
complicates the separation of zinc from cadmium. The best mode of
operation is the following: Both copper sulphate and zinc dust are weighed
separately, the former is dissolved in water and the latter introduced
into the solution of copper sulphate in small portions until it appears
colorless. During the operation the vessel is freely shaken, lumps are
comminuted with a glass rod, and a few drops of the liquid are ultimately
tested with hydrogen sulphide or ammonia. The remainder of zinc dust is
then weighed, and its value deducted from the original weight. Zinc and
cadmium of the filtrate are determined as above. On repeating this method
several times most satisfactory results are obtained.
Another mode of operating is to employ an excess of copper sulphate and to
determine the copper dissolved in the filtrate. The separation of copper
from cadmium being difficult and laborious, and the volumetric estimation
with potassium cyanide not practicable, it is not prudent to apply this
method.
When calcined zinciferous pyrites have to be examined, the estimation of
zinc is similar to that employed in the analysis of zinc ore. The sample
is exhausted with water, filtered, and, to eliminate calcium sulphate and
basic iron sulphate, evaporated to dryness. It is then dissolved in a
small quantity of alcohol and water, refiltered, and the filtrate
decomposed with ammonium carbonate. The original residue is treated with a
solution of ammonium carbonate, which dissolves arsenious acid and basic
zinc sulphate, filtered, and united with the first filtrate. When iron and
manganese are present, the filtrates are treated with bromine. The united
filtrates are boiled or examined volumetrically with sodium sulphide.
* * * * *