PREPARATION OF IRON (III) OXIDE PIGMENTS
20170267544 · 2017-09-21
Assignee
Inventors
Cpc classification
B82Y30/00
PERFORMING OPERATIONS; TRANSPORTING
C01P2004/64
CHEMISTRY; METALLURGY
International classification
B82Y30/00
PERFORMING OPERATIONS; TRANSPORTING
Abstract
The present invention relates to an improved process for producing iron oxide red pigments by the Penniman process using nitrate (also referred to as nitrate process or direct red process).
Claims
1. A process for producing haematite pigments, the process comprising contacting iron, an aqueous haematite nucleus suspension containing haematite nuclei which have a particle size of 100 nm or less and a specific BET surface area of from 40 m.sup.2/g to 150 m.sup.2/g (measured in accordance with DIN 66131), and an iron(II) nitrate solution in the presence of at least one oxygen-containing gas at temperatures of from 70 to 99° C., to produce a suspension comprising haematite pigment.
2. The process according to claim 1, further comprising at least one of: adding the iron(II) nitrate solution to the iron to produce a first mixture, and subsequently adding the haematite nucleus suspension to the first mixture to produce a reaction mixture; adding the iron(II) nitrate solution and the haematite nucleus suspension simultaneously to the iron to produce a reaction mixture; and adding the haematite nucleus suspension to the iron to produce a first mixture, and subsequently adding the iron(II) nitrate solution to the first mixture to produce a reaction mixture; in such a way that the concentration of iron(II) nitrate in the reaction mixture from 0.1 to 25 g/l of liquid phase, preferably from 1 to 20 g/l of liquid phase, calculated on the basis of anhydrous Fe(NO.sub.3).sub.2.
3. The process according to claim 1, wherein the reaction is carried out until the haematite pigment has a desired colour shade.
4. The process according to claim 1, further comprising separating the haematite pigment from the haematite pigment suspension by conventional methods.
5. The process according to claim 1, wherein the haematite pigment has the α-Fe.sub.2O.sub.3 modification.
6. The process according to claim 1, wherein at least iron is initially charged and the haematite nucleus suspension is then added to the iron.
7. The process according to claim 1, wherein a mixture of iron is initially charged and the haematite nucleus suspension is then added to the mixture of iron.
8. The process according to claim 1, wherein the haematite nucleus suspension is firstly added to at least iron at a temperature of from 10 to 99° C. and, after the addition is complete, the iron(II) nitrate solution is added at a temperature of the reaction mixture of from 70 to 99° C.
9. A process for producing iron oxide red pigments, the process comprising contacting iron, haematite nucleus suspension containing haematite nuclei having a particle size of 100 nm or less and a specific BET surface area of from 40 m.sup.2/g to 150 m.sup.2/g (measured in accordance with DIN 66131), and iron(II) nitrate solution in the presence of at least one oxygen-containing gas at temperatures of from 70 to 99° C., wherein: the haematite nucleus suspension is added to at least iron, the iron(II) nitrate solution is added, either before the addition, after the addition, or simultaneously with the addition of the haematite nucleus suspension, in such a way that the concentration of iron(II) nitrate is from 0.1 to 25 g/l of liquid phase, preferably from 1 to 20 g/l of liquid phase, calculated on the basis of anhydrous Fe(NO.sub.3).sub.2, the reaction is carried out with mixing by means of mechanical and/or hydraulic mixing, and the at least one oxygen-containing gas is introduced into the reaction mixture with a gas introduction volume of 6 m.sup.3 of gas volume/m.sup.3 of batch volume/hour or less, preferably from 0.2 to 6 m.sup.3of gas volume/.sup.3 of batch volume/hour, particularly preferably from 0.2 to 5 m.sup.3 of gas volume/m.sup.3 of batch volume/hour, very particularly preferably from 0.2 to 3 m.sup.3 of gas volume/m.sup.3 of batch volume/hour, to produce a suspension of the haematite pigment.
10. The process according to claim 9, wherein the iron(II) nitrate solution is added before the addition of the haematite nucleus suspension, in such a way that the concentration of iron(II) nitrate is 1 to 20 g/l of liquid phase, calculated on the basis of anhydrous Fe(NO.sub.3).sub.2.
11. The process according to claim 9, wherein the iron(II) nitrate solution is added after the addition of the haematite nucleus suspension, in such a way that the concentration of iron(II) nitrate is 1 to 20 g/l of liquid phase, calculated on the basis of anhydrous Fe(NO.sub.3).sub.2.
12. The process according to claim 9, wherein the iron(II) nitrate solution is added simultaneously with the addition of the haematite nucleus suspension, in such a way that the concentration of iron(II) nitrate is 1 to 20 g/l of liquid phase, calculated on the basis of anhydrous Fe(NO.sub.3).sub.2.
13. The process according to claim 9, wherein the at least one oxygen-containing gas is introduced into the reaction mixture with a gas introduction volume of 0.2 to 6 m.sup.3 of gas volume/m.sup.3 of batch volume/hour.
14. The process according to claim 9, wherein the at least one oxygen-containing gas is introduced into the reaction mixture with a gas introduction volume of 0.2 to 3 m.sup.3 of gas volume/m.sup.3 of batch volume/hour.
Description
[0029] The invention further comprises apparatuses suitable for carrying out the process of the invention. These are described in more detail below with the aid of
[0030]
[0031] In
A Oxygen-containing gas
Fe Iron
[0032] AQ—Fe(NO.sub.3).sub.2 Iron(II) nitrate solution
S—Fe.sub.2O.sub.3 Haematite nucleus suspension
PAQ—Fe.sub.2O.sub.3 Haematite pigment suspension
H.SUB.2.O Water
[0033] NOX Nitrogen oxide-containing stream (offgas from the production of the haematite pigment suspension)
1 Reactor for producing haematite pigment suspension
11 Reaction vessel
12 Support for iron
13 Gas introduction unit
111 Inlet for iron(II) nitrate solution, haematite nucleus suspension
112 Outlet for NOX
[0034] 113 Outlet for haematite pigment suspension
114 Outlet for liquid phase
115 inlet for liquid phase
2 Stirring device
21 Drive
[0035] 22 Connection between drive and stirrer
23 Stirrer
31 Pump
[0036] Reactor 1 typically comprises one or more reaction vessels made of materials which are resistant to the starting materials. Single reaction vessels can be, for example, brick-lined or tiled vessels let into the earth. The reactors also comprise, for example, vessels made of glass, nitric acid-resistant plastics such as polytetrafluoroethylene (PTFE), steel, e.g. enamelled steel, plastic-coated or painted steel, stainless steel having the material number 1.44.01. The reaction vessels can be opened or closed. In preferred embodiments of the invention, the reaction vessels are closed. The reaction vessels are typically designed for temperatures in the range from 0 to 150° C. and for pressures of from 0.05 MPa (0.05 megapascal correspond to 0.5 bar) to 1.5 MPa (1.5 megapascal correspond to 15 bar).
[0037] A preferred embodiment of a reactor 1 is shown in
[0038] A further preferred embodiment of a reactor 1 has at least reaction vessel 11, support 12 for iron, gas introduction unit 13 for the at least one oxygen-containing gas A, inlet 111 for at least iron(II) nitrate solution and haematite nucleus suspension, outlet 112 for a nitrogen oxide-containing stream NOX, outlet 113 for the haematite pigment suspension, a stirring device 2 comprising a drive 21, a connection between drive and stirrer 22 and a stirrer 23.
[0039] A further preferred embodiment of a reactor 1 has at least reaction vessel 11, support 12 for iron, gas introduction unit 13 for the at least one oxygen-containing gas A, inlet 111 for at least iron(II) nitrate solution and haematite nucleus suspension, outlet 112 for a nitrogen oxide-containing stream NOX, outlet 113 for the haematite pigment suspension, outlet for liquid phase 114, inlet for liquid phase 115 and a pump 31.
[0040] The process of the invention is described in more detail below. The aqueous haematite nucleus suspensions used in the process of the invention and the haematite nuclei present therein are known from the prior art. On the subject, reference is made to the description of the prior art. The haematite nuclei present in the water-containing haematite nucleus suspensions comprise nuclei having a particle size of 100 nm or less and a specific BET surface area of from 40 m.sup.2/g to 150 m.sup.2/g (measured in accordance with DIN 66131). The criterion of the particle size is satisfied when at least 90% of the haematite nuclei have a particle size of 100 nm or less, particularly preferably from 30 nm to 90 nm. The aqueous haematite nucleus suspensions used in the process of the invention typically comprise haematite nuclei having a round, oval or hexagonal particle shape. The finely divided haematite typically has a high purity. Foreign metals present in the iron scrap used for producing the haematite nucleus suspension are generally manganese, chromium, aluminium, copper, nickel, cobalt and/or titanium in a variety of concentrations, which can also be precipitated as oxides or oxyhydroxides and incorporated into the finely divided haematite during the reaction with nitric acid. The haematite nuclei present in the water-containing haematite nucleus suspension typically have a manganese content of from 0.1 to 0.7% by weight, preferably from 0.4 to 0.6% by weight. Strongly coloured red iron oxide pigments can be produced using nuclei of this quality.
[0041] The iron(II) nitrate solutions used in the process of the invention are known from the prior art. On the subject, reference is made to the description of the prior art. These iron(II) nitrate solutions typically have concentrations of from 50 to 150 g/l of Fe(NO.sub.3).sub.2 (reported as Fe(NO.sub.3).sub.2 based on water-free matter). Apart from Fe(NO.sub.3).sub.2, the iron(II) nitrate solutions can also contain amounts of from 0 to 50 g/l of Fe(NO.sub.3).sub.3. However, very small amounts of Fe(NO.sub.3).sub.3 are advantageous.
[0042] As iron, use is usually made in the process of the invention of iron in the form of wire, sheet, nails, granules or coarse turnings. The individual pieces can have any shape and usually have a thickness (e.g. measured as diameter of a wire or as thickness of a sheet) of from about 0.1 millimetre to about 10 mm. The size of wire bundles or of sheets used in the process usually depends on practicabilities. Thus, the reactor has to be able to be filled without difficulty with this starting material, which is generally effected through a manhole. Such iron is produced, inter alia, as scrap or as by-product in the metal processing industry, for example stamping sheets.
[0043] The iron used in the process of the invention generally has an iron content of >90% by weight. Impurities present in this iron are usually foreign metals such as manganese, chromium, silicon, nickel, copper and other elements. However, iron having a higher purity can also be used without disadvantages. Iron is typically used in an amount of from 20 to 150 g/l based on the volume of the reaction mixture at the beginning of the reaction according to the invention. In a further preferred embodiment, the iron, preferably in the form of stamping sheets or wires, is distributed on the iron support over the area thereof with a preferred bulk density of less than 2000 kg/m.sup.3, particularly preferably less than 1000 kg/m.sup.3. The bulk density can, for example, be achieved by bending sheets of at least one iron grade and/or by targeted laying of the iron. This leads to typically more than 90% by volume of the oxygen-containing gas blown in under the iron support to pass through the iron support without the oxygen-containing gas building up under the iron support.
[0044] The iron support, for example support 12, makes exchange of suspension and gas through the openings present in the iron support possible. Typical embodiments of the iron support can be sieve trays, perforated trays or meshes. In one embodiment, the ratio of the cumulated area of openings to the total support area is from 0.1 to 0.9, preferably from 0.1 to 0.3. The holes or openings required for exchange of suspension are typically selected so that falling of the iron through the iron support is largely prevented. The iron support, for example support 12, can correspond to the diameter of the internal diameter of the reactor, for example the internal diameter of the reaction vessel 11, or be made smaller. In the latter case, a wall is preferably installed at the side of the iron support device so as to prevent iron from falling down. This wall can be permeable to the suspension, for example configured as mesh, or impermeable to the suspension and have, for example, the shape of a tube or a cuboid open at the top.
[0045] In the process of the invention, the reaction of at least iron, haematite nucleus suspension and iron(II) nitrate solution in the presence of at least one oxygen-containing gas is carried out at temperatures of from 70 to 99° C.
[0046] The at least one oxygen-containing gas is preferably selected from among air, oxygen, air heated to above ambient temperature or air enriched with steam.
[0047] In one embodiment of the process of the invention, iron is placed on the support and the haematite nucleus suspension is then added to this iron. In a further embodiment of the process of the invention, a mixture or iron and water is initially charged and the haematite nucleus suspension is then added to the mixture of iron and water. In a further embodiment, the temperature of the resulting mixture can be from 10 to 99° C.
[0048] In further embodiments of the process of the invention, iron(II) nitrate solution is, after the addition or simultaneously with the addition of the haematite nucleus suspension, added to the mixture of at least iron and haematite nucleus suspension in such a way that the concentration of iron(II) nitrate is from 0.1 to 25 g/l of liquid phase, preferably from 1 to 20 g/l of liquid phase, calculated on the basis of anhydrous Fe(NO.sub.3).sub.2. In further embodiments of the process of the invention, iron(II) nitrate solution is, before addition of the haematite nucleus suspension, added to at least iron in such a way that the concentration of iron(II) nitrate is from 0.1 to 25 g/l of liquid phase, preferably from 1 to 20 g/l of liquid phase, calculated on the basis of anhydrous Fe(NO.sub.3).sub.2. In a preferred embodiment, the temperature of the reaction mixture during the addition of the iron(II) nitrate solution is from 70 to 99° C. The content of iron(II) nitrate is typically determined indirectly via measurement of the iron(II) content of a sample of the liquid phase by potentiometric titration of a sample of the liquid phase which has been acidified with hydrochloric acid using cerium(III) sulfate. The maximum concentration of iron(II) nitrate based on the liquid phase is determined by the rate of addition of the iron(II) nitrate solution to the mixture of at least iron and haematite nucleus suspension. During the process of the invention, iron(II) nitrate is consumed by the reaction with the iron. In a preferred embodiment, the process of the invention is carried out in such a way that the concentration of iron(II) nitrate of from 0.1 to 25 of liquid phase, preferably from 1 to 20 g/l of liquid phase, calculated on the basis of anhydrous Fe(NO.sub.3).sub.2, is maintained over from 70 to 100%, more preferably from 80 to 100%, of the total reaction time. In a further preferred embodiment, the process of the invention is carried out in such a way that the concentration of iron(II) nitrate of from 0.1 to 25 g/l of liquid phase, preferably from 1 to 20 g/l of liquid phase, calculated on the basis of anhydrous Fe(NO.sub.3).sub.2, is maintained over from 70 to 100%, more preferably from 80 to 100%, of the range of the reaction time from 0 to 50 hours. According to the invention, the beginning of the reaction time is defined as the beginning of the addition of the iron(II) nitrate solution and the end of the reaction time is defined as the end of the introduction of the at least one oxygen-containing gas. The beginning of the addition of the iron(II) nitrate solution is, according to the invention, defined as the point in time at which 1% by weight of the total amount of iron(II) nitrate has been added to the mixture of at least iron and haematite nucleus suspension. The end of the introduction of the at least one oxygen-containing gas is, according to the invention, defined as the point in time at which the rate of introduction of the at least one oxygen-containing gas is less than 0.1 m.sup.3 of gas volume/m.sup.3 of batch volume/hour.
[0049] During the process of the invention, the pigment is built up on the haematite nucleus present in the liquid phase, producing a haematite pigment suspension whose colour values, preferably a* and b* values, in the surface coating test change during the reaction as a result of the changing particle size and/or morphology during the pigment buildup. The point in time at which the process of the invention is stopped is determined by measuring the colour values of the haematite pigment present in the haematite pigment suspension. The process of the invention is stopped when the haematite pigment has the desired colour shade, preferably the desired a* and b* values in full shade or in reduction, in the surface coating test. This is effected by stopping the introduction of gas, optionally by simultaneous cooling of the reaction mixture to a temperature below 70° C. Typical reaction times for the reaction according to the invention are from 10 to 150 hours, depending on the desired colour shade.
[0050] In a preferred embodiment, the haematite pigment is separated off from the haematite suspension by customary methods, preferably by filtration and/or sedimentation and/or centrifugation, after the reaction according to the invention. Washing of the filter cake obtained after the separation and subsequent drying of the filter cake are preferably likewise carried out. One or more sieving steps, particularly preferably using different mesh openings and decreasing mesh openings, are likewise preferably carried out before separation of the haematite pigment from the haematite pigment suspension. This has the advantage that foreign bodies, for example pieces of metal, which would otherwise contaminate the haematite pigment are thereby separated off from the haematite pigment suspension.
[0051] The separation of the haematite pigment from the haematite pigment suspension can be carried out using all methods known to those skilled in the art, e.g. sedimentation with subsequent removal of the aqueous phase or filtration by means of filter presses, for example by means of membrane filter presses.
[0052] In a preferred embodiment of the process of the invention, at least one sulfate salt, for example iron(II) sulfate and/or an alkali metal or alkaline earth metal sulfate, preferably iron(II) sulfate and/or sodium sulfate, can be added to the haematite pigment suspension during or before sieving and/or during or before the separation. This has the advantage that the sedimentation of the haematite pigment from the haematite pigment suspension is accelerated. This makes the subsequent isolation of the haematite pigment easier.
[0053] At least one wash of the sediment or filter cake separated off in this way is then optionally carried out. Drying of the haematite pigment separated off in this way, for example by means of filter dryers, belt dryers, kneading dryers, spin flash dryers, drying ovens or spray dryers, is optionally carried out after the separation and/or washing. Drying is preferably carried out by means of belt dryers, plate dryers, kneading dryers and/or spray dryers.
[0054] It has surprisingly been found that significantly more haematite pigment is produced per amount of Fe(NO.sub.3).sub.2 used in the process of the invention compared to the processes of the prior art in which the iron(II) nitrate solution is added all at once at the beginning of the buildup of the pigment. Compared to the processes of the prior art, a greater proportion of the Fe.sup.3+ present in the haematite pigment comes from the iron and a smaller proportion of the Fe.sup.3+ present in the haematite pigment comes from the Fe(NO.sub.3).sub.2 in the process of the invention. In the process according to the prior art, in which the iron(II) nitrate solution is added all at once to the mixture of haematite nucleus suspension, iron and water and the amount of gas introduced is, for example, 10 m.sup.3 of air/m.sup.3 of batch volume/hour, 1.7 kg of Fe.sub.2O.sub.3 or less are usually produced per kg of Fe(NO.sub.3).sub.2. However, if the amount of gas introduced is, for example, reduced to 2 m.sup.3 of air/m.sup.3 of batch volume/hour, only 0.6 kg of Fe.sub.2O.sub.3 or less is formed per kg of Fe(NO.sub.3).sub.2. However, if the pigment buildup is, according to the invention, carried out with mechanical and/or hydraulic mixing and with an amount of gas introduced of likewise 2 m.sup.3 of air/m.sup.3 of batch volume/hour and if the iron(II) nitrate solution is added all at once at the beginning of the buildup of the pigment, 1.2 kg of Fe.sub.2O.sub.3 or more are produced per kg of Fe(NO.sub.3).sub.2, preferably from 1.2 to 2.2 kg of Fe.sub.3O.sub.3 per kg of Fe(NO.sub.3).sub.2.
[0055] In a further embodiment, the process of the invention comprises the reaction of iron, haematite nucleus suspension containing haematite nuclei having a particle size of 100 nm or less and a specific BET surface area of from 40 m.sup.2/g to 150 m.sup.3/g (measured in accordance with DIN 66131) and iron(II) nitrate solution in the presence of at least one oxygen-containing gas at temperatures of from 70 to 99° C., characterized in that the reaction is carried out with mixing of the liquid phase by means of mechanical and/or hydraulic mixing and the introduction of at least one oxygen-containing gas is carried out using a gas introduction volume of 6 m.sup.3 of gas volume/m.sup.3 of batch volume/hour or less, preferably from 0.2 to 6 m.sup.3 of gas volume/m.sup.3 of batch volume/hour, particularly preferably from 0.2 to 5 m.sup.3 of gas volume/m.sup.3 of batch volume/hour, very particularly preferably from 0.2 to 3 m.sup.3 of gas volume/m.sup.3 of batch volume/hour.
[0056] The process of the invention is described in more detail below. For the purposes of the present invention, mechanical and/or hydraulic mixing is the mixing of the liquid phase by means of suitable devices. According to the invention, the liquid phase also contains solids suspended therein, for example the haematite nuclei or the haematite pigment or else further solids such as iron particles. Suitable devices for the mechanical mixing encompass stirring devices, for example axial stirrers, radial stirrers and tangential stirrers. Stirring devices such as the stirring device 2 in
[0057] In one embodiment, baffles are additionally present on the interior wall of the reaction vessel 1 in the case of radially acting stirrers. Corotation of the liquid phase and the resulting formation of vortices is thus avoided.
[0058] The degree of mechanical mixing is defined by the outer circumferential velocity of the stirrer, for example the stirrer 23. Preferred circumferential velocities are 0.5-15 m/s, measured at the circumference of the circle formed by the diameter of the stirrer. The power input into the liquid phase, which can be derived from the power uptake of the stirrer, is, according to the invention, from 0.1 to 5 kW per m.sup.3 of batch volume, preferably from 0.4 to 3 kW per m.sup.3 of batch volume. The ratio of stirrer diameter to the internal diameter of the reactor is preferably from 0.1 to 0.9. The power input into the liquid phase is given by the power uptake of the stirrer multiplied by the efficiency of the stirrer in percent. Typical efficiencies of stirrers used in the process of the invention are in the range from 70 to 90%.
[0059] For the purposes of the invention, circumferential velocities of from 1 to 15 m/s and a power input of at least 0.4 kW/m.sup.3 of batch volume are particularly preferred.
[0060] Hydraulic mixing is carried out by means of a pump, for example pump 31, which takes the liquid phase from the reactor at an outlet, for example outlet 114, and feeds it back into the reactor at another place by an inlet, for example inlet 115. Flows are produced at the inlet and outlet and also in the entire reaction mixture. For the purposes of the invention, pump circulation volumes of from 0.1 to 20 batch volumes/hour are preferred. For example, the amounts circulated by pumping at a batch volume of 30 m.sup.3 and a value of 5 batch volumes/hour is 150 m.sup.3/hour. In a further embodiment, amounts circulated by pumping which generate a flow velocity at the inlet, for example inlet 115, of at least 0.05 m/s, preferably from at least 0.06 to 15 m/s, are preferred. Here, the flow velocities are measured at the inlet directly at the transition of the line from which the pumped liquid phase flows into the reaction mixture in the interior of the reactor. In a further embodiment, the flow is directed from the inlet, for example inlet 115, onto the iron support, for example iron support 12, preferably directed from underneath the iron support onto the iron support at a distance of less than 2 m, preferably less than 1 m. In a further embodiment, the inlet, for example inlet 115, is configured as a pipe or as a two-fluid sprayer or as nozzle.
[0061] It may be remarked at this point that the scope of the invention encompasses all possible combinations of the general ranges or preferred ranges mentioned above and in the following for components, value ranges or process parameters.
[0062] According to the process of the invention, the reaction of at least iron, haematite nucleus suspension and iron(II) nitrate solution is carried out with at least the liquid phase present during the reaction being mixed by means of mechanical and/or hydraulic mixing. Since suspended haematite is present in the liquid phase, the mechanical and/or hydraulic mixing is preferably carried out in such a way that the haematite suspended in the liquid phase remains uniformly dispersed in the liquid phase and does not accumulate in the lower part of the liquid phase.
[0063] According to the process of the invention, the reaction of at least iron, haematite nucleus suspension and iron(II) nitrate solution is carried out with introduction of at least one oxygen-containing gas at a gas introduction volume of 6 m.sup.3 of gas volume/m.sup.3 of hatch volume/hour or less, preferably from 0.2 to 6 m.sup.3 of gas volume/m.sup.3 of batch volume/hour, particularly preferably from 0.2 to 5 m.sup.3 of gas volume/m.sup.3 of batch volume/hour, very particularly preferably from 0.2 to 3 m.sup.3 of gas volume/m.sup.3 of batch volume/hour. According to the invention, the reaction mixture comprises all starting materials and the solid, liquid and gaseous products formed therefrom. A nitrogen oxide-containing stream NOX is formed during the reaction. In a preferred embodiment, the nitrogen oxide-containing stream NOX is discharged from the reactor, for example via the outlet 112 of reactor 1. The batch volume is, according to the invention, defined as total volume of the liquid and solid constituents of the reaction mixture which is present in the reaction vessel, for example in reactor 1, at a particular point in time during the reaction. The batch volume can, for example, be determined at any point in time during the reaction via a fill level indicator on the reactor in which the reaction is carried out.
[0064] The introduction of at least one oxygen-containing gas is preferably carried out by introducing at least one oxygen-containing gas into the liquid phase of the reaction mixture underneath the iron support, for example support 12. The introduction of the gas is preferably carried out using a gas introduction unit, for example gas introduction unit 13, e.g. sparging ring, nozzles, (two)-fluid sprayers or a pipe ring provided with holes, which is located within the reaction mixture. For this purpose, the at least one oxygen-containing gas must have a sufficient pressure to counter the hydrostatic pressure of the liquid column of the reaction mixture.
[0065] It has surprisingly been found that significantly more haematite pigment is produced per amount of Fe(NO.sub.3).sub.2 used in the process of the invention compared to the process of the prior art in which the buildup of pigment takes place at small amounts of introduced gas without mechanical and/or hydraulic mixing. Compared to the process of the prior art, a larger proportion of the Fe.sup.3+ present in the haematite pigment originates from the iron and a smaller proportion of the Fe.sup.3+ present in the haematite pigment originates from the Fe(NO.sub.3).sub.2 in the process of the invention. In the process of the prior art in which the gas introduction amounts used are 6 m.sup.3 of gas volume/m.sup.3 of batch volume/hour of reaction time or less but no mechanical and/or hydraulic mixing occurs, 1.0 kg of Fe.sub.2O.sub.3 per kg of Fe(NO.sub.3).sub.2 or less is usually produced. However, in the process of the invention, 1.2 kg of Fe.sub.2O.sub.3 per kg of Fe(NO.sub.3).sub.2 or more, preferably from 1.2 to 2.5 kg of Fe.sub.2O.sub.3 per kg of Fe(NO.sub.3).sub.2, are produced. As a result, the process is more economical since less iron(II) nitrate solution, which in contrast to the iron used has to be produced separately, is required for the production process. In addition, a significantly smaller amount of nitrogen oxides is discharged into the gas phase in the process of the invention due to the smaller gas introduction volumes compared to the prior art. In the process according to the prior art, in which a large amount of introduced gas of greater than 6 m.sup.3 of gas volume/m.sup.3 of batch volume/hour of reaction time of oxygen-containing gas is used but no mechanical and/or hydraulic mixing occurs, 80 g of nitrous gases such as NO and NO.sub.2 (always calculated as NO.sub.2) or more per kilogram of pigment produced and 40 g of dinitrogen monoxide or more per kilogram of pigment produced are typically given off from the reaction mixture into the ambient air. In addition, the nitrogen oxides dissolved in the liquid phase themselves serve as oxidant like the at least one oxygen-containing gas which oxidizes iron to Fe.sup.3+. Here, the nitrogen oxides in which the nitrogen has the oxidation numbers +1 to +5 are reduced either to nitrogen, i.e. to N.sub.2, which has the oxidation number 0 or to ammonium compounds in which the nitrogen has the oxidation number −3. As a result, significantly smaller amounts of nitrogen oxides and/or ammonium compounds, which have to he removed in a complicated fashion by gas scrubs or other gas or wastewater purification methods, arise in the process of the invention. Furthermore, significantly less energy is carried out from the reaction mixture heated to from 70 to 99° C. into the gas phase due to the lower gas introduction volumes compared to the prior art. Since the amount of Fe.sub.2O.sub.3 formed per kg of Fe(NO.sub.3).sub.2 significantly increased for the same amount of introduced gas, the amount of iron nitrate used in the buildup of the pigment can accordingly be reduced to the same degree without decreases in yield of haematite pigment.
[0066] In a further embodiment, the process for producing iron oxide red pigments comprises the reaction of iron, haematite nucleus suspension containing haematite nuclei having a particle size of 100 nm or less and a specific BET surface area of from 40 m.sup.2/g to 150 m.sup.2/g (measured in accordance with DIN 66131) and iron(II) nitrate solution in the presence of at least one oxygen-containing gas at temperatures of from 70 to 99° C., characterized in that the reaction [0067] is carried out with mixing of the liquid phase by means of mechanical and/or hydraulic mixing and [0068] the introduction of at least one oxygen-containing gas is carried out with a gas introduction volume of 6 m.sup.3 of gas volume/m.sup.3 of batch volume/hour car less, preferably from 0.2 to 6 mm.sup.3 of gas volume/m.sup.3 of batch volume/hour, particularly preferably from 0.2 to 5 m.sup.3 of gas volume/m.sup.3 of batch volume/hour, very particularly preferably from 0.2 to 3 m.sup.3 of gas volume/m.sup.3 of batch volume/hour, and [0069] the haematite nucleus suspension is added to at least iron and [0070] the iron(II) nitrate solution is, before the addition, after the addition or simultaneously with the addition of the haematite nucleus suspension, added to the mixture of at least haematite nucleus suspension and iron in such a way that the concentration of iron(II), nitrate is from 0.1 to 25 g/l of liquid phase, preferably from 1 to 20 g/l of liquid phase, calculated on the basis of anhydrous Fe(NO.sub.3).sub.2.
producing a suspension of the haematite pigment.
[0071] In this embodiment of the process of the invention, 1.5 kg of Fe.sub.2O.sub.3 per kg of Fe(NO.sub.3).sub.2 or more, preferably from 1.8 to 3 kg of Fe.sub.2O.sub.3 per kg of Fe(NO.sub.3).sub.2, are surprisingly produced. Furthermore, in this embodiment, 20 g of nitrous gases such as NO and NO.sub.2 (always calculated as NO.sub.2) or less per kilogram of haematite pigment produced and 20 g of dinitrogen monoxide or less per kilogram of haematite pigment produced are typically given off from the reaction mixture into the ambient air.
EXAMPLES AND METHODS
[0072] Titration of Iron(II) and Iron(III) Determination:
[0073] The content of iron(II) nitrate can be determined indirectly by measuring the iron(II) content by a potentiometric titration of a sample solution acidified with hydrochloric acid using cerium(III) sulfate.
[0074] NO.sub.x Measurement
[0075] NO.sub.x measurements were carried out using a gas analyzer PG 250 from Horriba (chemiluminescence method). Information about NO.sub.x formation were reported as a ratio to the pigment yield (calculated as NO.sub.2, in g of NO.sub.2/kg of pigment). The NO.sub.x emission arising in the production of the starting materials haematite nucleus and iron nitrate are not included.
[0076] N.sub.2O Measurement
[0077] Laughing gas measurements were carried out by means of a quantitative gas-chromatographic determination and/or by infrared measurement. Information on N.sub.2O formation was reported as a ratio to the pigment yield (g of N.sub.2O/kg of pigment). The N.sub.2O emission arising in the production of the starting materials haematite nucleus and iron nitrate are not included.
Examples 1-8
[0078] Examples 1 to 8 were carried out in the same reactor on a comparable scale (amounts of iron used from 55 to 60 kg), with the identical conditions and the identical relevant ratios between the amounts of starting materials and the volumes of the solutions being set. The iron used is generally present in excess. Parameters varied were: amount of gas introduced per unit volume; stirring: yes or no, stirrer speed, pump circulation: yes or no, amount circulated by pumping, flow velocity. These parameters are given for each example in Table 1.
[0079] A detailed description of the experiment is given below for Example 7.
[0080] 55 kg of iron sheet having a thickness of about 1 mm were placed in a 1 m.sup.3 reactor equipped with sieve trays (measure opening about 10 mm), sparging, ring (at the bottom of the reactor), circulation by pumping and inclined-blade stirrer. The sparging ring and the stirrer are installed underneath the sieve tray, the outlet of the pump circulation is located at the side of the iron bed and the intake of the pump circulation is located at the bottom of the reactor. The iron sheet was distributed uniformly on the sieve tray with a bulk density of 0.6-0.8 kg/l. Water, iron(II) nitrate solution (corresponding to 25.2 kg of Fe(NO.sub.3).sub.2 calculated as anhydrous Fe(NO.sub.3).sub.2, Fe(NO.sub.3).sub.2 concentration=120 g/l) and haematite nucleus suspension (corresponding to 16.1 kg of Fe.sub.2O.sub.3, concentration=130 g/l) were subsequently added in such amounts that a batch volume of 700 litres is attained and the concentration of nucleus (calculated as anhydrous Fe.sub.2O.sub.3) is 23 g/l and the concentration of iron nitrate (calculated as anhydrous Fe(NO.sub.3).sub.2) is 36 g/l. The mixture was heated to 85° C. with the stirrer switched on (140 rpm, 33 m/s, inclined-blade stirrer, 50 cm diameter) and pump circulation switched on and after reaching the temperature sparged with 2 m.sup.3 of air/batch volume/hour for 70 hours until an iron(II) nitrate concentration below 0.1 g/l of reaction mixture was reached. Stirring and pumped circulation were continued during the entire reaction time. After the introduction of gas had been ended, an iron(II) nitrate concentration of <0.1 of liquid phase was measured. The reaction mixture was then filtered through a filter press and the haematite pigment obtained was washed with water. The haematite pigment was subsequently dried at 80° C. to a residual moisture content of less than 5% by weight. The dried filter cake was subsequently broken up mechanically by means of a shredder. The haematite pigment was in this way obtained in powder form in a yield of 67.0 kg. The amount of Fe.sub.2O.sub.3 nuclei used is 16.1 kg, and the amount of freshly formed Fe.sub.2O.sub.3 (67.0 kg-16.1 kg) is 50.9 kg. The amount of iron(II) nitrate reacted is 25.2 kg. This gives a calculated ratio of 50.9 kg/25.2 kg=2.0 kg of Fe.sub.2O.sub.3 per kg of Fe(NO.sub.3).sub.2.
[0081] Table 1 shows the process parameters which were varied for Examples 1-3 (comparative examples) and Examples 4 to 8 (according to the invention) and also the ratio of kg of Fe.sub.2O.sub.3 per kg of Fe(NO.sub.3).sub.2. The results for the amounts of offgases NO.sub.x and N.sub.2O formed are shown in Table 2.
TABLE-US-00001 TABLE 1 Amount of gas kg of introduced Fe.sub.2O.sub.3 per unit formed/kg Pump volume of Fe(NO.sub.3).sub.2 Example Stirring circulation [m.sup.3/m.sup.3/h] consumed 1 (for No No 1 0.3 comparison) 2 (for No No 2 0.6 comparison) 3 (for No No 10 1.7 comparison) 4 50 rpm (1.3 m/s No 2 1.3 circumferential velocity) (Power input 0.19 kW/m.sup.3) 5 80 rpm (2.1 m/s No 2 1.5 circumferential velocity, power input 0.3 kW/m.sup.3) 6 140 rpm (3.7 m/s No 2 1.8 circumferential velocity, power input 0.71 kW/m.sup.3) 7 140 rpm (3.7 m/s 12 m.sup.3/h 2 2.0 circumferential velocity, power input 0.71 kW/m.sup.3) 8 No 12 m.sup.3/h 2 1.4
TABLE-US-00002 TABLE 2 Amount of NO/NO.sub.2 Amount of N.sub.2O [calculated as NO.sub.2 [in g/kg of in g/kg of haematite haematite Example pigment] pigment] 3 (for 114 57 comparison) 7 9 14
Examples 9-12
[0082] Examples 9 to 12 were carried out in the same reactor on a comparable scale (amounts of iron used from 55 to 60 kg), with the identical conditions and the identical relative ratios between the amounts of starting materials and the volumes of the solutions being set. The iron used is generally present in excess. Parameters varied were: amount of gas introduced per unit volume; stirring: yes or no, pumped circulation: yes or no, introduction of iron(II) nitrate solution: yes or no. These parameters are given for each example in Table 1.
[0083] A detailed description of the experiment is given below for Example 11.
[0084] 55 kg of iron sheet having a thickness of about 1 mm were placed in 1 m.sup.3 reactor equipped with sieve tray (mesh opening about 10 mm), sparging ring (at the bottom of the reactor), pump circulation and inclined-blade stirrer. The sparging ring and the stirrer are installed underneath the sieve tray, the outlet of the pump circulation is located at the side of the iron bed and the intake of the pump circulation is located at the bottom of the reactor. The iron sheet was uniformly distributed on the sieve tray. Water and haematite nucleus suspension (corresponding to 16.1 kg of Fe.sub.2O.sub.3 calculated as anhydrous Fe.sub.2O.sub.3) is subsequently added in such amounts that a volume of 490 litres is attained. The mixture was heated to 85° C. with the stirrer switched on (140 rpm, 3.7 m/s, inclined-blade stirrer, 50 cm diameter) and pump circulation switched on (12 m.sup.3/m.sup.3 of batch volume/hour) and after reaching this temperature sparged with 2 m.sup.3 of air/m.sup.3 of batch volume/hour. After the temperature of 85° C. had been reached, an iron(II) nitrate solution (210 litres, concentration=120 g/l, calculated on the basis of anhydrous Fe(NO.sub.3).sub.2) was introduced over a period of 24 hours in such a way that the Fe(NO.sub.3).sub.2 concentration does not exceed 20 g/l of liquid phase through to the end of the Fe(NO.sub.3).sub.2 addition. The volume of the iron(II) nitrate solution after introduction had been ended was 700 litres. During the addition of the iron(II) nitrate solution and for a further 46 hours afterwards, sparging was continued so that gas was introduced for a total of 70 hours. Stirring and pump circulation were continued during the entire reaction time. After introduction of gas for 70 hours, an iron(II) nitrate concentration of <0.1 g/l of reaction mixture was attained. The reaction mixture was then filtered through a filter press and the haematite pigment obtained was washed with water. The haematite pigment was subsequently dried at 80° C. to a residual moisture content of less than 5% by weight. The dried filter cake was subsequently broken up mechanically by means of a shredder. The haematite pigment was in this way obtained in powder form in a yield of 78.9 kg.
[0085] The amount of Fe.sub.2O.sub.3 nuclei used was 16.1 kg, and the amount of freshly formed Fe.sub.2O.sub.3 (76 kg-16.1 kg) is 59.9 kg. The amount of iron(II) nitrate reacted was 25.2 kg. This gives a calculated ratio of 59.9 kg/25.2 kg=2.4 kg of Fe.sub.2O.sub.3 per kg of Fe(NO.sub.3).sub.2.
[0086] Table 3 shows the ratios of kg of Fe.sub.2O.sub.3 per kg of Fe(NO.sub.3).sub.2 for Example 7 and also the results for the amounts of offgases NO.sub.x and N.sub.2O formed.
TABLE-US-00003 TABLE 3 kg of Fe.sub.2O.sub.3 formed/kg of Introduction of Fe(NO.sub.3).sub.2 Example iron(II) nitrate consumed 7 Total amount was 2.0 added all at once, initial concentration: 36 g/l 11 Introduction so that 2.4 the iron(II) nitrate concentration is below 20 g/l
TABLE-US-00004 TABLE 4 Amount of gas NO/NO.sub.2 introduced calculated per unit as NO.sub.2 in N.sub.2O in volume Fe(NO.sub.3).sub.2 g/kg of g/kg of Example Mixing m.sup.3/m.sup.3/h addition pigment pigment 3 None 10 (air) All into 114 57 the initial charge 9 None 10 (air) Below 20 g/l 73 46 by slow addition 7 Stirrer: 2 (air) All into 9 14 140 rpm the initial (3.7 m/s) charge Pump circulation: 12 m.sup.3/h 11 Stirrer: 2 (air) Below 20 g/l 5 10 140 rpm by slow (3.7 m/s) addition Pump circulation: 12 m.sup.3/h 12 Pump 6 (air) All into 78 44 circulation: the initial 12 m.sup.3/h charge
[0087] An amount circulated by pumping of 12 m.sup.3/hour corresponds in the test reactor to an an amount circulated by pumping of 17 batch volumes/hour and a flow velocity at the outlet of 1.4 m/s. An amount circulated by pumping of 6 m.sup.3/hour corresponds in the test reactor to an amount circulated by pumping of 8.5 batch volumes/h and a flow velocity at the outlet of 0.7 m/s.