JPS6324024A - Sintered product - Google Patents

Abstract

(57) [Abstract] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to sintered articles, particularly sintered iron articles and precursors thereof. Sintered iron articles, such as pellets, are used in the steel industry as a convenient handling form of iron and have been proposed as catalysts for hydrogenation reactions, such as ammonia synthesis and nitrile hydrogenation. See for example British patent 1484864.

Shaped iron articles are typically made by reduction of precursors, usually in the form of sintered iron oxide compositions formed from finely divided iron oxide compositions. To help densify the sintered precursor and/or increase catalytic activity if the sintered iron article is desired to be used as a catalyst, oxidative substances that cannot be easily reduced or upon heating may be added. Small amounts of compounds that decompose into such oxide-like materials are often incorporated into the composition. Among such common additives are alumina,
There are compounds that can be decomposed into magnesia, calcium oxide, and alkali metal oxides or similar oxide-like substances. For example, for use as an ammonia synthesis catalyst precursor, the sintered iron oxide composition typically contains 1 to 5% alumina, 0.3 to 2% alkali metal, especially potassium, w/w. oxide, up to 5% calcium oxide, and/or up to 2% magnesia. The iron oxide composition may also contain reducible oxides, such as oxides of metals such as cobalt, typically in proportions up to 40% w/w. For the precursor of the nitrile hydrogenation catalyst, the iron oxide composition should be
Typically contains 0.5 to 5% by weight alumina, magnesia, and/or calcium oxide, but
Generally does not contain alkali metal oxides. For non-catalytic applications, calcium oxide is often incorporated as a flux.

Like the molten product, sintered iron oxide articles have low surface area and high density. However, since the sintered product has a completely different structure from that of the molten product, it can be easily distinguished from the molten product using an optical microscope and/or a scanning electron microscope. In particular, the sintered product shows a significant proportion of evidence of the finely divided particles used in its manufacture, although many of those fine particles have agglomerated. Sintered iron oxide articles can also be distinguished from molten products by measurements of pore volume distribution. For example, the sintered product will generally have at least 0.01. especially at least 0.02
The porosity of the sintered product is around 9cm3, 10μ
It is shown in the form of pores with a radius of less than m. In contrast, the molten product exhibits a negligible pore volume in the form of pores with a radius of less than 10 μm, typically totaling less than 0.005 cm −1 . Pore volume distribution can be measured at various pressures by mercury borosimetry.

The sintered product also exhibits a distinctly different pore volume distribution than the molten material when the pore volume is measured after reduction of iron oxide to iron. In this case, both the sintered product and the molten product are 100 to 100 OA (0,
It has considerable porosity in the form of pores with radii ranging from 0.01 to 0.01 μm). However, the reduced molten article exhibits only a small amount of porosity, less than about 0.015 cm"g-', in the form of pores of radius between 0.1 and 1011 m, whereas the sintered product is at least 0 after reduction
．． The porosity of 02crn3·g−1 is generally expressed in the form of pores with a radius between 0.1 and 10 μm.

The sintered iron oxide composition preferably comprises at least 50% by weight iron oxide (expressed as F'e203), and
The combined weight of iron oxide (expressed as F'e203) and any cobalt oxide (expressed as COO) present is preferably at least 70%, especially at least 85% by weight of the composition.

The iron oxide used to make the sintered article may be magnetite or hematite. However, if sintering is carried out as usual in an oxidizing atmosphere, e.g. air,
The iron oxide in the sintered product is in the form of hematite unless very high sintering temperatures in excess of 1370°C are used. Sintered iron oxide articles in which the iron oxide is in the form of magnetite can be produced if the sintering is carried out in a carefully controlled non-oxidizing atmosphere at a sintering temperature of about 1300°C or less, or if the sintering is carried out in the form of hematite. can only be obtained if carried out at temperatures above the temperature at which it is converted to magnetite, 1370°C.

As mentioned above, it is often desirable for the composition to include calcium oxide. One disadvantage of incorporating calcium oxide into compositions that are sintered iron oxides and hematite is that during the reduction of hematite to metallic iron, the calcium ferrite that is probably formed during the sintering step is As a result of the reduction, there is a tendency for the sintered article to crack, weaken, and even collapse. This problem of cracking occurs when the iron oxide in the sintered product is in the form of magnetite, unless the calcium oxide level is very high.
It doesn't seem to happen. This is believed to be because calcium oxide dissolves into the magnetite crystal lattice and only at high levels of calcium oxide a separate calcium ferrite phase is formed.

We have discovered that incorporating small amounts of magnesium aluminate spinel (magnesium aluminate spinel) into iron oxide compositions provides a significant reduction in this propensity to crack.

The invention therefore provides a sintered iron oxide-containing article containing a small proportion of calcium oxide, the composition of which is characterized in that it also contains a small proportion of magnesium aluminate spinel.

The invention also provides sintered iron articles obtained by reduction of such sintered iron oxide-containing articles.

It has been discovered that the magnesium aluminate alloy needs to be added as is to the composition from which the sintered iron oxide containing article is made. Thus, adding magnesia and alumina as separate components does not have the desired effect. Of course, the composition can also contain alumina and/or magnesia in addition to spinel.

The amount of magnesium aluminate spinel required is relatively small, with 0.1 to 1 mole of spinel per mole of calcium oxide generally being sufficient.

Sintered iron oxide-containing articles are finely divided granules containing particles of iron oxide, such as hematite or magnetite, together with other components including magnesium aluminate spinel and calcium oxide or calcium compounds that can decompose to calcium oxide upon heating. by forming a composition, shaping the finely divided composition into the desired shape, and then heating the composition to convert the metal compound to the oxide form if necessary and sinter the particles together. You can make it.

It is preferred that essentially all of the particles in the composition have a size of 50 μm or less, preferably 20 μm or less. In particular, at least 50%, preferably at least 90% by weight of the particles
preferably has a dimension of 10 μm or less.

The molding process may be performed wet or dry.

If complex shapes are required, they may be formed by techniques such as isobaric molding. If the desired shape is in the form of a let, the particulate composition may be compressed into a sheet or ribbon by passing through counter-rotating rolls, and the compressed sheet or ribbon is then compressed in one plane. It is characterized by two opposing surfaces formed by contact compression and the other surface formed by fracture, which can also be fractured into the shape of a let. Instead of forming sheets or ribbons by compression, individual lets, which may be elongated, e.g. in the shape of a cigarette, may be formed by roll compression using a suitably powerful die-cut roll. A binder such as starch or a stearate of a metal such as aluminum or magnesium is added to the roll compaction, typically from 0.5 to 3% by weight of the granular composition.
may be added in amounts in the range of 0 to 100% to assist in the formation of agglomerated compacts. Some water may also be included, especially if the binder is starch.

Alternatively, and preferably, when a pellet form is desired, the particulate composition is formed by wet extrusion, in which one or more organic, + A liquid is added to the particulate composition prior to extrusion. When the liquid is water, the binder is a hydrophilic polymer that is self-adhesive, such as a solubilized starch, especially in admixture with a shear-thinning hydrophilic r'+J mom - such as corn starch or polyvinyl alcohol. be. The use of such binders for wet extrusion of oxidic materials is described in EP 134,138. Extrusion is conveniently carried out at room temperature by forcing the composition through a suitable goo.

One advantage of using extrusion methods is that by using an extrusion gouge with a suitable core, it is possible to produce shaped articles of uniform cross-section with longitudinally extending passages. Articles with a large number of through passages are advantageous when it is desired to produce a product with a large geometric surface fR per unit volume of droplet of the article. The present invention is based on the I
It is particularly advantageous when manufacturing articles with a large number of small cross-section through passages in the crn opening. The manufacture of such articles is disclosed in European patents 222541.222542 and 2234
It is described in 39.

After extrusion, the extruded product is cut to the desired length after or preferably before sintering.

The cutting technique employed is of course such that none of the passages become obstructed during the cutting process. A suitable cutting technique is described in EP223445. Alternatively, articles can be made by the tablet method.

In this case, the walls of the tableting goo and/or all the cores have a slight slope, for example up to 3 degrees, to aid in demolding the unit object from the goo. Alternatively, granulation techniques widely used, for example in the steel industry, can be used.

After shaping, and if the shaping method is by extrusion, after cutting the extrudate to the desired length, drying the article if necessary, and then applying a binder with an organic component to the shaping step. When used in a calcination step, the organic component is burnt off in an oxygen-containing gas, such as air, preferably at 250 to 500°C. Such a firing step may also be advantageous even when no organic binder is used. The article is then preferably sintered at a temperature above 1100°C, especially above 1200°C. Preferably the sintering temperature is 1350°C
lower. Sintering is preferably carried out at a temperature such that no significant conversion of hematite to magnetite occurs.

If the sintered iron oxide article also contains an alkali metal compound, such a compound or a compound that can decompose into such a compound during the firing or sintering step may be incorporated as a powder, or It may be incorporated by impregnating the particulate iron oxide-containing composition with an aqueous solution containing the desired alkali metal compound or compound decomposable thereto afterward, but preferably before firing and sintering.

Reduction of the sintered iron oxide article to an iron article is carried out by passing a stream of reducing gas such as hydrogen and/or carbon monoxide over the sintered iron oxide article at a temperature ranging from 300 to 1200°C. It is convenient to do so. If the iron article is to be used as a catalyst, the temperature for its reduction is preferably as low as 500° C. and the reducing gas is preferably hydrogen or, in some cases, the process gas used in the catalytic reaction. For example, it may be ammonia synthesis gas, and its reduction may be carried out in a reactor in which the catalytic reaction is to take place. Avoid back-diffusion of water vapor that comes into contact with the iron formed by reduction, and ensure that the iron oxide is -
Care should be taken to prevent overheating once reduced. Alternatively, the precursor may be reduced outside the reactor in which it is used, passivated by cold oxygen diluted with an inert gas such as nitrogen, and then thoroughly reduced before loading it into the reactor. It can be returned. The reduction of iron oxide to iron is preferably from 1 to 300, especially from 20 to 1
It is carried out at pressures in the range of 20 bar (absolute value).

These sintered iron articles can be used as ammonia synthesis catalysts, especially for ammonia synthesis under the following conditions: temperatures from 300 to 500°C, preferably from 350 to 430°C, pressures from 20 to 250, preferably from 40 to 120 bar (absolute). , up to 3.1, especially 2.5 to 3.0, or 1.5 to 2.0 as described in U.S. Pat. No. 4,383,982.
It has particular utility when operating with hydrogen/nitrogen mixtures containing 3 moles of hydrogen/mol of nitrogen at-.

The invention is illustrated by the following examples.

Example 1 Hematite was prepared with a median particle size of 3 μm and a total particle size of 10 μm.
It was ground into a fine powder smaller than m.

958 parts by weight of powdered hematite powder was then mixed with 31 parts by weight of alumina trihydrate and 11 parts by weight of calcium carbonate, the latter two of which had been pre-milled to the same degree of fineness as the hematite. It was something that was done.

To this mixture was then added 10 parts by weight of a high molecular weight polysaccharide [[Zsoplast PS I J, Zimmer Land Schwarz, Lahnstein am Main.

(West Germany)], 440 wt. corn starch (“Koldek”, brand GO8010, CPCUK Ltd., Industrial Division, (Traffobi Park, Manchester, UK)
] and approximately 130 parts by weight of 969
71, an aqueous solution containing potassium carbonate, was added and mixed into a homogeneous paste form.

The mixture was then extruded at room temperature through a circular die with 13 wires of 0.7 mm diameter suspended as the core. A cylindrical extrusion with 13 holes extending through its length was cut to length, dried at 30°C for 12 hours in a controlled atmosphere, and then heated to 400°C at 200°C/hour. and held at 400° C. until the organic components were completely burnt out. These shaped articles were then sintered at 1300°C in an air atmosphere for 4 hours and then cooled to room temperature over 6 hours.

A sintered shaped article with a 13 sil hole of 0.6 trrm diameter extending through it with a length of 6.5 rrvn and a diameter of 6.5 rran has a particle density of 4, measured with respect to volume in mercury at atmospheric pressure. .2'j·cm-3, and the porosity was 0.043 m3·g-1.

Chemical analysis showed that the sintered unit body had the following composition, expressed as weight percent:

F'e O96.9% Ca0 06% A12032.0% to 20 0.5% The above procedure was repeated with various proportions of magnesium aluminate spinel ground to the same fineness as hematite. Repeated with iron oxide composition added prior to formation.

To evaluate the reducing properties of the sintered iron oxide articles, a large number of them were loaded into a cylindrical reactor with a diameter of 27.5 sn and a length of 70 m to form a randomly packed bed with a volume of about 40 rttl.

In the first set of experiments, A, 75% (volume/volume)
The temperature was increased to 350° C. over 3 hours and then further increased to 475° C. over 8 hours while passing a mixture of hydrogen and nitrogen containing 2,502 hours of hydrogen through the bed.

In the second set of experiments, B, nitrogen was added to the bed at 2501
The bed was heated to 475° C. while passing at a rate of 7:00, and the nitrogen stream was then replaced by a hydrogen/nitrogen mixture containing 75% (vol/vol) hydrogen until the iron oxide was completely reduced to iron. kept at temperature.

In both sets of experiments, after cooling to room temperature under a hydrogen/nitrogen solution, the hydrogen/nitrogen mixture was replaced with nitrogen flowing at a rate of 200,137 hours, and then the nitrogen was gradually replaced with air over a period of 30 minutes. Replaced. These shaped articles were then inspected. The results are shown in the table below.

For comparison, a composition was also made in which the magnesium aluminate spinel was replaced by magnesium.

When reduction procedure A was carried out on unitary articles made from compositions omitted entirely from lime, the original integrity of the shaped articles remained intact. However, the disadvantage of omitting lime is that the catalytic properties of the catalyst are adversely affected and also the sintering temperature required to achieve the desired density of the sintered article is increased.

Example 2 A composition containing 1% (w/w) magnesium aluminate spinel was used and arranged in a circular ring of 29.14 and 6 hard cores around a central core. Extrudates were made according to the procedure of Example 1 using an extrusion die with 50 wire cores. The gou and core were sized such that after sintering, the extruded article had a length and diameter of 8.5 mm and a through passage of approximately 0.48 mm. The particle density was 4.09/demand-3.

The chemical composition of the article by weight was: F'e203 96.4% AI! 203 2.3% CaO 0.6% M2C 0.3% 20 0.4% An adiabatic reactor was used to evaluate the total activity of the catalyst produced by reduction of shaped articles. These shaped articles f23.7
Loaded into a catalyst bed with a volume of 1 and an outer diameter of 203 mm and an inner diameter of 8 mm.
It filled an annular space with a length of 1015 mm. The shaped article was exposed to 2.0% hydrogen and nitrogen at a pressure of 80 bar (absolute).
3007TL3 with a gas mixture containing a molar ratio of 35
/hour (under standard conditions). The gas inlet temperature was initially 350°C and the temperature was gradually increased until reduction was complete as evidenced by the cessation of water vapor formation. The rate of temperature rise was adjusted to maintain the water concentration below 2000 ml by volume. Once the reduction is complete, the inlet temperature is reduced to 350°C, and after steady state is achieved, the ammonia concentration in the gas leaving the bed is 8.3% (wt/wt).
The temperature rise across the bed was 95°C.

After passivation as described above, the discharged catalyst R-let remained in its original form.

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Claims (10)

[Claims] (1) A sintered iron oxide-containing article containing a small proportion of calcium oxide, characterized in that it also contains a small proportion of magnesium aluminate spinel. (2) A sintered article according to claim 1, comprising from 0.1 to 1 mole of magnesium aluminate per mole of calcium oxide. (3) A sintered article according to claim 1 or 2, containing up to 5% by weight of calcium oxide. (4) A sintered article according to claim 3, comprising from 0.5 to 5% by weight of calcium oxide. (5) The sintered product according to any one of claims 1 to 4, which contains alumina and/or magnesia in addition to magnesium aluminate spinel. (6) Forming a finely divided particulate composition containing iron oxide and a small proportion of calcium oxide or a calcium compound that decomposes to calcium oxide upon heating and magnesium aluminate into a shaped article and baking the composition. A method of manufacturing a sintered oxide-containing article, the method comprising: (7) Forming a finely divided particulate composition containing iron oxide and a small amount of calcium oxide or a calcium compound that decomposes to calcium oxide upon heating and magnesium aluminate into a shaped article and sintering the composition. A method of making a sintered iron article comprising: forming a sintered oxide-containing article; and then reducing the iron oxide in the sintered iron oxide-containing article to metallic iron. (8) Flow of reducing gas onto the sintered iron oxide-containing product for 30 minutes.
8. A process according to claim 7, wherein the reduction is carried out by passing at a temperature of 0 to 1200<0>C. (9) A sintered iron article comprising calcium oxide and magnesium aluminate spinel, produced by the method according to claim 7 or 8. (10) A sintered iron article comprising a product obtained by reducing iron oxide in the sintered iron oxide-containing article according to any one of claims 1 to 5 to metallic iron.