PROCESSING OF METAL CHLORIDE SOLUTIONS AND METHOD AND APPARATUS FOR PRODUCING DIRECT REDUCED IRON

BACKGROUND OF THE INVENTION

1. Field of the Invention

[0001] The present invention relates to a method and apparatus for processing metal chloride liquors, such as spent acid liquors from either acid leaching of metalliferous minerals or acid pickling of metals, and to a method and apparatus for production of direct reduced iron (DRI).

2. Description of Related Art

[0002] There are several important industrial processes which produce as a by-product metal chloride liquors, in the form of spent acid liquors which contain metal chloride in solution.

[0003] Such metal chloride liquors include:

• Spent leach liquors resulting from the acidic digestion of a mineral species such as, but not necessarily restricted to, the mineral known as ilmenite. The acid in this case may be, but is not necessarily restricted to, hydrochloric acid.

• So-called 'pickle liquor' resulting from an acid treatment or pickling, or etching of metallic surfaces for cleaning of rust or oxide scale. The acid in this case may be, but is not necessarily restricted to, hydrochloric acid.

[0004] In the first of these cases, ilmenite, a titanium mineral, is leached in hydrochloric acid to yield a synthetic rutile and spent leach liquor containing iron and other chlorides in solution, together with a small excess of hydrochloric acid, according to the generalised reaction equation given below:

FeO ₁ TiO ₂ + 2HC1 → FeCl ₂ + TiO ₂ + H ₂ O

[0005] Elemental oxide constituents of the ilmenite (other than iron) which are soluble in hydrochloric acid are similarly dissolved to form their respective chlorides. The resultant solution from this type of reaction would typically have a composition similar to the example given in the table below:

[0006] Table 1 - Typical analysis of spent metal chloride liquor from acid leaching of ilmenite.

[0007] In the second case, steel products that have acquired an oxide or rust coating are often subjected to a process referred to as 'pickling' wherein the oxide or rust coating is removed by passing the steel through a hot solution of hydrochloric or

other acid. In the case of the use of hydrochloric acid, the reactions involved may be simplified to the following:

Fe ₂ O ₃ + Fe + 6HCl → 3 FeCl ₂ + 3 H ₂ O

[0008] Known processes for treatment of such metal chloride liquors concentrate on regeneration and recovery of the acid, for recycling to the acid leaching or pickling process.

[0009] This acid regeneration is commonly done by pyrohydrolysis, in which the metal chloride reacts with water and oxygen to recover the acid, producing a metal oxide as a by-product.

[0010] In the case of iron chloride, the predominant reactions taking place during pyrohydrolysis may be expressed as follows:

2FeCl ₂ + 2H ₂ O + ¹ AO ₂ → Fe ₂ O ₃ + 4HCl

[0011] Pyrohydrolysis is conducted at temperatures which may range from 600°C to 1200 ⁰ C but preferably in the range of 850°C to 95O ⁰ C. The fluidising gas is typically air. Sufficient fuel is added to maintain reaction temperature and to control oxygen potential. The off-gas, containing products of combustion and hydrochloric acid vapour, is treated for recovery of the hydrochloric acid component.

[0012] Pyrohydrolysis may be carried out by one of two well-known methods utilised in the current art, namely:

• In a spray tower, wherein the liquor is sprayed into a void chamber and reacted with hot gases derived from the burning of a liquid or gaseous

• fuel. In this instance the metal oxide product is in the form of a fine powder.

• In a fluidised bed reactor, wherein the liquor is injected into a fluidised bed of previously manufactured metal oxide, and the mass maintained at high temperature by the injection of a gaseous, liquid or solid fuel or

any mixture thereof. In this type of equipment, the metal oxide residue may,be in the form of small pellets with a nominal size of lmm to 2mm in diameter.

[0013] As a further development, as instanced in International Patent Application PCT/AU93/00056 (WO93/16000), the metal chloride liquor is first evaporated to a dry pelleted form before feeding to a pyrohydrolysis reactor of the fluid bed type. The evaporator in this instance may preferably be of the fluid bed type or alternatively rotary equipment may be used, according to normal practice for the thermal evaporation of water from salts in solution.

[0014] The process of WO93/16000 is particularly advantageous for acid regeneration in acid leaching processes as it results in a highly concentrated, superazeotropic, acid solution for return to the leaching step.

[0015] The contents of WO93/16000 are incorporated herein by reference.

[0016] Other known acid regeneration processes include the KeramChemie process, in which the metal chloride liquor undergoes pyrohydrolysis without pre- concentration, resulting in a sub-azeotropic acid solution.

SUMMARY OF THE INVENTION

[0017] The present invention aims to provide a process of improved commercial viability, and apparatus for conducting the process.

[0018] In one form, the invention provides a process for regeneration of acid and metal from spent acid liquor containing metal chloride in aqueous solution, including the steps of, in sequence:

• Optionally, concentrating the liquor to form a liquor concentrate or solidifying the liquor concentrate to form a solid metal chloride;

• Pyrohydrolysing the liquor, concentrated liquor or solid metal chloride in a reactor to regenerate the acid and form a metal oxide; and

• Reducing the metal oxide to metal.

[0019] Preferably, the metal is iron or is predominantly iron.

[0020] In one preferred form, the reduction step is conducted as a two stage reduction reaction, including a first stage reduction reaction to a lower oxidation state oxide, using a partially combusted fuel as a reducing agent, and a second stage reduction reaction in which the lower oxidation state oxide is converted to the metal.

[0021] Preferably, the first reduction stage is conducted in a first fluid bed reduction reactor in which the metal oxide is contacted with the fuel and a sub- stoichiometric amount of oxygen.

[0022] Alternatively, the fuel and oxygen may be contacted prior to the first reduction stage.

[0023] Preferably also, the off-gas of the first reduction stage is used as the reducing agent for the second reduction for the second stage.

[0024] In a further preferred form, the off-gas from the second stage reduction reactor is oxidized to provide energy for the concentration or pyrohydrolysis steps of the process. Preferably the hot gas required for the evaporation of the metal chloride liquor is provided by the off-gases from the oxide pellet metallising stage.

[0025] In a further form, the invention provides a process for treatment of a metal oxide feed, including the steps of:

• Contacting the metal oxide feed with a reducing agent comprising a fuel and a sub-stoichiometric amount of oxygen, in a first stage reduction to form a lower oxidation state oxide; and

• Feeding the lower oxidation state oxide formed by the first stage reduction and off-gases from the first stage to a second reduction stage wherein the lower oxidation state oxide is reduced to the metal.

[0026] Preferably, the first and second stages reduction reactions are carried out in respective first and second fluid bed reduction reaction chambers.

[0027] Preferably, the metal oxide feed is formed by pyrohydrolysis of a spent acid liquor containing a metal halide, preferably chloride.

[0028] Further aspects of the invention are as set out in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Further preferred embodiments of the invention will now be described with reference to the accompanying drawings, in which:

[0030] Fig. 1 is a flowchart of processing of a spent metal chloride liquor in accordance with a first embodiment of the invention;

[0031] Fig. 2 is a flowchart of processing of a spent metal chloride liquor in accordance with a second embodiment of the invention;

[0032] Fig. 3 is a flowchart of processing of a spent metal chloride liquor in accordance with a third embodiment of the invention, and

[0033] Fig. 4 is a flowchart of processing of an iron ore to iron metal.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0034] Fig. 1 illustrates a first embodiment of the invention, which includes evaporation and pelletisation of the metal chloride (MeCl) liquor prior to pyrohydrolysis.

[0035] Spent leach liquor derived from the leaching of iron or other metallic oxide or spent pickle liquor derived from a steel or metal finishing process is injected into an evaporator 110 of a fluid bed, rotary kiln or other suitable type. The

iron or metal chloride product discharged from the evaporator is preferentially a pelleted solid. The hot gas providing the heat source for this evaporation is derived from the off-gas of the two metallising stages hereinafter described, further oxidised in an afterburner section 112 of the evaporator.

[0036] The evaporator product is fed to a pyrohydrolysis reactor 114, normally of the fluid bed type, where the iron or other metal chlorides are converted to metallic oxides by reaction with water at a temperature preferably in the range of 600°C to 1200 ⁰ C. The water for this reaction may be provided by combustion of the fuel used and by residual water of crystallisation in the reactor feed from the evaporator.

[0037] The concentration, pelletisation and pyrohydrolysis steps and their equipment and operating parameters are known per se, and are described in more detail in WO93/16000.

[0038] It should be noted that spent liquor or concentrated spent liquor may be substituted for part or all of the pelletised material fed to the pyrohydrolysis reactor.

[0039] Metal oxide pellets are discharged at a high temperature from the pyrohydrolysis reactor into the a fluid bed gasifying reactor (gasifier) 116, which serves as the first stage reduction reactor, operating at approximately 1000°C ± 100 ⁰ C. The gasifier may be supplied with either a solid, liquid or gaseous hydrocarbon fuel such as is appropriate to local availability and cost structures. Suitable fuels may include coal, oil, or natural gas.

[0040] In the gasifier, the fuel is converted to a reducing gas predominantly consisting of hydrogen and carbon monoxide.

[0041] As a further embodiment, the feed to the first stage reduction unit may be augmented or (as discussed later with reference to Fig. 4) entirely replaced by iron ore or metallic oxide wastes such as mill scales and baghouse dusts.

Alternatively, or in addition, metallic oxide wastes may mixed with the metal chloride liquor before the evaporation step.

[0042] The gasifying reactions involved are quite complex but have been exhaustively studied and reported in the literature. Examples of some of the more simple reactions involved are given below:

C + O ₂ → CO ₂

2C + O ₂ → 2CO

2C + I ¹ Z ₂ O ₂ → CO + CO ₂

2C + 3H ₂ O → CO + CO ₂ + 3H ₂

CH ₄ + 2O ₂ → CO ₂ + 2H ₂ O

CH ₄ + H ₂ O → CO + 3H ₂

[0043] The oxygen for these reactions may be supplied either as ambient or preheated air and with or without oxygen enrichment. The aim is to ensure that only sufficient oxygen is used to provide enough heat of combustion to maintain the endothermic reactions that generate the CO and H ₂ components necessary for the metal oxide reduction reactions occurring simultaneously. For example, it has been found that satisfactory results may be obtained with an oxygen supply of between 30% to 50% of the full stoichiometric combustion requirement.

[0044] The bed of FeO acts as a catalyst for the gasification reactions.

[0045] hi the gasifier, the metal oxide is converted rapidly, in the case of iron (III) oxide, into an oxide of lower valence, Fe(II), without metallising. It is important that the reduction takes place rapidly so that the formation of FeO is predominant and the formation of the intermediate oxide Fe ₃ O ₄ is minimised, as too high a proportion OfFe ₃ O ₄ may lead to incipient fusion taking place and the resultant 'stickiness' leading to de-fluidisation of the bed.

[0046] The reactions taking place in the first stage reduction are:

3Fe ₂ O ₃ + H ₂ → 2Fe ₃ O ₄ +Tf ₂ O 3Fe ₂ O ₃ + CO → 2Fe ₃ O ₄ + CO ₂ Fe ₃ O ₄ +H ₂ → 3FeO + H ₂ O Fe ₃ O ₄ + CO → 3FeO + CO ₂

[0047] From the gasifier, partially reduced solid oxide pellets - predominantly FeO - are elevated by means of a pneumatic "J-valve" fluid power or other suitable device into the metallising fluid bed reactor train 118. The gases exiting the first stage reactor are used as the fluidising medium in the second stage or metallising reactor and contain sufficient residual reducing gases H ₂ and CO for the . required reaction. Where coal is used as the fuel for the first reduction stage, char may be formed in the first stage and is carried with the oxide pellets into the second stage. The temperature of the second reactor chamber is slightly lower than in the first reactor - for example about 900 ⁰ C ± 100 ⁰ C- and the residence time in this reactor is of the order of one hour for maximum conversion of oxide to metal.

[0048] The reactions in the second stage reduction are:

FeO + CO → Fe + CO ₂

FeO + H ₂ → Fe + H ₂ O

[0049] By using the off-gases from the first stage for the second stage reduction, the minor proportion OfFe ₃ O ₄ in the reaction products from the first stage is reduced by reaction with the CO.

Fe ₃ O ₄ +CO → 3FeO + CO ₂

FeO + CO → Fe + CO ₂

[0050] The product from the metallising stage is indirectly cooled under such conditions as to exclude air and so avoid any reoxidation of the product which could occur whilst the material is at an elevated temperature. Nominally, the metallised pellets must be cooled to less than 200° C or lower before contact with air is allowed.

[0051] The indirect solids cooler 20 may be of any suitable type, as known in the art. Where air is used as an indirect cooling medium, as it is in the illustrated cooler, the resultant hot air may be used as the air feed to the gasifier 116.

[0052] As mentioned above, the hot gas required for the evaporation of the metal chloride liquor is provided by the off-gases from the oxide pellet metallising stage. The gases are first passed through the afterburner 112 where any excess carbon monoxide and hydrogen are converted to extra heat for use in the evaporator. Furthermore, heat may be recovered from the hot metal pellets for use in preheating the air feed to the first stage reduction reactor or elsewhere in the plant.

[0053] As a result, it is expected that the metallisation step adds less than 20%, and most likely only about 10%, to the total fuel requirements of the process, compared to the process of WO93/16000, but with a substantial increase in the economic value of the end products.

[0054] Furthermore, the above process provides a single solution to the steel industry for processing of pickle liquor and recycling of iron oxide wastes such as mill scale and baghouse dust, producing regenerated acid and direct reduced iron as valuable products. Higher zinc baghouse dusts may be added to the spent liquor stream for processing and bound in the inert iron oxide pellets produced by the pyrohydrolysis step for disposal without metallisation.

[0055] The acid regeneration plant also has the capability to handle waste water streams generated during normal steelmaking operations or from site storm water run-off. This water may contain fine oxides, oil (from rolling mills), fine coal / carbon or chlorides and would be fed into the evaporator or used for acid absorption, depending on the contaminants present.

[0056] The process produces no solid or liquid effluents, and dioxins and furans have been below the level of detection during test operations.

OPERATING DATA

[0057] During a bench-scale trial with spent liquor derived from the hydrochloric acid leaching of an ilmenite, the following data was recorded:

[0058] Liquor Feed to the Evaporator

[0059] The composition of the feed liquor to the evaporator has been quoted in Table 1 above

[0060] Evaporation Stage

[0061] Evaporation and drying of the solid iron chloride was taken to the point where the so-called water of crystallisation corresponded to an empirical formula of FeCl ₂ .1.5H ₂ O, thus providing sufficient water for the pyrohydrolysis reaction

[0062] Pyrohydrolysis

[0063] Pyrohydrolysis was conducted at a nominal temperature of between 85O ⁰ C to 950°C

[0064] Reduction

[0065] Iron oxide pellets from pyrohydrolysis were first reduced to the monovalent state and then fully reduced to the metallised state. The relevant data are given in Table 2 below.

[0066] The temperature in the first stage was 95O ⁰ C and the retention time was half an hour.

[0067] The temperature in the second stage was 930 ⁰ C and the retention time was two and a half hours.

[0068] Table 2: Reduction Feed and Product analyses

[0069] The iron product from this trial was non-pyrophoric; therefore no briquetting or special handling would be required for subsequent safe use.

[0070] The chemical composition and physical state were such that it would be considered satisfactory feed for molten metal or steel production, for example as feedstock for an electric arc furnace.

[0071] Operating Pressures

[0072] This embodiment of the invention operates at essentially atmospheric or low pressure and none of the major process vessels or equipment need be certified according Australian Standard AS 1210-1997 and any amendments thereof.

[0073] Fig. 2 illustrates a second embodiment of the invention, in which the spent metal chloride liquor is fed directly to the pyrohydrolysis reactor 214 without prior evaporation and pelletisation.

[0074] hi the embodiment of Fig. 2, the illustrated process are constructed and operated in similar manner to their analogously numbered components described above with reference to Fig. 1, except that the pyrohydrolysis reactor typically will need to be larger due to the more dilute metal chloride feed liquor.

[0075] In the arrangement of Fig. 2, the off-gases from the stage 2 reduction are fed to an afterburner 212 and waste heat recovery unit 213 and the waste heat is recovered for use elsewhere in the plant.

[0076] In the arrangement of Fig. 3, the off-gases from the stage 2 reduction 318 are fed directly to the pyrohydrolysis unit 314 for further combustion and recovery of heat values.

[0077] In Figs. 2, 3 and 4 the reference numerals are allocated by analogy to those of Fig.l, with '200-series' numerals used in Fig. 2, '300-series' numerals used in Fig. 3 and '400-series' numerals used in Fig.4.

[0078] Fig. 4 illustrates an alternative embodiment, in which iron ore fines or iron oxide wastes are reduced to iron.

[0079] The iron ore or oxide fines - for example of diameter about 3 mm - are fed to a fluid bed preheater 422, using the residual chemical and thermal energy in the off-gases from the second stage reduction reactor 418. The off-gases, which

contain CO and H ₂ , are fed to an afterburner 424 in the base of the preheater 422 and the combustion gases fed to the fluid bed.

[0080] The preheated feed is then fed to the two-stage co-current reduction reactor 416, 418, which operates as described above with reference to Figs. 1 to 3.

[0081] Though the present invention has been described above with respect to a particular embodiment thereof it is to be understood that the invention is not limited thereto but is capable of variation within the knowledge of a person skilled in the art. All changes which come within the meaning and range of equivalency of the claims are intended to be embraced therein.

[0082] In this specification, the word "comprising" is to be understood in its "open" sense, that is, in the sense of "including", and thus not limited to its "closed" sense, that is the sense of "consisting only of. A corresponding meaning is to be attributed to the corresponding words "comprise", "comprised" and "comprises" where they appear.

[0083] It will further be understood that any reference herein to known prior art is not, unless the indicated to the contrary, an admission that such prior art is commonly known by those skilled in the art to which the invention relates.