There is a high demand for titanium dioxide in the global market,
which is expected to continue growing. Additionally, there is a
constant increase in the worldwide need for inorganic pigments.
According to experts’ estimates, in recent years, titanium dioxide
pigment accounted for 65-70% of the total consumption of inorganic
pigments and over 90% of the consumption of white pigments.
In producing pigmentary titanium dioxide by the nor mal sulfate
process, objectionable quantities of iron, chromium, vanadium
and/or niobium impurities are car ried over into the final product.
By effecting precipitation of the titanium values from sulfuric
acid solution in the form of a titanyl sulfate slurry or discrete
titanyl sulfate particles, and separating and redissolving the
titanyl sul fate in an aqueous liquid, a solution is provided from
which hydrous titanium dioxide can be precipitated by hydrolysis
with increased purity of product. Precipitation of titanyl sulfate
as a solid mass is avoided by adding sulfuric acid to the titanyl
sulfate solution shortly after initiation of precipitation.
The present invention relates to a process for the pro duction of
pigmentary titanium dioxide. Pigmentary titanium dioxide is
commonly made by the sulfate process wherein an impure
iron-containing titani ferous material, for example ilmenite, is
digested with a limited amount of hot concentrated sulfuric acid
and the resulting solid is then dissolved, normally in water or
dilute sulfuric acid. The solution thus formed is then subjected to
various treatments such as reduction of the iron content to the
ferrous state; removal of undissolved solids by clarification and
of some of the dissolved iron, for exam ple by concentration and
cooling. Ultimately a substan tially clear solution is obtained
having acceptable iron/ titanium and acid/titanium ratios. To this
solution are added or are formed in situ, rutile- or
anatase-inducing nuclei and the resulting mixture is heated to
hydrolyze the titanium sulfate in solution and to precipitate hy
drous titanium dioxide. The latter is filtered off and subjected to
various wash ing procedures to remove excess acid and various color
forming impurities such as iron or chromium compounds. To the
purified hydrous titanium hydroxide are added various conditioning
agents, for example compounds of zinc, antimony or tin, if rutile
titanium dioxide is to be produced and heat-decomposable alkali
metal salts or phosphates (e.g. as the ammonium salt). The titanium
dioxide is then calcined, for example at a temperature in the range
of about 800 to 1100° C., wet and/or dry milled, classified
following which it may be coated, dried and finally milled. It may
also be coated with hydrous oxides or other metals or metalloids,
for example of aluminum, titanium, silicon, zirconium and/ or
cerium at some stage before recovery. Titanium dioxide pigment
produced by the above proc ess frequently contains coloring
impurities which prevent the development of the maximum brightness
of which titanium dioxide pigments are capable. It is believed that
this inferior brightness is due, at least in part, to the presence
of small amounts of elements such as chromium, vanadium, niobium
and/or iron in the pigment derived from impurities in the
iron-containing titaniferous mate rial. It has proved impossible to
remove all the coloring impurities and to de