Iron oxide red pigments, process for the production of iron...

Details Iron oxide red pigments, process for the production of iron... Iron oxide red pigments, process for the production of iron...

1998-10-15

2001-01-30

Marcheschi, Michael (Department: 1755)

Compositions: coating or plastic

Materials or ingredients

Pigment, filler, or aggregate compositions, e.g., stone,...

C106S459000, C423S633000

Reexamination Certificate

active

06179908

ABSTRACT:

The present invention relates to iron oxide red pigments, a process for the production of iron oxide red pigments by calcining iron oxide yellow pigments or iron oxide black pigments, and the use thereof.
Precipitation processes for the production of iron oxides have been known for a long time. The typical course of these processes is described, for example, in Ullmanns Enzyklop{umlaut over (a)}die der technischen Chemie, 5th edition, volume A 20, pages 297 ff. Iron(II) sulfate formed during the pickling of steel plates or iron(II) sulfate formed during the production of titanium dioxide by the sulfate process is usually used as the raw material.
An increased trend towards using hydrochloric acid as pickle has been observed for years in the pickling industry, and so large quantities of FeCl
2
are formed. Furthermore, the so-called chloride process is increasingly being used worldwide to produce titanium dioxide. Large quantities of FeCl
2
are formed during the production of synthetic rutile for TiO
2
production. Solutions containing iron(II) chloride, which should as far as possible be converted into useful substances, are thus increasingly being formed from these processes.
One common process for utilising FeCl
2
waste solutions is the spray roasting process, in which iron(II) chloride or iron(III) chloride is subjected to oxidative hydrolysis at high temperatures (typically at more than 1000° C.). Iron oxides, typically haematite, and hydrochloric acid, which can also be recycled as a useful substance, e.g. back into the pickling process, are the end products of this process. An iron oxide which is generally suitable for producing hard ferrites is obtained from this process without any special purification steps. If soft ferrites are to be produced from iron oxides obtained in this way, the iron chloride solution used has to undergo additional purification operations beforehand, which makes the process considerably more expensive. Since more and more iron chloride, largely of inferior quality, is being produced and the take-up capacity of the ferrite market is limited, an alternative method of producing a useful substance from these iron chlorides is being sought. Direct landfill or discharge at sea of the iron chloride solutions is not possible for ecological reasons.
The object of the present invention was therefore to provide a process allowing iron chloride solutions to be converted cheaply and as simply as possible into high-quality useful substances.
It was possible to achieve this object through the iron oxide red pigments according to the invention and the process according to the invention.
The present invention provides iron oxide red pigments with a lightness L* (full shade) of 36.0 to 44.0 CIELAB units, an a* value of 24 to 30 CIELAB units and a b* value of 14 to 26 CIELAB units and with a lightness L* (with brightening) of 59 to 66 CIELAB units and an a* value of 18 to 30 CIELAB units and a b* value of 4 to 26 CIELAB units and with an Mn content of 0.012 to 0.12 wt. %, based on pigment, a chloride content of 0.05 to 0.59 wt. %, based on pigment, and a chromium content of less than 40 mg/kg pigment.
The invention also provides a process for the production of iron oxide red pigments by calcining iron oxide yellow pigments at temperatures of 350 to 1000° C., preferably of 600 to 900° C., in the presence of an oxygen-containing gas and optionally by subsequent grinding, characterised in that the iron oxide yellow pigment was produced as follows, in that
a) an alkaline component is added to an acidic Fe(II) chloride solution with an FeCl
2
content of 50 to 450 g/l, stirring intensively, in a quantity sufficient to adjust the pH of the solution to between 3 and 5,
b) a flocculation aid is optionally also added to the solution before or after adding the alkaline component or together with the alkaline component,
c) oxidation is optionally carried out after adding the alkaline component and the optional flocculation aid,
d) the solid formed after the treatment in a) to c) is separated from the solution,
e) the solution formed in d) is added to an &agr;- or &bgr;-FeOOH seed suspension produced by the precipitation process in a quantity corresponding to 4 to 8 times the molar quantity of iron in the seed suspension overall (i.e. iron from iron oxide hydroxide and iron from unreacted iron compound),
f) the suspension formed in e) is heated to a temperature of 30 to 95° C., preferably of 30 to 85° C., particularly preferably of 55 to 75° C., mixing thoroughly,
g) oxidation is then carried out using an oxidising agent, the oxidising agent being added such that 0.5 to 10 mole % iron per hour, preferably 0.5 to 2.0 mole % iron per hour, is oxidised and at the same time the pH is increased, using an alkaline component, at a rate of 0.01 to 0.4 pH units/hour to a final pH of 3.0 to 5.0,
h) oxidation is optionally continued at a constant pH of between 3.0 and 5.0,
i) oxidation is stopped as soon as the Fe(II) content of the suspension is less than 1 mole %,
j) the solid from i) is finally separated off, washed, dried and ground.
The invention also provides a process for the production of iron oxide red pigments by calcining iron oxide black pigments at temperatures of 350 to 1000° C., preferably of 600 to 900° C., in the presence of an oxygen-containing gas and optionally by subsequent grinding, characterised in that the iron oxide black pigment was produced as follows, in that
a) an alkaline component is added to an acidic Fe(II) chloride solution with an FeCl
2
content of 50 to 450 g/l, stirring intensively, in a quantity sufficient to adjust the pH of the solution to between 3 and 5,
b) a flocculation aid is optionally also added to the solution before or after adding the alkaline component or together with the alkaline component,
c) oxidation is optionally carried out after adding the alkaline component and the optional flocculation aid,
d) the solid formed after the treatment in a) to c) is separated from the solution,
e) the solution formed in d) is added to a solution which contains a precipitant reacting as an alkali, at a temperature of 50 to 95° C., in a quantity such that the equivalent ratio of iron(II) to precipitant is 0.5 to 0.65, preferably 0.5 to 0.58,
f) oxidation is then carried out using an oxidising agent,
g) oxidation is stopped as soon as the FE(II) content of the suspension obtained in f) is less than 1 mole %.
Sodium hydroxide, sodium carbonate, magnesium carbonate, magnesium oxide, magnesium hydroxide, calcium oxide, calcium hydroxide or ammonia is preferably used as the alkaline component or precipitant reacting as an alkali.
Iron(II) chloride solutions from steel pickling or else iron(II) chloride solutions from TiO
2
production by the chloride process may be used.
The iron oxide yellow pigments or iron oxide black pigments obtained as intermediates in the process according to the invention have manganese contents of 0.011 to 0.11 wt. %, based on pigment, and chloride contents of 0.07 to 0.7 wt. %, based on pigment, and chromium contents of 5 to 40 mg/kg, based on pigment.
When producing iron oxide red via iron oxide black in accordance with the invention, it is advantageous to maintain an equivalence ratio of iron(II) to precipitant of 0.5 to 0.65.
It is generally useful to grind after calcining. Grinding is preferably carried out with a jet mill, a pendulum roller mill, a roller mill, an attritor, with an impact mill or with a bead mill. Dry grinding with the addition of a grinding aid is particularly preferred.
The process according to the invention may advantageously be carried out as follows:
Sodium hydroxide solution is added to an iron(II) chloride solution with an FeCl
2
content of between 50 and 450 g/l, stirring intensively, in a quantity such that the pH is adjusted to 3 to 5. Another alkaline component, such as e.g. Ca(OH)
2
, Na
2
CO
3
or ammonia etc., may be used instead of the sodium hydroxide solution. In addition, the sedimentation properties of the hydroxide or carbonate slurry formed may be improved by adding a flocculation

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