Chemistry of inorganic compounds – Treating mixture to obtain metal containing compound – Alkaline earth metal
Reexamination Certificate
2000-09-15
2002-08-27
Bos, Steven (Department: 1754)
Chemistry of inorganic compounds
Treating mixture to obtain metal containing compound
Alkaline earth metal
C423S140000
Reexamination Certificate
active
06440377
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a process for the production of high-grade gypsum and also of iron-oxide pigments from waste acid that accumulates in the course of the production of titanium dioxide in accordance with the sulfate process.
The utilisation or at least harmless elimination of waste acid is prescribed at sites in Europe and at most other sites for the production of titanium dioxide, so that over the course of time various processes have been developed with a view to utilisation: in printed publication EP-A 577 272 it is disclosed that a usable gypsum can be obtained from the waste acid by partial neutralisation with calcium carbonate. The metal-sulfate solution remaining after separation of this material, also designated as “white gypsum”, is then brought to a pH value of about 9 by addition of CaO or CaCO
3
, the material obtained in the process, also designated as “red gypsum”, having to be disposed of. In so doing, on the one hand the opportunity for recycling of valuable raw materials is lost, and on the other hand valuable landfill area is extensively used up, since, depending on the titanium raw material, 1 to 2.5 t of this waste accumulates per tonne of TiO
2
pigment produced.
Another process, described in EP-A 0 133 505, for reprocessing the waste acid avoids these disadvantages by practically the entire amount of waste acid accumulating being re-used for the production of TiO2, the waste acid being firstly concentrated and, after separation of the filter salts that are precipitated out in the process, the 65-% to 85-% sulfuric acid being employed again for the purpose of decomposing ore. SO
2
is obtained from the filter salts by thermal dissociation, and pure sulfuric acid or oleum, which is likewise re-used for the purpose of decomposing ore, is obtained from the SO
2
. Although this process minimises the consumption of raw materials, it is very energy-intensive and therefore costly.
A further process for reprocessing waste acid, described in printed publication U.S. Pat. No. 3,016,286, involves neutralisation of the waste acid and precipitation and separation of the hydroxides of Ti, Al, Cr and V, as well as subsequent precipitation of magnetite with ammonia. However, the disadvantages of this process are that, on the one hand, large quantities of ammonia are consumed for the purpose of neutralising the free sulfuric acid and that the magnetite which is precipitated out of the solution containing a considerable amount of ammonium sulfate displays no pigment properties.
In the case of a modification, described in printed publication EP-A 638 515, of the process in the form of an extraction of the magnesium from the solution containing ammonium sulfate, pure ammonium sulfate can subsequently be obtained by crystallisation and can be used as fertiliser. Although the proportion of usable material is increased by this means, the economy of the process is not satisfactory, by reason of the additional process steps and the inferior quality of the magnetite obtained from the solution containing a considerable amount of ammonium sulfate.
Another variant for reprocessing the waste acid consists, according to U.S. Pat. No. 4,137,292 and DE-A 24 56 320, in that gypsum and magnetite are precipitated out simultaneously by neutralisation of the waste acid with calcium compounds, whereby for utilisation of the two compounds a mechanical separation, for example by means of a hydrocyclone or by magnetic separation, has to be undertaken which, despite elaborate process steps, results neither in pure gypsum nor in a pure magnetite pigment. An optimisation of the process according to GB-A 1 421 773, to the effect that ammonium salts or alkali-metal salts are present simultaneously in the course of precipitation of the gypsum with calcium compounds, also does not avoid the aforementioned principal disadvantages of this process.
The object was therefore to develop a process that makes it possible for the waste acid accumulating during production of high-quality products to be made available as extensively as possible for meaningful utilisation.
Surprisingly it has been found that both a high-grade gypsum and high-grade iron-oxide pigments can be produced from the waste acid accumulating in the course of the production of titanium dioxide in accordance with the sulfate process if in a first step the waste acid is caused to react with a calcium compound to form gypsum and the latter is separated, optionally directly, from the remaining solution. In a second step, by increasing the pH value the elements Al, Ti, Cr, V and Fe(III) are at least partially precipitated out of the solution so obtained and are separated. In a third step an iron-oxide pigment is formed from the remaining solution containing iron(II) sulfate and is subsequently separated from the rest of the solution.
SUMMARY OF THE INVENTION
The invention provides a process for the production of gypsum and also of an iron-oxide pigment from the waste acid accumulating in the course of the production of titanium dioxide in accordance with the sulfate process, characterised in that in a first stage a partial neutralisation of the waste acid with a calcium compound is effected subject to precipitation and optionally direct separation of gypsum, subsequently the remaining solution is neutralised further in a second stage subject to precipitation of a deposit containing Ti, Al, Cr, V and optionally Fe(III), and the solution containing iron-sulfate that is obtained after separation of the solids is converted in a third stage into an iron-oxide pigment by adding alkaline compounds and also by addition of an oxidising agent.
DETAILED DESCRIPTION OF THE INVENTION
Separation of the gypsum obtained in the first stage may be effected directly after the precipitation, as a result of which a particularly high quality is obtained; however, separation of the gypsum may also be effected together with the solids precipitated in the second stage which contain Ti, Al, Cr, V and optionally Fe(III), as a result of which the process is simplified. Direct separation of the gypsum obtained in the first stage is preferred.
Partial neutralisation of the free sulfuric acid in the first stage is preferably effected by addition of calcium carbonate, calcium hydrogencarbonate, calcium oxide or calcium hydroxide or alternatively of other alkaline-reacting substances that contain one or more of the cited compounds, for example dolomite. However, partial neutralisation of the free sulfuric acid in the first stage may also be effected by means of a combination of individual substances cited above. Use is preferably made of calcium carbonate, calcium oxide or calcium hydroxide as calcium source. The use of finely ground lime (calcium carbonate) is particularly preferred, since the CO
2
arising in the process can also be utilised.
It is furthermore possible, simultaneously with the neutralisation of the waste acid, to raise the iron content with a view to increasing the amount of iron-oxide pigment through neutralisation of the free sulfuric acid being undertaken in part with substances that contain metallic iron or iron(II), for example with scrap iron, mill scale, turnings or cast-iron filings. Further neutralisation of the free sulfuric acid can then be effected with calcium carbonate.
It is often expedient to dilute the waste acid prior to the reaction with the calcium compound, in order to reduce the viscosity of the reaction mixture. For the purpose of dilution, use may be made either of fresh water or alternatively of a process water that accumulates in the further course of the process. Dilution is preferably effected in a 1:1 ratio (parts by weight of waste acid to parts by weight of water). It is advantageous in this connection to slurry the Ca compound that is used for the purpose of neutralisation with a part of the dilution water and to neutralise the waste acid with this suspension.
Partial neutralisation of the waste acid in the first stage by addition of the calcium compound is preferably effected until a pH value f
Auer Gerhard
Lailach Günter
Meisen Ulrich
Schuy Werner
Bayer Aktiengesellschaft
Bos Steven
Gil Joseph C.
Roy Thomas W.
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