Compositions: coating or plastic – Materials or ingredients – Pigment – filler – or aggregate compositions – e.g. – stone,...
Reexamination Certificate
2003-03-12
2004-09-14
Koslow, C. Melissa (Department: 1755)
Compositions: coating or plastic
Materials or ingredients
Pigment, filler, or aggregate compositions, e.g., stone,...
C252S062560, C502S338000, C423S632000, C423S633000, C423S634000
Reexamination Certificate
active
06790274
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an improved method for producing highly pure iron oxides, usable in particular as pigments and as precursors of catalysts or of materials for electronic devices.
BACKGROUND OF THE INVENTION
The method most commonly used to produce iron oxides used as pigments employs, as its source material, pickling liquids that contain ferrous sulfate or chloride originating from the iron industry or from the process for producing titanium dioxide.
The acid aqueous solution of ferrous salts is first neutralized by dissolving iron scrap and is then treated in conditions of oxidation with sodium hydroxide to precipitate FeOOH nuclei which are then pumped into a reactor that contains iron scrap and is maintained in air stream.
The resulting ferric sulfate (chloride) hydrolyzes forming FeOOH or Fe
2
O
3
; the sulfuric or hydrochloric acid that is released reacts with the iron to form ferrous sulfate or chloride, which are then oxidized to ferric salts. The reaction time varies from a few days to several weeks, depending on the reaction conditions and on the type of pigment that is sought.
The advantage of the method, with respect to others, is the limited use of alkali and of ferrous sulfate or chloride. The small quantity of ferrous salt that is required initially is renewed continuously during the process by the dissolving of the iron by the sulfuric or hydrochloric acid released in the reaction.
The disadvantage of the method is the difficulty in eliminating, even after thorough washing, the impurities of sulfate and chloride anions that are present in the oxides, which have a negative effect on the quality of the pigments.
For example, in order to reduce these anions to values that are acceptable for the production of high-quality red pigments, it is necessary to treat the precipitated oxides with concentrated solutions of NaOH (U.S. Pat. No. 5,614,012).
GB 1226876 describes a method for producing highly pure FeOOH suitable for producing ferrites for use in electronic devices, wherein electrolytic iron with average dimension between 20 and 140 microns is reacted, in conditions of oxidation with air made to flow at high speed in order to maintain a uniform aqueous suspension of the iron particles, with an acid chosen among sulfuric acid, hydrochloric acid, nitric acid and acetic acid, used at a molar concentration of less than 0.01 and in a molar ratio with the iron of more than 0.02 and preferably between 0.26 and 0.55. Iron is used in an amount not exceeding 25 g/l and the weight ratio between the solution and the iron is at least 40.
The reaction temperature is between 50 and 70° C.: at temperatures above 70° C., there is an undesirable production of oxides such as spinel, which also form at temperatures below 70° C. if the iron concentration is higher than 25 g/l.
At temperatures below 50° C., the oxide particles that form are too fine and difficult to filter and wash in order to achieve values of impurities due to acid radicals of less than 0.1% by weight.
The sought dimensions of FeOOH are a few microns in length and more than 0.3 and 0.1 microns in width and thickness, respectively.
If the concentration of the acid is too high (more than 0.25 mol in the case of sulfuric acid), the FeOOH yield decreases even considerably due to the dissolving of iron ions in the mother liquor. The productivity of the method is 20-26 g of FeOOH per liter of suspension per hour.
SUMMARY OF THE INVENTION
The aim of the present invention is to provide a method for producing iron oxides that is capable of overcoming the difficulties of the methods of the prior art.
It has now been found unexpectedly that it is possible to reduce or eliminate the drawbacks of the processes of the prior art and obtain, with high productivities that are distinctly higher than those of hitherto known processes, and with substantially complete conversion of the iron to oxides that are practically free from alkaline, alkaline-earth and ammonium ions and in which after washing there are relatively small quantities of impurities related to anions of carboxylic acids, which however can be eliminated by heating in the step for converting the oxides to Fe
2
O
3
oxides.
DETAILED DESCRIPTION OF THE INVENTION
The method according to the invention comprises the following steps:
a) reaction, in an agitated aqueous solution, of an aliphatic and/or aromatic carboxylic acid containing one or more carboxylic groups, with a pKa of 0.5 to 6 at ambient temperature with reference to the first carboxyl, capable of decomposing by heating in air at a temperature of 200 to 350° C., forming carbon dioxide and water, and of forming ferrous salts soluble in the aqueous solution in the reaction conditions, with microspheroidal iron having an average particle diameter of no more than 250 microns, preferably between 30 and 200 microns, or, always acting under agitation, with iron turning scraps or cuttings having dimensions such that their surface is greater than 0.01 m
2
per kg of iron and per liter of solution, using ratios between moles of acid and g-atoms of iron from 0.03 to 1.5 and water/iron weight ratios from 1 to 20, working at temperatures between 0 and 100° C.;
b) oxidation to ferric carboxylate of the ferrous carboxylate formed in a) with an oxidizing agent, used in excess of the stoichiometric value with respect to the oxidation of the ferrous salt to ferric salt, selected from oxygen, gaseous mixture containing oxygen, for example air, hydrogen peroxide, ozone, organic peroxides and hydroperoxides.
As an alternative, it is also possible to subject to oxidation preformed ferrous carboxylates obtained from the acids indicated in a) or mixtures thereof with one or more of said acids, using a molar ratio between the anion contained in the salt and in the acid (if mixtures with acids are used) and g-atoms of iron from 0.03 to 1.5. All the other conditions of the alternative related to the type of iron, the water/iron weight ratio, the reaction temperature, the oxidizing agent and the agitation of the medium are the ones indicated in a) and b).
Preferably, the temperature in a) and b) is between 5 and 70° C., the ratio between moles of acid and g-atoms of Fe is 0.05 to 0.8, and the water/iron weigh ratio is 2 to 10.
The oxygen and the air are used in high excess of the stoichiometric value for oxidation of the ferrous ion to ferric ion; the hydrogen peroxide, the ozone and the peroxides are in an excess of 2-4 times or more.
During step a), it is preferred to work in a stream of inert gas, such as nitrogen, in order to avoid the formation of explosive mixtures of oxygen with the hydrogen generated during the iron dissolution reaction.
It is also possible to perform steps a) and b) simultaneously: this is true in particular when using hydrogen peroxide, peroxides or hydroperoxides as oxidizing agents.
When using air or oxygen, one operates in a strong stream of gas in order to remove the generated hydrogen, thus avoiding the forming of explosive mixtures.
The step of oxidation of the ferrous salt to ferric salt can be performed in the presence of nuclei of FeOOH prepared separately according to known methods.
The presence of nuclei allows to reduce the dimensions of the particles. For example, by using an inoculum of goethite in an amount between 4 and 10% by weight, and by working with oxygen at 30° C. and with an HCOOH/Fe ratio of 0.5 and total conversion of iron, a drastic reduction of the particle size is obtained.
The iron is preferably used in a microspheroidal form, with an average particle diameter of preferably 40 to 150 microns; the carbon content is preferably less than 0.15% by weight; elements such as manganese, nickel, copper and silicon, if present, are in quantities that are preferably lower than 1% by weight.
The manganese and nickel content is preferably lower than 0.05% by weight, and copper content is preferably lower than 0.2%.
Usable representative microspheroidal iron is obtained by metallurgical reprocessing, according to known methods, of scrap from the working of meta
Conca Esterino
Petrini Guido
Rubini Carlo
Koslow C. Melissa
Manlove Shalie
SUD Ehemie MT S.r.L.
Welsh & Katz Ltd.
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