Process for the production of black iron oxide pigments

Details Process for the production of black iron oxide pigments Process for the production of black iron oxide pigments

2000-08-18

2001-10-16

Green, Anthony (Department: 1755)

Compositions: coating or plastic

Materials or ingredients

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

C423S632000, C423S633000

Reexamination Certificate

active

06302952

ABSTRACT:

This invention relates to a process for the production of black iron oxide pigments and, more particularly, to the production of black iron oxide pigment from hematite.
Pigmentary black iron oxide has the formula Fe
3
O
4
and a cubic crystal structure typical of a magnetite. It may be used, for example, in the pigmentation of building materials such as concrete, mortar, plaster of Paris, roofing tiles, paving bricks, artificial stone products or other similar products.
A great quantity of somewhat impure iron (II) chloride is produced as a by-product in the steel pickling industry and it would be of value to utilise this by-product as a raw material. According to the well known “Ruthner”, or “spray-roast”, process pickle mill waste aqueous iron chloride is converted into an iron oxide having the formula Fe
2
O
3
and a hexagonal crystal structure typical of a hematite by a spray-roasting process. The product of such a process and raw material is a red pigment having an appreciable content of chloride and of a wide range of other impurities. Typically this product may contain more than 90% by weight of Fe
2
O
3
, a restricted quantity of FeO, for example from 0.005% to 0.5% by weight, a substantial quantity of chloride ion, for example from 0.002% to 1% of more, possibly up to 5% by weight, an appreciable quantity of Mn
2
O
3
, for example from 0.1% to 1% by weight as well as, usually, oxides of some or all of aluminium, chromium, magnesium, titanium, zinc, lead, copper, arsenic and vanadium and oxides also of calcium silicon and phosphorus and a content of carbon.
It would be a useful contribution to the pigments industry to enable the production of a black iron oxide from a natural or synthetic hematite. However, hematite can be a relatively unreactive material and no aqueous slurry process enabling this has been known. Both natural mineral hematite and synthetic spray-roast hematite have passed through a period of elevated temperature which induces a degree of inertness due to calcination.
French Patent No. 2244716-A of Ruthner Akt. describes the production of a black iron oxide pigment powder from Fe
2
O
3
prepared by the thermal dissociation of an iron chloride solution by means of a high temperature solid-vapour reaction comprising heat treatment under oxidising, reducing or neutral conditions at a temperature above 400° C. and then heat treatment under reducing conditions at a temperature under 550° C. The product is Fe
3
O
4
having a magnetite structure containing 0.02% chloride. This product is then ground in a vibratory mill to give a particle size substantially below 0.045 mm.
The present invention provides an alternative process for the production of a black iron oxide pigment from Fe
2
O
3
hematite iron oxide, which may have been produced by the “Ruthner” process or by other means, or which may be a mineral hematite. The process may be used to reconvert poor quality synthetic or natural hematites to black iron oxide or to improve their performance as red pigments by conversion to black pigment and subsequent oxidation of the magnetite so produced back to hematite. The process of the present invention uses a low temperature aqueous slurry reaction.
The present invention provides a process for the production of black iron oxide pigment from iron oxide having the hematite crystal structure the process being characterised in that the hematite is contacted with a soluble source of Fe(II) ions in the presence of a soluble source of Fe(III) ions and in an aqueous alkaline medium to convert the same to Fe
3
O
4
.
It has been found that the strength of the black pigment may be very considerably enhanced by the introduction of a quantity of a soluble source of Fe(III) ions, for example ferric sulphate, into the aqueous alkaline medium as is shown in Examples 5a to 5C and 6a to 6c below.
In the practice of the invention the iron oxide having a hematite structure is preferably a spray-roast product produced from iron chloride as described above. Such a product will normally be in the form of particles of which at least 50% by weight are at least 5 micrometers, for example from 5 to 20 micrometers in diameter.
The soluble source of Fe(II) ions may be a water soluble iron salt such as the chloride or the sulphate. Preferably the iron salt is iron sulphate which may suitably be in the form of ferrous sulphate heptahydrate, or copperas. The soluble source of Fe(II) ions is suitably included in the reaction mixture as a solution in water, preferably a concentrated solution, for example, in the case of copperas, a solution containing about 450 g/l or up to the solubility limit of copperas at the temperature used. Preferably the quantity of the soluble source of Fe(III) ions is at least 1 g/l and, for example up to 20 g/l of the soluble source of Fe(II) ions or an equivalent quantity.
The alkalinity of the medium may suitably be provided by a base such as ammonium hydroxide or sodium carbonate but is preferably an alkali metal hydroxide such as sodium hydroxide. As a general procedure for carrying out the invention the hematite may be suspended in water using a suitable degree of agitation, and if required a suspending agent, the Fe(II) compound may be added to the suspension in the form of an aqueous solution or may be added as a solid and dissolved in situ, the reaction mixture may be heated to reaction temperature, the base may then be added and the resulting reaction mixture may be maintained for a period to allow the reaction to proceed and the required crystal habit to form.
In a standard process according to the invention the initial solids concentration of the hematite may suitably be from 50 to 150 g/l, the temperature of the reaction mixture may suitably be maintained at, at least, 60° C., and suitably below the boiling temperature at ambient pressure, but preferably from 70° C. to 100° C., the agitation may suitably be by stirring at a rotor tip speed of at least 0.3 m/sec., suitably up to 3.5 m/sec., but preferably from 0.5 to 2 m/sec. the ratio of Fe(II)/FE(III), that is the ratio of soluble Fe(II) to the Fe(III) content of the hematite, may suitably be at least 0.25, suitably up to 1, the content of added source of soluble Fe(III) ions in the aqueous medium is at least 5 g/l, the base may suitably be introduced over from 1 minute to 60 minutes but preferably over 5 to 30 minutes, the base may suitably be in the form of an at least 100 g/l, suitably up to 600 g/l solution, the pH may suitably be at least 7.5, suitably up to 11 but preferably below 9.0 for example below 8.75, to augment the black strength of the pigment and the reaction mixture may suitably be maintained for from 2 to 24 hours after the addition of the base.
In order to enhance the pigmentary properties of the product of this invention the following process features thereof are preferably employed, individually or in any combination.
It has been found that the strength of the black pigment may be enhanced by controlling the particle size of the hematite. Preferably, the particle size is controlled at at least 97% by weight below 15, particularly preferably below 10 micrometers in diameter. Preferably at least 90% by weight of the particles are below 10, particularly preferably below 5 micrometers in diameter. Preferably at least 10% by weight of the particles are below 0.75, particularly preferably below 0.5 micrometers in diameter. This feature is illustrated in Examples 3a, 3b and 3c hereafter.
It has also been found that the black strength of the pigment may be enhanced by closely controlling the Fe(II)/Fe(III) ratio defined above. Preferably, the ratio is from 0.4 to 0.55, particularly preferably 0.45 to 0.55 suitably 0.5+/−0.025. This feature is illustrated in Examples 6a to 6c hereafter.
It has also been found that the strength of the black pigment may be enhanced by the addition of a suspending agent to the aqueous alkaline medium before or after the suspension of the oxide having the hematite crystal structure is formed or to the oxide itself. Preferably the suspending agent is a p

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