PREPARATION OF RED IRON OXIDE PIGMENT

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) and apparatuses for carrying out the process.

Claims

1. Apparatus for producing haematite from iron(II) nitrate, haematite nuclei, iron, and oxygen, the apparatus comprising: a reaction vessel for contacting iron(II) nitrate, haematite nuclei, iron, and oxygen to produce haematite pigment and nitrogen oxide, support for iron, the support comprising a base and an outer delimitation disposed around at least a portion of the base, a holder retaining the support relative to the reaction vessel, at least one gas introduction unit disposed relative to the support for introducing oxygen-containing gas to contact iron supported in the support, an inlet for introducing a liquid reaction medium of iron(II) nitrate solution, haematite nucleus suspension, and optionally water into the reaction vessel, an outlet for removal of a nitrogen oxide-containing stream (NOX), and an outlet for removal of haematite pigment suspension.

2. The apparatus for producing haematite according to claim 1, wherein the outer delimitation comprises an impermeable wall, a wall provided with openings, mesh rods, a sieve, or a combination thereof.

3. The apparatus for producing haematite according to claim 1, wherein the outer delimitation has a vertical height dimension extending from the base and comprising a lower region and an upper region, and is impermeable to the liquid reaction mixture in at least the lower region, preferably from 10 to 50% of the height of the delimitation, measured from base.

4. The apparatus for producing haematite according to claim 3, wherein the outer delimitation is permeable to the liquid reaction mixture in the upper region, preferably from 50 to 90% of the height of the delimitation, measured from base.

5. The apparatus for producing haematite according to claim 1, wherein the base defines a circumference, and the outer delimitation is joined with the base around the full circumference.

6. The apparatus for producing haematite according to claim 1, wherein the base defines a circumference, and the outer delimitation is joined with the base around part of the circumference and the other part of the circumference joined to an interior wall of the reaction vessel.

7. The apparatus for producing haematite according to claim 1, wherein: the base of the support comprises openings therethrough defining a gas introduction portion for passage of gas into the support, and the area formed by the gas introduction portion represents a gas introduction area (GA); the reaction vessel comprises an inner surface that defines a wall circumference, and the area formed by the circumference of the inner surface represents a reactor area (RA); and a ratio of the gas introduction area (GA) to the reactor area (RA) is designed so that a gas introduction volume of from 8 to 32 m.sup.3, preferably from 8 to 24 m.sup.3, of oxygen-containing gas per m of gas introduction area per hour is obtained at a gas introduction volume of from 0.5 to 2 m.sup.3 of oxygen-containing gas per m.sup.3 of reaction volume per hour, or a gas introduction volume of from 8 to 16 m.sup.3, preferably from 8 to 12 m.sup.3, of oxygen-containing gas per m.sup.2 of gas introduction area per hour is obtained at a gas introduction volume of from 0.5 to 1 m.sup.3 of oxygen-containing gas per m of reaction volume per hour, or a gas introduction volume of from 16 to 32 m.sup.3, preferably from 24 to 32 m.sup.3, of oxygen-containing gas per m.sup.3 of gas introduction area per hour is obtained at a gas introduction volume of from 1 to 2 m.sup.3 of oxygen-containing gas per m.sup.3 of reaction volume per hour.

8. A process for producing haematite, the process comprising contacting iron; a water-containing 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/g to 150 m.sup.2/g, (measured in accordance with DIN 66131); an iron(II) nitrate solution and oxygen-containing gas at temperatures of 70 to 120 C., preferably from 70 to 99 C., to a liquid reaction mixture which contains at least one liquid phase and haematite suspended therein, wherein the contacting comprises introducing the oxygen-containing gas into the liquid reaction mixture at a gas introduction volume of from 0.5 to 2 m.sup.3 of oxygen-containing gas per m.sup.3 of reaction volume per hour and a gas introduction volume of from 8 to 32 m.sup.3, preferably from 8 to 24 m.sup.3, of oxygen-containing gas per m.sup.2 of gas introduction area per hour, or a gas introduction volume of from 0.5 to 1 m.sup.3 of oxygen-containing gas per m.sup.3 of reaction volume per hour and a gas introduction volume of from 8 to 16 m.sup.3, preferably from 8 to 12 m.sup.3, of oxygen-containing gas per ma of gas introduction area per hour, or a gas introduction volume of from 1 to 2 m of oxygen-containing gas per m.sup.3 of reaction volume per hour and a gas introduction volume of from 16 to 32 m.sup.3, preferably from 24 to 32 m.sup.3, of oxygen-containing gas per m.sup.3 of gas introduction area per hour.

9. The process for producing haematite according to claim 8, wherein the contacting is carried out without additional mechanical mixing and/or without additional hydraulic mixing.

10. The process for producing haematite according to claim 8, wherein the contacting is carried out with additional mechanical mixing and/or with additional hydraulic mixing.

11. The process according to claim 8, wherein the contacting is carried out until the haematite pigment has a desired colour shade.

12. The process according to claim 8, further comprising separating the haematite pigment from the reaction mixture.

13. The process according to claim 8, wherein the haematite pigment produced has the modification -Fe.sub.2O.sub.3.

14. The process according to claim 8, wherein the contacting is carried out in an apparatus comprising: a reaction vessel for contacting iron(II) nitrate, haematite nuclei, iron, and oxygen to produce haematite pigment and nitrogen oxide, a support for iron, the support comprising a base and an outer delimitation disposed around at least a portion of the base, a holder for retaining the support relative to the reaction vessel, at least one gas introduction unit disposed relative to the support for introducing oxygen-containing gas to contact iron supported in the support, an inlet for introducing a liquid reaction medium of iron(II) nitrate solution, haematite nucleus suspension, and optionally water into the reaction vessel, an outlet for removal of a nitrogen oxide-containing stream (NOX), and an outlet for removal of haematite pigment suspension.

15. Use of the apparatus according to claim 1 to produce haematite pigments.

16. Apparatus for the production of haematite, the apparatus comprising: a first vessel for containing a liquid reaction medium comprising iron(II) nitrate solution, haematite nucleus suspension, and optionally water; a support device configured for supporting iron, and being disposed in fluid communication with the reaction medium in the first vessel, the support device comprising: a base comprising a plurality of inlet orifices therethrough, a wall portion extending vertically from the base; and an outlet portion at least adjacent a top of the wall portion; a gas introduction unit disposed at at least one of: within the base of the support device; and below the base of the support device, and configured for passing oxygen-containing gas upwardly through the support device and iron disposed within the support device, wherein the gas introduction unit, support device and first vessel are configured such that upward movement of the gas through the support device circulates reaction medium to flow from an area at least adjacent the bottom of the first vessel, upwardly through the inlet orifices, upwardly through iron disposed within the support device, out of the support device through the outlet portion thereof, back into the first vessel at a location above the area at least adjacent the bottom of the first vessel, and downwardly to the area at least adjacent the bottom of the first vessel.

17. The apparatus according to claim 16, wherein the support device comprises at least one of: a support device within the first vessel; and a separate vessel exterior to the first vessel.

18. The apparatus according to claim 17, wherein: the support device comprises a support device within the first vessel, with the base thereof disposed adjacent the bottom of the first vessel, and the wall extending upwardly from the base; the first vessel comprises a wall defining a cross-sectional dimension; and the support device has a cross-sectional dimension smaller than a cross-sectional dimension of the first vessel such that upward flow of the gas and reaction medium occurs through the support device, and downward flow of the reaction medium occurs between the wall portion of the support device and the wall of the first vessel.

19. The apparatus according to claim 18, wherein the wall of the support device has a vertical height dimension extending upwardly from the base and comprising a lower region adjacent the base and an upper region above the lower region, wherein the wall is impermeable to the liquid reaction medium in the lower region extending from 10 to 50% of the height of the wall, measured from base, and is permeable to the liquid reaction medium in the upper region extending from 50 to 90% of the height of the wall, measured from the base.

20. The apparatus according to claim 19, wherein: the orifices of the support device define a gas introduction portion for passage of gas into the support device, and the area formed by the gas introduction portion represents a gas introduction area (GA); the wall of the first vessel defines a wall circumference, and the area formed by the circumference represents a reactor area (RA); and a ratio of the gas introduction area (GA) to the reactor area (RA) is configured so that: a gas introduction volume of from 8 to 32 ms, preferably from 8 to 24 m.sup.3, of oxygen-containing gas per m.sup.2 of gas introduction area per hour is obtained at a gas introduction volume of from 0.5 to 2 m.sup.3 of oxygen-containing gas per m.sup.3 of reaction volume per hour, or a gas introduction volume of from 8 to 16 m.sup.3, preferably from 8 to 12 m.sup.3, of oxygen-containing gas per m.sup.2 of gas introduction area per hour is obtained at a gas introduction volume of from 0.5 to 1 m.sup.3 of oxygen-containing gas per m.sup.3 of reaction volume per hour, or a gas introduction volume of from 16 to 32 m.sup.3, preferably from 24 to 32 m, of oxygen-containing gas per m.sup.2 of gas introduction area per hour is obtained at a gas introduction volume of from 1 to 2 m.sup.3 of oxygen-containing gas per m.sup.3 of reaction volume per hour.

Description

[0040] The invention additionally encompasses apparatuses suitable for carrying out the process of the invention. These are described in more detail below with the aid of FIGS. 1 to 7.

[0041] FIGS. 1, 2, 5 and 6 depict an embodiment of a reactor according to the invention as side view of a central vertical cross section.

[0042] FIG. 3 shows a plan view of the horizontal cross section of the reaction vessel 11 of FIG. 1 at the height of the support for iron 14.

[0043] FIG. 4 shows a plan view of the horizontal cross section at the areas GA and RA at the height of the planes a of the reaction vessel 11.

[0044] FIG. 7 shows a compartment 2 of a reactor 1.

[0045] The invention thus provides an apparatus comprising at least one reactor 1, in which the process of the invention is carried out. The abovementioned embodiments are described in more detail for the reactor 1.

[0046] In the figures, the symbols have the following meanings: [0047] A oxygen-containing gas [0048] Fe iron [0049] AQ-Fe(NO.sub.3).sub.2 iron(II) nitrate solution [0050] SFe.sub.2O.sub.3 haematite nucleus suspension [0051] PAQ-Fe.sub.2O haematite pigment suspension [0052] H.sub.2O water [0053] NOX nitrogen oxide-containing stream (offgas from the production of the haematite pigment suspension) [0054] GA gas introduction area [0055] DGA diameter of the gas introduction area GA [0056] RA internal area of the reaction vessel 11 [0057] DRA Internal diameter of the reactor [0058] 1 reactor for producing haematite pigment suspension [0059] 11 reaction vessel [0060] 12 outer delimitation [0061] 13 holder for 12 and 14 [0062] 14 support for iron [0063] 15 gas introduction unit [0064] 21 compartment [0065] 111 inlet for iron(II) nitrate solution, haematite nucleus suspension and optionally water [0066] 112 outlet for NOX [0067] 113 outlet for haematite pigment suspension [0068] 211 feed conduit from compartment 21 to the reaction vessel 11 [0069] 212 discharge conduit from compartment 21 to the reaction vessel 11

[0070] Reactor 1 typically comprises one or more reaction vessels made of materials which are resistant to the starting materials. Simple reaction vessels can be, for example, masonry-lined or tiled vessels let into the earth. The reactors also comprise, for example, containers made of glass, plastics which are resistant to nitric acid, e.g. 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 open 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 corresponds to 0.5 bar) to 1.5 MPa (1.5 megapascal corresponds to 15 bar).

[0071] One embodiment of a reactor 1 is shown in FIG. 1. Reactor 1 has at least one reaction vessel 11, outer delimitation 12 of the support for iron 14, holder 13 for 12 and 14, gas introduction unit 18 for the at least one oxygen-containing gas A, inlet 111 for iron (II) nitrate solution, haematite nucleus suspension and optionally water, outlet 112 for a nitrogen oxide-containing stream NOX and outlet 113 for the haematite pigment suspension.

[0072] In one embodiment, the outer delimitation 12 is typically formed by an impermeable wall, a wall provided with openings, by mesh rods, a sieve or a combination thereof. Possible openings in the delimiting wall should be designed so that iron is prevented from falling through. Preference is given to a wall which is impermeable to the liquid reaction mixture, at least in the lower region, for example 10-50% of the height of the delimitation 12, measured from the support for iron 14. In the upper region, for example from 50% to 90% of the height of the delimitation 12, measured from the support for iron 14, lateral openings, e.g. In the form of meshes, holes, which prevent iron from falling down and make exchange of suspension possible, can be present. The delimitation is typically designed so that when carrying out the process of the invention, not more than 10% by volume of the nitrogen oxide-containing stream gets from the inside of the outer delimitation 12 through the openings of the outer delimitation 12 to the other side of the outer delimitation 12. However, this is generally prevented by the airlift pump effect brought about by the upwards-flowing gas in the interior space formed by the outer delimitation 12.

[0073] The upper edge of the outer delimitation 12 is preferably below the surface of the reaction mixture which adjoins the gas space located above the reaction mixture, in particular when it has no lateral openings (mesh, holes, sieve) for mass transfer. When the upper edge of the outer delimitation 12 is above the surface of the reaction mixture which adjoins the gas space above the reaction mixture and the outer delimitation 12 is suspension-permeable, no exchange of the liquid reaction mixture between the volumes within and outside the delimitation 12 is possible. This leads to the proportions of the liquid reaction mixture located within and outside the outer delimitation only being able to mix with one another via the support for iron, which could lead to a deterioration in the space-time yield, the product yield and the product quality. According to the invention, the gas introduction volume per unit area per unit time in the unit m.sup.3 of oxygen-containing gas per m.sup.2 of gas introduction area per hour based on the area formed by the support for iron 14 is measured.

[0074] In FIGS. 1, 5 and 6, which depict a side view of a vertical section of the reactor 1, the direction of view denoted by the marking a is marked by an arrow. FIG. 3 shows a horizontal cross section of the reaction vessel 11 of FIG. 1 as plan view at the height of the support for iron 14. In FIG. 1, the outer delimitation 12 has the shape of a cylinder open at the top and the bottom. Here, the support for iron 14 has the area GA with the diameter DGA. The area GA defines the gas introduction area.

[0075] In various embodiments, the support for iron 14 and thus also the outer delimitation 12 can have various shapes, for example circular, oval, triangular, quadilateral, pentagonal or polygonal (in each case based on the horizontal cross section of the outer delimitation 12). The gas introduction area GA thus also has the corresponding shape. The support for iron 14 and the outer delimitation 12 and thus also the gas introduction area GA are preferably circular or oval. The support for iron 14 and the outer delimitation 12 are likewise preferably orthogonal to one another. The area of the support for iron and the horizontal area within the outer delimitation above which 90% of the volume enclosed by the support for iron and the outer delimitation is located are in this case identical.

[0076] In further embodiments, the outer delimitation 12 can have different diameters at different heights. For example, in FIG. 2 the outer delimitation 12 widens conically in an upwards direction and forms a funnel open at the top. In these cases, the support for iron 14 and the outer delimitation 12 are not orthogonal. Here, the horizontal area within the outer delimitation 12 above which 90% of the volume enclosed by the support for iron and the outer delimitation is located defines the parameter gas introduction area.

[0077] FIG. 4 depicts a plan view of the horizontal cross section of the reaction vessel 11 of FIG. 1 and of the outer delimitation 12 at the height of the support for iron 14, which is formed by the inner edge of the outer delimitation 12 (direction of view a). The area formed by the support for iron 14 is the gas introduction area GA which has a diameter DGA and is shown as cross-hatched area in the upper part of FIG. 4. The area formed by the circumference of the inner surface of the reaction vessel 11 is the reactor area RA which has a diameter DRA and is depicted as hatched area in the lower part of FIG. 4.

[0078] The support for iron 14 allows exchange of at least the liquid reaction mixture and the oxygen-containing gas through openings present in the support. Typical embodiments of the support for iron 14 can be sieved trays, perforated trays or meshes. The ratio of the cumulated area of openings to the total area of the support for iron is typically at least 0.1. The upper value of the ratio of the cumulated area of openings to the total area is determined by the technical boundary conditions which are set down by the iron present on the support for iron 14, for example size and shape of the iron parts and weight of the iron bed. The ratio of the cumulated area of openings to the total area of the support for iron 14 is preferably as great as possible. The openings required for the reaction mixture to flow through the support for iron are typically suitable for the selection of the iron raw material. Falling of the iron through the support is typically largely avoided thereby. The gas introduction area GA is preferably smaller than the area RA.

[0079] The gas introduction area is configured so that a gas introduction volume of from 8 to 32 m, preferably from 8 to 24 m.sup.3, of oxygen-containing gas per m.sup.2 of gas introduction area per hour is achieved at a gas introduction volume of from 0.5 to 2 m.sup.3 of oxygen-containing gas per m.sup.3 of reaction volume per hour, or a gas introduction volume of from 8 to 16 m.sup.3, preferably from 8 to 12 m.sup.3, of oxygen-containing gas per m.sup.2 of gas introduction area per hour is achieved at a gas introduction volume of from 0.5 to 1 m.sup.3 of oxygen-containing gas per m.sup.3 of reaction volume per hour, or a gas introduction volume of from 16 to 32 m.sup.3, preferably from 24 to 32 m.sup.3, of oxygen-containing gas per m.sup.2 of gas introduction area per hour is achieved at a gas introduction volume of from 1 to 2 m.sup.3 of oxygen-containing gas per m of reaction volume per hour. Depending of the dimensions of the reactor, the percentage of the reactor area made up by the gas introduction area is different. The lower the reactor volume, the smaller does the percentage of the gas introduction area based on the reactor area have to be. The percentage of the gas introduction area based on the reactor area can be calculated by dividing the intended value m.sup.3 of oxygen-containing gas per m.sup.2 of gas introduction area per hour, for example 8 m.sup.3 of oxygen-containing gas per m.sup.2 of gas introduction area per hour, by the intended value m.sup.3 of oxygen-containing gas per m.sup.3 of reaction volume per hour, for example 1 m.sup.3 of oxygen-containing gas per m.sup.3 of reaction volume per hour, with a value of 8 m being obtained in this example and the height of the reaction volume in the reactor being divided by this value. At an illustrative height of the reaction volume of 1.05 m at a reaction volume of 1 m=, the percentage of the gas introduction area based on the reactor area is 13% (calculated by 1.05 m:8 m). At an illustrative height of the reaction volume of 5.09 m at a reaction volume of 100 in, the percentage of the gas introduction area based on the reactor area is 64% (calculated by 5.09 m:8 m).

[0080] The gas introduction unit 15 is located underneath the support for iron 14 and consists, for example, of one or more sparging rings, perforated pipes, ring-shaped pipes, pipes installed in a star shape, two-fluid sprayers (injectors, ejectors) or gas introduction mats. The gas introduction unit 15 can also be integrated into the support for iron 14. Integration of the gas introduction unit 15 into the support for iron 14 is, for example, effected by the sparging unit being mechanically joined directly to the support or being configured as a mesh which is made up of perforated tubes and simultaneously serves as support for iron.

[0081] The introduction of gas preferably takes place underneath the support for iron 14, so that the gas flowing out from the gas introduction unit 15 goes through the iron bed. Relative to the reactor height, the gas introduction unit is preferably located in the lower half, preferably in the lower third, of the reactor.

[0082] In the embodiment shown in FIG. 1, the gas introduction unit 15 is installed underneath the support for iron 14. This ensures that 90 percent by volume or more of the at least one oxygen-containing gas A flows through the support for iron 14 and through the iron Fe and only less than 10 percent by volume of the at least one oxygen-containing gas A flows through the holder 13 and then between the wall of the reaction vessel 11 and the outer delimitation of the gas introduction area 12 in the direction of the surface of the liquid reaction mixture. However, this is generally prevented by the airlift pump effect which is brought about by the upwards-flowing gas in the interior space formed by the outer delimitation 12. The introduction of the at least one oxygen-containing gas underneath the support for iron 14 results in formation of a gas stream directed in the direction of the surface of the reaction mixture, which leads to convection of the liquid reaction mixture past the iron located on the support for iron 14. The at least one oxygen-containing gas flowing into the reaction mixture partly dissolves in the reaction mixture. The proportions of the at least one oxygen-containing gas dissolved in the reaction mixture partially react with the other components of the reaction mixture, as a result of which dissolved ammonium compounds and/or nitrogen oxides are also formed in the reaction mixture. Part of the nitrogen oxides in turn reacts with the reaction components. Part of the at least one oxygen-containing gas and of the nitrogen oxides formed leave the reaction mixture as nitrogen oxide-containing stream NOX.

[0083] In one embodiment, the support for iron 14 is typically a sieve or mesh which is mechanically joined to the holder 13 and the outer delimitation 12.

[0084] In a further embodiment, the holder 13 is a wall which is partly liquid- and/or gas-impermeable, preferably consisting partly or entirely of a mesh or sieve.

[0085] In a further embodiment, the holder 13 consists of struts which are joined to the bottom or the side wall of the reaction vessel 11.

[0086] In a further embodiment, the support for iron 14 is mechanically joined over its entire circumference to the outer delimitation 12 and the holder 13, as depicted in FIG. 1. While the process of the invention is being carried out, an upward-directed stream arises within the outer delimitation 12 and a downward-directed stream arises in the space between the outer delimitation 12 and the inner wall of the reactor 11, so that the liquid reaction mixture is circulated past the iron.

[0087] In a further embodiment, the support for iron 14 is mechanically joined at only part of its circumference to the outer delimitation 12 and the holder 13 and mechanically joined around the other part of its circumference directly to the reaction vessel 11, as shown in FIG. 5. While the process of the invention is being carried out, an upward-directed stream arises between the outer delimitation 12 and the wall of the reaction vessel 11 above the support for iron 14 and a downward-directed stream arises between the outer delimitation 12 and the wall of the reaction vessel 11 at the side of the support for iron 14, so that the reaction mixture is circulated past the iron.

[0088] In a further embodiment, the support for iron 14 and the gas introduction unit 15 are present in at least one separate compartment 21 of the reaction vessel 11, as depicted in FIGS. 6 and 7. The at least one separate compartment 21 is connected by a feed conduit 211 and a discharge conduit 212 to the reaction vessel 11. The feed conduit 211 and the discharge conduit 212 are preferably located underneath the surface of the reaction mixture which adjoins the gas space in the reactor. In this embodiment, the gas introduction area GA is, as in the above-described embodiments as per FIGS. 1 and 5, identical to the area of the support for iron 14. If a plurality of separate compartments are present, the parameter m.sup.3 of gas introduction volume of oxygen-containing gas per m.sup.2 of gas introduction area per hour is calculated from the ratio of the sums of the individual gas introduction volumes of the individual compartments to the sum of the gas introduction areas of the individual compartments.

[0089] In alternative embodiments, inlet 111 and/or outlet 112 and/or outlet 113 in the reaction vessel 11 can also be configured so that they are formed by the same opening, by in each case an inlet or outlet for each individual starting material and/or product or by any combination thereof.

[0090] In further embodiments, the reactor 1 additionally comprises a mixer (not depicted in the figures), for example a propeller mixer, and/or a pump which withdraws the liquid phase from the reactor at an outlet and feeds it back into the reactor at an inlet at another point via a conduit. In a further preferred embodiment, heating (not depicted in the figures) can be provided directly in the reaction vessel 11, for example as heating mantle, heating plug and/or as heating coil, or be effected by direct introduction of hot water and/or pressurized steam into the liquid reaction mixture.

[0091] In the following, the procedure for carrying out the process of the invention will be described by way of example. To carry out the process of the invention, the starting materials iron, optionally water, iron(II) nitrate solution and haematite nucleus suspension are introduced via an inlet, for example the inlet 111, into the reaction vessel, for example reaction vessel 11. The reaction mixture is heated to a temperature of from 70 to 120*C, preferably from 70 to 99 C. Haematite is precipitated onto the haematite nucleus by oxidation by means of oxygen-containing gas, with the oxygen-containing gas being introduced into the liquid reaction mixture at a gas introduction volume of from 0.5 to 2 m.sup.3 of oxygen-containing gas per m.sup.3 of reaction volume per hour and a gas introduction volume of from 8 to 32 m.sup.3, preferably from 8 to 24 m.sup.3, of oxygen-containing gas per m.sup.2 of gas introduction area per hour, or a gas introduction volume of from 0.5 to 1 m.sup.n of oxygen-containing gas per m.sup.3 of reaction volume per hour and a gas introduction volume of from 8 to 16 m.sup.3, preferably from 8 to 12 m.sup.3, of oxygen-containing gas per m.sup.3 of gas introduction area per hour, or a gas introduction volume of from 1 to 2 m.sup.3 of oxygen-containing gas per m.sup.3 of reaction volume per hour and a gas introduction volume of from 16 to 32 m.sup.3, preferably from 24 to 32 ms, of oxygen-containing gas per m.sup.2 of gas introduction area per hour, and the size of the haematite nucleus increases during the reaction until the iron oxide red pigment has attained the desired colour shade. The haematite pigment suspension PAQ-Fe.sub.2O produced in this way is either temporarily stored in an optional storage vessel (not depicted in the figures) and/or transported directly through an outlet, for example outlet 113, and via a conduit which communicates with the reaction vessel into a separation apparatus (not shown in the figures) in which the pigment is separated from the reaction mixture. In a preferred embodiment, the additional mechanical mixing, for example by means of a propeller stirrer, and/or by means of additional hydraulic mixing, for example by pumped circulation of the liquid reaction mixture of the liquid reaction mixture is excluded or not.

[0092] 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 the a* and b* values in surface coating testing, change during the reaction as a result of the changing particle size and/or morphology during 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 with reduction, in surface coating testing. This is effected by ending the introduction of gas, optionally by simultaneous cooling of the reaction mixture to a temperature of less than 70 C. Typical reaction times for the reaction according to the invention are from 10 to 150 hours, depending on the desired colour shade.

[0093] The haematite pigment suspension produced in this way for example the haematite pigment suspension PA-Fe.sub.2O.sub.3, is either temporarily stored in an optional storage vessel (not depicted in the figures) and/or transported directly through an outlet, for example the outlet 113, and via a conduit into a separation apparatus (not depicted in the figures) in which the pigment is separated from the reaction mixture.

[0094] In a preferred embodiment, the haematite pigment is separated from the haematite suspension by conventional 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 likewise preferably 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 metal pieces, which would otherwise contaminate the haematile pigment are separated off from the haematite pigment suspension.

[0095] The separation of the haematite pigment from the haematite pigment suspension can be carried out using al methods known to those skilled in the art, e.g. sedimentation with subsequent removal of the aqueous phase or filtration through filter presses, for example membrane filter presses.

[0096] In a preferred embodiment of the process of the invention, at least one sulphate salt, for example iron(II) sulphate and/or an alkali metal sulphate or alkaline earth metal sulphate, preferably iron(II) sulphate and/or sodium sulphate, can be added to the haematite pigment suspension during or before sieving and/or during or before the separation. This has the advantage that sedimentation of the haematite pigment from the haematite pigment suspension is accelerated. This assists the subsequent isolation of the haematite pigment. Furthermore, the buildup reaction can be carried out further when iron(II) sulphate is used. Precipitation of residual iron by means of sodium hydroxide solution subsequently takes place, with the pH being set while introducing air by addition of an alkaline precipitant (e.g. NaOH, KOH, CaCO.sub.3, NaCO.sub.3, K.sub.2CO.sub.3, etc.) to pH 3.5 to 6, preferably 4-5, until the iron(II) content is <0.1 g/l. After precipitation is complete, the introduction of gas is stopped and the pH is set to pH 4-6 by further addition of the alkaline precipitant.

[0097] At least one wash of the sediment or filtercake separated off in this way is then optionally carried out. Drying of the haematite pigment which has been 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 wash. Drying is preferably carried out by means of belt dryers, plate dryers, kneading dryers and/or spray dryers.

[0098] It has surprisingly been found that a significantly smaller amount of nitrogen oxides are given off into the gas phase in the process of the invention than in the process according to the prior art in which the pigment buildup is carried out at significantly greater gas introduction volumes per reaction volume per hour. In addition, significantly less energy, compared to the prior art, is carried out into the gas phase from the reaction mixture which has been heated to from 70 to 120 C., preferably from 70 to 99 C., and then has to be introduced again as external energy, for example in the form of steam, into the reaction mixture due to the smaller gas introduction volumes. In the process according to the prior art, in which gas introduction amounts of 10 m.sup.3 of gas volume/m.sup.3 of reaction volume/hour of reaction time are used, 1.7 kg of Fe.sub.3O.sub.3 are usually produced per kg of Fe(NO.sub.3).sub.2. Compared to the process of the prior art, at least the same proportion or a greater proportion of the Fe.sup.3+ present in the haematite pigment comes from the iron and either the same proportion or 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. More than 1.7 kg of Fe.sub.2O.sub.3% per kg of Fe(NO.sub.3).sub.2, preferably from 1.8 to 2.5 kg of Fe.sub.2O.sub.3 per kg of Fe(NO.sub.3).sub.2, are produced by means of the process of the invention. If the proportion of the Fe.sup.3+ originating from the iron in the haematite pigment is greater, 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 producing the same amount of haematite.

[0099] The process of the invention and the apparatus of the invention in which the process of the invention is carried out thus make it possible to produce iron oxide red pigments by the Penniman process using nitrate in high quality, in high yields, in an energy efficient way and with avoidance of offgases which contain undesirable reaction products such as nitrous gases or laughing gas.

EXAMPLES AND METHODS

Titration of Iron(I) and Iron(III) Determination:

[0100] 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) sulphate.

NO.SUB.x .Measurement

[0101] NO.sub.x measurements were carried out using a gas analyser PG 250 from Horriba, (chemiluminescence method). Information about NO.sub.x formation was reported as a ratio to the pigment yield (calculated as NO.sub.2, in g of NO.sub.2/kg of pigment). The gas sample is dewatered by means of a cold trap in the gas analyser. The NO.sub.x emission arising in the production of the starting materials haematite nucleus and iron nitrate is not included.

N.SUB.2.O Measurement

[0102] For sample preparation, a sample of the gas to be measured is firstly passed through a cooling apparatus, for example a gas wash bottle cooled with ice water, so that the dried gas has a temperature of not more than 40C. Here, the proportion of water typically drops to from 40 to 50 g of water vapour/m.sup.3 of air. Laughing gas measurements were carried out by means of a quantitative gas-chromatographic determination and/or by infrared measurement. Information about 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 is not included.

O.SUB.2 .Measurement

[0103] For sample preparation, a sample of the gas to be measured is firstly passed through a cooling apparatus, for example a gas wash bottle cooled with ice water, so that the dried gas has a temperature of not more than 40 C. Here, the proportion of water typically drops to from 40 to 50 g of water vapour/m of air. The measurement of the oxygen content in the dried nitrogen oxide-containing stream is carried out, for example, by means of an electrochemical sensor which can selectively determine the oxygen concentration in the gas mixture. Measurement of the oxygen content in the dried nitrogen oxide-containing stream can also be carried out by other methods. Since the oxygen content is an absolute quantity which can be determined absolutely by comparison with reference samples, a person skilled in the art will here use only methods which have been validated by means of reference samples.

N.SUB.2 .Measurement

[0104] For sample preparation, a sample of the gas to be measured is firstly passed through a cooling apparatus, for example a gas wash bottle cooled with ice water, so that the dried gas has a temperature of not more than 40 C. Here, the proportion of water typically drops to from 40 to 50 g of water vapour/m of air. The measurement of the nitrogen content in the dried nitrogen oxide-containing stream is carried out by gas chromatography. For this purpose, gas samples are taken, e.g. by filling evacuated gas sample bottles with offgas, and determined quantitatively by gas chromatography. Measurement of the nitrogen content in the dried nitrogen oxide-containing stream can also be carried out by other methods. Since the nitrogen content is an absolute quantity which can be determined absolutely by comparison with reference samples, a person skilled in the art will here use only methods which have been validated by means of reference samples.

Example 1: (Comparison)

[0105] 69.9 kg of iron sheet having a thickness of about 1 mm were placed in a 1 m.sup.3 reactor (internal diameter 1.0 m) equipped with circular sieve trays (mesh opening about 10 mm) having a diameter of 0.98 m and a resulting area of 0.75 m.sup.2 and a sparging ring. The sparging ring (20 cm diameter) was installed underneath the sieve tray, with the iron sheet being uniformly distributed on the sieve tray. Water, iron(II) nitrate solution and haematile nucleus suspension were subsequently introduced in such amounts that a volume of 1000 litres was attained and the concentration of nucleus (as Fe.sub.2O.sub.3) was 21 g/l and the concentration of iron nitrate (calculated as anhydrous Fe(N.sub.2).sub.2) was 44 g/l. The mixture was heated to 85 C. and after reaching the temperature sparged with 10 m.sup.3/h of air (13 m.sup.3 of air/m of gas introduction area/h) for 94 hours. The reaction mixture is subsequently filtered through a filter press and the haematite pigment obtained is washed with water. The haematite pigment is subsequently dried at 80 C. to a residual moisture content of less than 5% by weight. The dried filtercake is subsequently broken up mechanically by means of a shredder. The haematite pigment is in this way obtained in powder form in a yield of 93.1 kg (corresponds to 1.7 kg of Fe.sub.2O.sub.3 per kg of Fe(NO.sub.3).sub.2). The parameter kg of Fe.sub.2O.sub.3 per kg of Fe(NO.sub.3).sub.2 is determined by measuring the volume of the liquid reaction mixture and the concentration of Fe(NO.sub.3).sub.2 after the reaction is complete. The amount of Fe(NO.sub.3).sub.2 consumed in the reaction, which is reported as a ratio to the amount of haematite pigment obtained, is calculated from the amount of Fe(NO.sub.3).sub.2 introduced before the reaction and the amount of Fe(NO.sub.3).sub.2 remaining alter the reaction.

Examples 2 to 7

[0106] Examples 2 to 7 were carried out in the same reactor on a comparable scale, with the identical conditions and the identical relative ratios of the amounts of the starting materials to the volumes of the solutions being set. The only parameter varied was the gas introduction volumes. The results of the experiments are reported in Table 1.

Example 8 (According to the Invention)

[0107] A cylindrically shaped insert (sidewalls without openings) having sieve trays (mesh opening about 10 mm) and a diameter of 0.28 m and a height of 0.8 m was installed in a reaction vessel having a useable volume of 1 m.sup.3 and a gas introduction unit was installed underneath this insert.

[0108] 25 kg of iron sheet were introduced into this tube which was open at the top and has the sieve tray and the gas introduction unit underneath. Water, iron(II) nitrate solution and haematite nucleus suspension were subsequently added in such amounts that a volume of 1000 litres was attained and the concentration of nucleus (as Fe.sub.2O.sub.3) was 21 g/l and the concentration of iron nitrate (calculated as anhydrous Fe(NO.sub.3).sub.2) was 44 g/l. The mixture was heated to 85 C. and after reaching the temperature sparged with 0.5 m.sup.3/h of air (corresponds to 0.5 m.sup.3 of air/m.sup.3 of reaction volume/h (corresponding to 8 m.sup.3 of air/m.sup.2 of gas introduction area/h) for 48 hours. The reaction mixture is subsequently filtered through a filter press and the haematite pigment obtained is washed with water. The haematite pigment is subsequently dried at 80 C. to a residual moisture content of less than 5% by weight. The dried filtercake is subsequently broken up mechanically by means of a shredder.

[0109] Examples 9 to 12 (according to the invention) were carried out in the same reactor on a comparable scale, with the identical conditions and also the identical relative ratios of the amounts of starting materials to the volumes of the solutions being set. The only parameter varied was the gas introduction volumes. The results of the experiments are shown in Table 2.

TABLE-US-00001 TABLE 1 Gas introduction volumes (comparative examples) Gas Gas introduction introduction volume per volume per kg of Fe.sub.2O.sub.3 reaction gas formed/kg volume introduction of per time area per time Pigment Fe(NO.sub.3).sub.2 Example [m3/m3/h] [m.sup.3/m.sup.2/h] quality consumed 1 1.3 1 Unsatisfactory 0.4 2 2.7 2 Unsatisfactory 0.6 3 10 7.5 Good 1.7 4 12.0 9 Good 2.1 5 17.3 13 Good 1.7 6 21.3 16 Good 1.8 7 28 21 Good 1.9

TABLE-US-00002 TABLE 2 Gas introduction volumes (according to the invention) Gas Gas introduction introduction volume per volume per kg of Fe.sub.2O.sub.3 reaction gas formed/kg volume introduction of per time area per time Pigment Fe(NO.sub.3).sub.2 Example [m3/m3/h] [m.sup.3/m.sup.2/h] quality consumed 8 0.5 8.0 Good 2.3 9 0.55 9.0 Good 2.3 10 0.8 13.0 Good 2.2 11 1 16.0 Good 2.1 12 1.3 21.0 Good 2.0