Compositions: coating or plastic – Materials or ingredients – Pigment – filler – or aggregate compositions – e.g. – stone,...
Reexamination Certificate
2000-09-15
2003-03-11
Koslow, C. Melissa (Department: 1755)
Compositions: coating or plastic
Materials or ingredients
Pigment, filler, or aggregate compositions, e.g., stone,...
C423S632000, C423S633000, C423S634000
Reexamination Certificate
active
06530987
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a process for the preparation of iron oxide pigments from waste acid occurring in the production of titanium dioxide by the sulfate process.
The preparation of iron oxide pigments from residues resulting from titanium dioxide production has already been known for a fairly long time, for example from DE-A 24 60 493 and JP-A 44 050 695. In particular, the iron sulfate heptahydrate (green salt) occurring in titanium dioxide production is used as a raw material for preparing iron oxide pigments. The typical procedure involved in this process is described for example in Ullmann's Encyclopedia of Industrial Chemistry, 5th ed., Vol. A 20, p. 297 et seq.
Waste acid, which also occurs in the production of titanium dioxide by the sulfate process, is however less suitable for preparing iron oxide pigments. Although iron oxides can also be obtained from waste acid in a similar way to processes normally used for the preparation of iron oxide pigments, the quality of these iron oxides falls far short of the requirements that high quality iron oxide pigments, in particular iron oxide red pigments or iron oxide yellow pigments, have to satisfy. The reason for this are undesirable constituents of the waste acid, such as compounds of Ti, Cr, V or Mn, which adversely affect the particle shape, particle size, stability and colour properties of the iron oxide pigments obtained therefrom. In addition the large amount of free sulfuric acid in the waste acid, which first of all has to be neutralised for the preparation of iron oxide pigments, adversely affects the profitability of the process.
The utilisation or at least harmless disposal of the waste acid is prescribed in Europe and in most other countries that produce titanium dioxide, with the result that various processes for utilisation have been developed over the course of time.
Printed specification EP-A 577 272 describes how a utilisable gypsum can be obtained from the waste acid by partial neutralisation with calcium carbonate. The metal sulfate solution remaining after separation of this material, which is also known as “white gypsum”, is adjusted to a pH of approximately 9 by addition of CaO or CaCO
3
, the material obtained thereby, which is also known as “red gypsum”, then having to be disposed of. On the one hand valuable raw materials are obtained from a recycling, but on the other hand a large amount of valuable disposal space is used up since, depending on the titanium raw material, 1 to 2.5 tonnes of this waste is formed per tonne of TiO
2
pigment that is produced.
Another process for working up the waste acid, described in EP-A 0 133 505, avoids these disadvantages by reusing practically all the occurring waste acid for the preparation of TiO
2
, wherein the waste acid is first of all concentrated and, after separating the filtration salts that are formed, the 65% to 85% sulfuric acid is reused to digest the ore. Sulfur dioxide is obtained from the filtration salts by thermal cracking and from this SO
2
pure sulfuric acid or oleum is obtained, which likewise are reused for digesting the ore. Although this process minimises the consumption of raw materials, it is very energy-intensive and therefore expensive to operate.
A further process for working up the waste acid, which is described in printed specification U.S. Pat. No. 3,016,286, involves the neutralisation of the waste acid and the precipitation and separation of hydroxides of Ti, Al, Cr and V, as well as the subsequent precipitation of magnetite with ammonia. The disadvantages of this process are however the fact that in the first place large amounts of ammonia are consumed in neutralising the free sulfuric acid, and the magnetite precipitated from the highly concentrated ammonium sulfate solution does not exhibit any pigment properties.
In a modification of the process described in printed specification EP-A 638 515 involving an extraction of magnesium from the ammonium sulfate-containing solution, pure ammonium sulfate can then be obtained by crystallisation and used as fertiliser. Although the amount of utilisable material is increased, the profitability of the process is however unsatisfactory on account of the additional process steps and the poor quality of the magnetite obtained from the highly concentrated ammonium sulfate solution.
Another variant for working up the waste acid comprises, according to U.S. Pat. No. 4,137,292 and DE-A 24 56 320, precipitating gypsum and magnetite simultaneously by neutralising the waste acid with calcium compounds, wherein in order to be able to utilise the two compounds a mechanical separation has to be carried out, for example by means of a hydrocyclone or by magnetic separation, which however provides neither pure gypsum nor a pure magnetite pigment despite the complicated and costly process steps. An optimisation of the process according to GB-A 1 421 773 is based on the fact that ammonium or alkali metal salts are simultaneously present in the precipitation of gypsum with calcium compounds, though this modification also does not avoid the aforementioned principal disadvantages of this process.
The presence of relatively large amounts of manganese in the iron salt solutions interferes in the preparation of iron oxide yellow or iron oxide red pigments. If the starting materials &agr;-FeOOH or magnetite that are used for the preparation of iron oxide red contain more than 0.11 wt. % manganese, referred to iron, then no high grade red pigment can be obtained. The presence of Cr, V, Ti and other chromophoric metal ions or metal ions affecting the precipitation process likewise interfere.
The object of the invention is accordingly to develop a process that enables the occurring waste acid to be processed so as to produce qualitatively high grade iron oxide pigments.
It has now surprisingly been found that high quality iron oxide pigments can be prepared from the waste acid resulting from the production of titanium dioxide by the sulfate process, that a partial neutralisation of the free sulfuric acid contained in the waste acid is carried out with metallic iron and/or iron compounds, the undesired elements Ti, Al, Cr and V are at least partially precipitated as hydroxides and separated, and the iron sulfate-containing solution obtained after their separation is processed further into iron oxide pigments.
SUMMARY OF THE INVENTION
The invention accordingly provides a process for the preparation of iron oxide pigments from the waste acid resulting from the production of titanium dioxide by the sulfate process, characterised in that in a first stage a partial neutralisation of the sulfuric acid contained in the waste acid is carried out with compounds from the group comprising metallic iron and/or iron compounds, the sulfuric acid is optionally neutralised further with a further alkaline compound, the precipitate containing the Ti, Al, Cr and V compounds is separated from the resultant reaction product, and an iron oxide pigment is precipitated from the resultant iron sulfate-containing solution by adding alkaline compounds as well as an oxidizing agent.
DETAILED DESCRIPTION OF THE INVENTION
The undesirable elements Ti, Al, Cr and V are precipitated either during the addition of the metallic iron or of the iron compounds if this stage is carried out at a sufficiently high pH; the increase in the pH necessary to precipitate the undesirable elements may however also be achieved after completing the reaction with iron or iron compounds, by adding other alkaline compounds.
The iron sulfate-containing solution obtained after separating the undesirable elements may be converted either to a magnetite pigment, which can then be baked in a manner known per se to an iron oxide red pigment or iron oxide yellow pigment, or alternatively an iron oxide yellow pigment (&agr;-FeOOH) can be prepared from the iron sulfate-containing solution also after the precipitation process, which pigment can then be baked in a manner known per se to an iron oxide red pigment.
Metallic iron or basic i
Auer Gerhard
Julius Udo
Lailach Günter
Meisen Ulrich
Schuy Werner
Akorli Godfried R.
Bayer Aktiengesellschaft
Koslow C. Melissa
Roy Thomas W.
LandOfFree
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