GB1599163A

GB1599163A - Ore reduction

Info

Publication number: GB1599163A
Application number: GB34430/77A
Authority: GB
Original assignee: Humphreys and Glasgow Ltd
Current assignee: Humphreys and Glasgow Ltd
Priority date: 1978-05-15
Filing date: 1978-05-15
Publication date: 1981-09-30

Description

(54) IMPROVMENTS IN OR RELATED TO ORE REDUCTION (71) We, HUMPHREYS & GLASGOW LIMITED, a British Company, of 22 Carlisle Place, London SW1P 1JA, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to a continous process for the reduction of ore, particularly iron ore.

Whilst the present invention can be applied to the reduction of a variety of ores, particular reference will be made hereinafter to the reduction of iron ore to yield metallic iron.

In conventional continuous processes iron ore is reduced to metallic iron in two stages in accordance with the following simplified equations: Fe2O3 + H2 < 2 FeO + H20 (1A) Fe2O3 + CO < 2 FeO + CO2 (1B) FeO + H2 < Fe + H2O (2A) FeO + CO < Fe + CO2 (2B) It has been found that the reactions described by equations 2A and 2B cannot be effectively conducted when the concentration of the reducing components. H2 and CO, in the process stream contacting the iron ore is less than about 70 mole %. Ideally the reducing gas stream should be composed entirely of these reductants, and contain no other diluting gases.However. the processess producing the reducing stream and the methods, employed to clean and to heat the stream to the required temperature often introduce diluents in the form of water vapour, N., CO2, CH4 and other gases.

Considerable amounts of energy are expended in circulating the reducing gases (or those process gases which eventually enter the reducing gas stream) through equipment which is essential for their manufacture, purification, heating (or cooling) and through the apparatus in which they are consumed.

Large amounts of heat are also required to raise the temperature of the reducing gas stream to the temperature required in the ore reduction apparatus. This is about 1000"C in many instances.

Generally the reducing components, H2 and CO, are produced from a hydrocarbon feedstock, e.g. natural gas, liquefied petroleum gas, naphtha, kersine, gas oils, crude oil, or the oil residues from oil refining operations, by either partial oxidation or steam reforming.

Presently known processes produce a purified reducing gas stream which contains about 98% of a mixture of H2 and CO; the remainder being water vapour and CO,.

Although the carbon monoxide, which is invariably formed in both the partial oxidation and the steam reforming processes, is a very useful reducing component for metallic ore, it does have the disadvantage that carbon dioxide is formed in the course of the reduction process which is difficult and expensive to remove. If the need to remove CO2 was eliminated from the reduction process a significant saving would be made both in the capital cost and process costs of the overall reduction process.

Conventionally the reducing gas stream is often produced at relatively low pressures by steam reforming the hydrocarbon feedstock at from 870 to 9000C with a steam/carbon mole ratio of aout 5:1. These conditions are generally selected to minimise the amount of methane formed since this component interferes with the reduction process. The temperature conditions do, however. put stringent demands on the reformer tube materials, and the process cost are generally high due to the large amounts of steam that must be raised with such a high stream ratio. Any reduction that may be possible in either of these conditions would be advantageous. The high temperatures needed for this reduction process are achieved in some cases from the combustion of the spent reducing gases leaving the reduction step.

In one known ore reduction process natural gas and steam are passed through a steel chamber, which contains refractory chequer bricks that have been heated to a very high temperature. A type of reforming reaction takes place yielding a gas containing hydrogen and carbon monoxide, which passes to the "Shaft-Kiln".

Gases returning from the "Shaft-Kiln" are passed back through a second chamber, which is identical to the other one referred to above. Air is admitted to burn any hydrogen and carbon monoxide which still remains in the gas. and sometimes additional natural gas is also admitted and burned in air. The heat of combustion raises the temperature of the refractory bricks, which are then ready to receive incoming natural gas and stem. A set of changeover valves allow the gases to be switched from one chamber to the other, when the refractory in the first chamber becomes too cool to effect the necessary reforming reaction and the bricks in the second chamber have become sufficiently heated to receive the incoming natural gas and stream. The avoidance of this method of heating the gases would be advantageous to the capital cost of the overall plant.

In accordance with the present invention there is provided a method for the continuous reduction of an ore which comprises providing a feed stream containing hydrogen and at least one oxide of carbon, purifying the feed stream to remove substantially all of the oxides of carbon present in the feed stream, and feeding the purified stream containing hydrogen gas through a reactor in contact with a stream of ore, whereby the reduction is effected substantially in the absence of any of the oxides of carbon.

Hydrogen gas in large volumes is currently produced either by the partial oxidation or the steam reforming of a hydrocarbon fuel, such as methane. The hydrogen thus produced generally contains carbon monoxide, carbon dioxide and methane. The oxides of carbon can be removed from the gas stream by various wet scrubbing processes, whilst the methane can be removed cryogenically. Carbon monoxide can be absorbed in a "copper liquor" based on one of a variety of copper salt/ammonia complexes, whilst carbon dioxide can be absorbed either in an aqueous alkali or in an amine solution.

In view of the small size of the hydrogen molecule, however, physical separation techniques can also be used selectively to remove the oxides of carbon from the hydrogen.

The present invention preferably uses a Pressure Swing Adsorption Unit (PSA unit) to remove these oxides. This unit is essentially a set of vessels (sometimes as many as ten) containing at least one molecular sieve; and incorporating interconnecting pipework and associated control valves. Such units function by adsorbing certain gases from gaseous mixtures whilst allowing others to pass through the apparatus. A change of pressure allows the adsorbed gases to be released from the molecular sieve.

In the present application a P.S.A. Unit is employed to remove impurities (CH4, H2O, CO2, CO and possibly other gases) from the hydrogen stream being fed to the ore reduction process.

The present process incorporates an apparatus for the preparation of gaseous mixtures containing hydrogen.

Such apparatus is designed to process conventional hydrocarbon materials by subjecting them to heating and vaporisation (when they are liquids), treatment to remove sulphur compounds, and thermal or catalytic reforming or partial oxidation.

Preferably the present invention uses a steam reforming process to produce a reducing stream which contains a maximum percentage of hydrogen, regardless of the amount of methane and carbon monoxide produced thereby.

Heating of the reducing gas can be performed wholly or partially within the gasification apparatus.

One way in which this may be achieved is by passing the reducing gas stream through the convection zone of a steam reformer. Advantage is taken of the fact that heat which is conventionally used for steam generation is now employed for heating reducing gas. This is a consequence of the fact that less steam is required in the gasfication process when gasification is followed by a P.S.A. Unit for purification.

A P.S.A Unit yields hydrogen which is of high purity (99.9 + % H2). Thus a reducing gas is obtained which is essentially 100% H2.

The use of pure H2 as a reducing gas means that the gases leaving the ore reducing apparatus contain mainly water vapour and unconsumed H2. Although some gases occluded in the ore will also appear. the volume of gas needed to be transported is minimized.

Water vapour is easily condensed out of the gases leaving the ore reducing apparatus when these gases are subjected to cooling and cleaning operations. Thus the gas which is recycled contains a high proportion of hydrogen. Furthermore the reducing gas is very dry which enhances the ore reduction process. Also the need to remove CO2 from the gases leaving the ore reducing apparatus in order to reuse the gases as reducing gas is obviated.

It is convenient (although not essential) to operate the P.S.A. Unit at a pressure of about 20 atmospheres. Reducing gas is circulated at approximately 4 atmospheres (although not essential). Gas-handling equipment can thus be smaller than currently used.

By allowing freshly produced hydrogen from the P.S.A. Unit to pass through an eductor (acting as the motive stream) together with the gases returned from the ore reducing apparatus, the pressure of the latter stream is raised partly towards the level required for feeding back to the ore reduction process.

In this manner the combined stream (fresh make-up gas plus recycled gas) can be circulated through the present process with the use of a compressor, but at a reduced rate of power consumption.

One embodiment of the invention will be described by way of example, with reference to the accompanying drawing which illustrates a simplified flow sheet for an iron ore reduction process.

A heavy naphtha hydrocarbon feedstock 1 is mixed with recycled hydrogen 2 and is fed to a disulphuriser 3. Here the mixed feedstock is first passed over a catalyst to convert any sulphur compounds contained therein to H.S and is then passed over a sulphur absorbent (usually, although not essentially, zinc oxide) to absorb the H,S so produced. In some instances steam 4 is blended with the desulphurised mixed feedstock before the combined stream is heated to a predetermined temperature and passed over a catalyst in a catalytic rich gas unit 5 where the hydrocarbon is decomposed to yield a gaseous mixture 6 containing methane, hydrogen, carbon monoxide, and carbon dioxide, together with unreacted steam.

Additional superheated steam 7 is introduced and the stream of reactants is fed to a steam reformer 8. In this example, it is a conventional tubular reformer whereby the reactants are passed over a nickel-based catalyst contained in alloy tubes, which are externally heated to about 850"C. A steam/carbon mole ratio of about 3:1 is generally used.

The gaseous mixture 9 from the reformer has similar constituents to the ingoing stream of reactants, except that the hydrogen and carbon monoxide contents are increased, mainly at the expense of the methane.

Heat is recovered, from the hot gas stream leaving the reformer, in a waste heat boiler 10 where steam is generated. Additional hydrogen is formed in the CO converter 11, where the carbon monoxide is reacted (shifted) to carbon dioxide.

More heat is recovered, from the gases leaving the CO converter, in a series of heat exchangers 12 where the gas is finally cooled to a temperature which is as close as possible to ambient. and the liquid water removed. This last stage of cooling may be effected either by cooling the gas against circulated cooling-water or alternatively by air cooling.

At this stage the process gas stream is rich in hydrogen and contains, in addition, carbon dioxide, methane, carbon monoxide, possibly nitrogen, and water vapour.

Within the P.S.A. Unit 13 the process gas stream is separated into two streams: (a) a product stream 14 which is essentially hydrogen of a high purity; and which is composed of the major portion of the hydrogen which entered the P.S.A. Unit. This stream is fed to an eductor 16.

(b) a tail-gas stream 15 composed of essentially all the carbon dioxide, methane, carbon monoxide possibly nitrogen, water vapour, and the balance of the hydrogen which does not appear in the product stream.

Approximately 75% of the hydrogen which enters the P.S.A. Unit appears in the product stream. This percentage is not constant, but depends upon a number of factors; the most important of which is the pressure at which the tail gas is released from the Unit.

In the eductor 16 the product stream 14 from the P.S.A. Unit (acting as the motive fluid) is mixed with a low pressure recycle gas stream 19, which is essentially hydrogen, returned from the shaft furnace 20.

At the discharge of the eductor the combined stream of hydrogen passes to the suction of compressor 24, in which it is boosted to a pressure which is sufficiently higher than that obtaining in the shaft furnace to overcome the pressure losses in the circuit.

This stream of reducing gas is heated first in the convection zone 17 of the reformer. Here it absorbs heat from the hot flue gases as they pass from the reformer furnace out to atmosphere.

Additional heating of the reducing gas is effected in a heater 18 which is fired with a suitable fuel. Here the reducing gas is heated to about 1000"C before being fed to the shaft furnace 20. In the furnace the reducing gas is passed upwards counter-currently to a downwardly flowing stream of iron ore. A product containing about 94% by weight of metallic iron is discharged from the base of the furnace. The spent reducing gas leaving the top of the furnace, which still contains a significant proportion of hydrogen, is recycled to the eductor 16.

A portion 21 of the product (hydrogen) stream from the P.S.A. Unit is led away to a gas compressor 22 where it is compressed to provide a source of hydrogen for the conversion of sulphur in the hydrocarbon feedstock.

The recycle gas stream 19 is cooled in cleaning unit 23 to condense out the water vapour produced in the furnace 20. Dust removal may also be performed in this unit if required.

Gas stream 19 may also be passed through a heat inter-changer in order to re-heat the recycle stream after cooling and before it is fed to the eductor 16.

The tail gas stream from the P.S.A. Unit is at a low pressure, and is returned to any part of the apparatus (typically the reformer furnace) which requires a source of fuel.

Heat recovery is also possible both from the reformer flue gas and from the flue gas of heater 18.

In one example of the particular process illustrated in the accompanying flow sheet, the process conditions of some of the more important streams are as follows: A Stream Composiflbns Steams 1, 2 Combined He 47.4 Hydrocarbon 52.6 100.0 Volume % Streams 6, 7 Combined Stream 9 H2 7.4 H 42.9 CO 0.4 CO 10.7 CH4 18.7 CO2 7.9 CO2 8.3 CH4 2.3 H2O 65.2 H2O 36.2 100.0 100.0 Volume C,,?) Volume % Stream 14 Stream 15 H2 99.9 H2 37 5 (CO,CH4.CO2) 0.1 CO 10.8 100.0 CO2 44.8 Volume % CH4 6.9 100.0 Volume % B Stream Conditions Streams 1, 2 Combined Temperature 450 "C Pressure 25 bar g.

Streams 6, 7 Combined Temperature 510 C Pressure 23 bar g.

Stream 9 Temperature 850"C Pressure 19 bar g.

Stream 14 Temperature 40"C Pressure 18 bar g.

Stream 15 Temperature 4() C Pressure 0.4 bar g.

C Approximate Flowrates (Dry Volumes Streams 1. 2 Combined 2800 Nm3/h Streams 6, 7 Combined 21700 Nm3/h Streunl 9 52100 Nm3/h Stream 14 29600 Nm3/h Stream 15 26400 Nm3/h Stream 21 1300 Nm3/h D Iron Production The figures quoted here relate to an iron production rate of approximately 50 tonnes/h.

WHAT WE CLAIM IS: 1. A method for the continuous reduction of an ore which comprises providing a feed stream containing hydrogen and at least one oxide of carbon. purifying the feed stream to remove substantially all of the oxides of carbon present in the feed stream, and feeding the purified stream containing hydrogen gas through a reactor in contact with a stream of ore, whereby the reduction is effected substantially in the absence of any of the oxides of carbon.

2. A method as claimed in claim 1 wherein the feed stream containing hydrogen and at least one oxide of carbon is purified using a pressure swing adsorption process to yield the said purified stream.

3. A method as claimed in claim 2 wherein the impure gas stream is formed by the steam reforming of a hydrocarbon feedstock.

4. A method as claimed in claim 3 wherein the steam reforming is carried but at about 85() C using a steam/carbon mole ratio of about 3:1.

5. A method as claimed in any one of claims 2 to 4 wherein the tail gases from the pressure swing adsorption process are consumed as fuel in the overall process.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. B Stream Conditions Streams 1, 2 Combined Temperature 450 "C Pressure 25 bar g. Streams 6, 7 Combined Temperature 510 C Pressure 23 bar g. Stream 9 Temperature 850"C Pressure 19 bar g. Stream 14 Temperature 40"C Pressure 18 bar g. Stream 15 Temperature 4() C Pressure 0.4 bar g. C Approximate Flowrates (Dry Volumes Streams 1. 2 Combined 2800 Nm3/h Streams 6, 7 Combined 21700 Nm3/h Streunl 9 52100 Nm3/h Stream 14 29600 Nm3/h Stream 15 26400 Nm3/h Stream 21 1300 Nm3/h D Iron Production The figures quoted here relate to an iron production rate of approximately 50 tonnes/h. WHAT WE CLAIM IS:

1. A method for the continuous reduction of an ore which comprises providing a feed stream containing hydrogen and at least one oxide of carbon. purifying the feed stream to remove substantially all of the oxides of carbon present in the feed stream, and feeding the purified stream containing hydrogen gas through a reactor in contact with a stream of ore, whereby the reduction is effected substantially in the absence of any of the oxides of carbon.

6. A method as claimed in anv one of the preceding claims wherein the gas stream

leaving the reactor is recycled to the reactor.

7. A method as claimed in claim 6 wherein an eductor driven by the said purified gas stream is used to assist recycling of the gas stream leaving the reactor.

8. A method as claimed in any one of the preceding claims wherein the ore is iron ore.

9. A method as claimed in claim 1 substantially as hereinbefore described with reference to the accompanying drawing.

tO. Ore when reduced in accordance with a method as claimed in any one of the preceding claims.