WO2018081856A1

WO2018081856A1 - Processing of zinc-containing waste materials

Info

Publication number: WO2018081856A1
Application number: PCT/AU2017/051200
Authority: WO
Inventors: John Winter
Original assignee: Austpac Resources N.L.
Priority date: 2016-11-01
Filing date: 2017-11-01
Publication date: 2018-05-11
Also published as: AU2016253551A1

Abstract

A process for treatment of zinc-containing waste materials includes the steps of: (a) Combining the zinc-containing waste material with iron chloride into a solid, pelletised form which includes iron oxide, zinc oxide and iron chloride, (b) Converting the iron chloride to iron oxide in solid form and recovering HCl, (c) Reducing the iron oxide to iron, and liberating zinc and impurities in a gas stream, and (d) Capturing the zinc and impurities from the gas stream and, optionally, reducing the captured zinc to zinc metal.

Description

PROCESSING OF ZINC-CONTAINING WASTE MATERIALS

Field of the invention

[001 ] The invention relates to a process and apparatus for the processing of electric arc furnace dust and other zinc-containing waste materials into one or more usable commodities.

Background of the invention

[002] Electric arc furnace (EAF) dust is waste stream from the processing of scrap galvanised steel in an electric arc furnace, and typically consists mostly of iron and zinc compounds, primarily oxides.

[003] The zinc is primarily in the form of oxides, typically zinc ferrites such as franklinite (zinc ferrite spinel ZnFe₂O₄), although other zinc forms such as ZnO may be present in smaller amounts. Depending on the scrap material feed to the EAF, the zinc content of the EAF dust may be from about 5-40%.

[004] Other components which may be present in minor amounts in EAF dust mixture include lead, cadmium, arsenic, manganese, chromium, copper, silver, calcium and carbon.

[005] EAF dust is classified as a hazardous waste, and disposal costs can be expensive.

[006] Australia produces approximately 25,000 tonnes of EAF dust per year, typically at high Zn concentrations. USA produces roughly 50 times that amount, often at lower Zn concentrations.

[007] Other zinc-rich waste materials include blast furnace dust (for example approx. 2-6% zinc) or basic oxygen steelmaking (BOS) furnace dust (for example about 5-15% zinc).

[008] Processes for recovery of Zn from EAF dust are known.

[009] In the Waelz process, the EAF dust is introduced with a reductant (coke) into a rotary kiln. The zinc compounds are reduced to elemental zinc, which volatilises and oxidises in the vapour phase to zinc oxide. The zinc oxide is collected from the kiln outlet exhaust.

[010] Another process (US Patent 5912402, Drinkard Metalox) dissolves components of the EAF dust in nitric acid to access the zinc compounds. [01 1 ] Another process (Nakayama, 2010, "New EAF Dust Treatment Process : ESRF", South East Asia Iron and Steel Institute (SEAISI) Conference 201 1 ), binds the EAF dust into the briquetted coke/limestone feed of an electric smelting reduction furnace.

[012] Any reference herein to known prior art does not, unless the contrary indication appears, constitute an admission that such prior art is commonly known by those skilled in the art to which the invention relates, at the priority date of this application.

Summary of the invention

[013] There is a need for a new process for treatment of EAF dust to recover the Fe and/or Zn.

[014] The invention relates to a process for recovery of iron- and zinc-rich metallurgical waste dust to recover the iron and/or zinc, whereby the metallurgical waste dust is bound with one or more iron compounds in solid form, from which the iron is subsequently reduced to form iron and a zinc stream for zinc recovery.

[015] In one example form, the invention provides a process for treatment of zinc-containing waste materials, such as furnace dust, including the steps of:

(a) Combining the zinc-containing waste material with iron chloride into a solid, pelletised form which includes iron oxide, zinc oxide and iron chloride,

(b) Converting the iron chloride to iron oxide in solid form and recovering HCI,

(c) Reducing the iron oxide to iron, and liberating zinc in a gas stream, and

(d) Capturing zinc oxide from the gas stream and, optionally, reducing to zinc metal.

[016] In one form, the zinc-containing waste material comprises a waste dust from a steelmaking furnace, such as EAF dust, BOS dust, or blast furnace dust, or a combination of two or more of these.

[017] In one form, the zinc-containing waste material comprises EAF dust, optionally mixed with blast furnace dust and/or BOS dust.

[018] In one optional form step (a) comprises combining the zinc-containing waste material with spent pickle liquor and evaporating water to form solid pellets. Optionally the pellets formed are multi-layered pellet having predominantly iron chloride layers, and combined iron oxide/zinc oxide layers.

[019] The zinc-containing waste material and spent pickle liquor may be fed

separately into an evaporator to minimize formation of zinc chloride.

[020] Optionally, in a preliminary step the zinc-containing waste material may be contacted with a hydrophobic agent, for example oil, to form a surface coating to minimize formation of ZnCI₂ in the evaporation/pelletisation step.

[021 ] The oil may also contribute to the fuel requirements for the subsequent pyrohydrolysis step.

[022] Further example aspects of the invention will be apparent from the following description, and from the claims.

Brief description of the drawings

[023] A detailed description of a preferred embodiment will follow, by way of example only, with reference to the accompanying figures of the drawings, in which:

[024] Fig. 1 is a flow diagram of an example process for recovering zinc and iron from electric arc furnace dust.

[025] Fig. 2 is a flow diagram of an alternative example process, without recovery of metallic zinc.

[026] Fig. 3 is a flow diagram of a further example process, incorporating use of a hydrophilic agent.

Detailed description of the embodiment or embodiments

[027] With reference to Fig. 1 , a process is described whereby iron and zinc compounds in an electric arc furnace dust are recovered as iron and as elemental zinc.

[028] The primary inputs for the process are EAF dust 1 , water 2, coal 3,4, optionally natural gas 5 or other fuel gas, and an iron chloride liquor 6.

[029] The EAF dust feed may also contain a proportion of, or alternatively be composed of BOS and/or blast furnace dust, however for convenience will be referred to hereinafter as "EAF dust".

[030] The iron chloride liquor may optionally be 'spent pickle liquor' (SPL) 13 from an acid treatment ("pickling"), or etching of metallic surfaces for cleaning of rust or oxide scale. Such spent pickle liquor from steel processing typically comprises about 10-20% - usually about 13-17% - iron chloride (predominantly iron (II) chloride FeCI₂), and about 1 -5% - usually about 2-4% - unreacted HCI, in aqueous solution.

Evaporation/Pelletisation

[031 ] The EAF dust - which is typically of particle size minus 10 μιη - is mixed with fine coal 3 (particle size less than 1 mm, preferably about d50 = 250μιη) and fed into a mixing tank 7 where it is slurried with water 2 to achieve a pumpable slurry, for example containing 60% by mass of EAF dust.

[032] Alternatively, some or all of the coal fraction can be slurried with the SPL, where it also acts as a reductant for any Fe³⁺ in the SPL.

[033] The EAF dust feed slurry is fed to an evaporator/pelletiser, optionally a fluidised bed evaporator/pelletiser 8 as illustrated, along with a feed of spent pickle liquor 6. Preferably the EAF dust feed slurry and SPL feeds are fed through separate feed nozzles and kept separate until fed to the evaporator, to limit reaction of free HCI in the SPL with the ZnO in the EAF dust, limiting formation of ZnCI₂ for reasons described below.

[034] Feed ratios of the SPL to the EAF dust slurry may be adjusted within a relatively wide range, for example 10% to 75% EAF dust by weight.

[035] The free HCI in the pickle liquor feed will volatilise and, in that form, will not substantially react with the zinc oxides in the EAF feed, and thus avoid formation of zinc chloride which would be undesirable in subsequent steps of the process. In particular, any zinc chloride formed would not pyrohydolyse to form zinc oxide and HCI at the temperatures of the subsequent pyrohydrolysis step, described below, but instead is stable and remains as zinc chloride and volatilises and contaminates the HCI produced during the pyrohydrolysis step.

[036] The energy source for the evaporator/pelletiser is combustion gases from a combustion chamber (after burner 9). Typically these gases 10 will be fed to the base of the evaporator/pelletiser at about 400-700^QC, to maintain the fluidised bed reactor at about 120C-200 ^QC, preferably about 140-160 ^QC.

[037] The gas feed also serves to fluidise the mass within the fluidised bed evaporator, allowing formation of solid pellets 1 1 formed primarily of layers of crystalline FeCl₂. H₂O from the SPL, and an amorphous mixture of iron and zinc oxides from the EAF dust agglomerated together.

[038] The FeC layer may be hydrated, typically from about FeCI₂.1 .5H₂O, to FeCI₂.4H₂O.

[039] The EAF dust is substantially unreacted, but is physically bound in the pellet layers.

[040] The average solids residence time in the evaporator/pelletiser 8 is quite long - typically 1 - 3 hours, for example about 1 .5 - 2.5 hours, or about 2 hours.

[041 ] The gas residence time on the other hand is quite short, typically about 1 -5 seconds, more preferably about 1 .5 - 3 seconds.

[042] Typical sizing for the solid iron chloride/iron oxide/zinc oxide pellets 1 1 produced by the evaporator/pelletiser are about 1 - 5 mm, for example about 2-3 mm.

[043] The proportions of EAF dust to iron chloride in the pellets can vary widely, up to a maximum where the physical integrity of the pellet is compromised, to a minimum of zero, or to where the quantities of EAF dust being processed are not commercially warranted.

[044] The iron chloride content of resultant pellets may range for example from 30-90 wt%, for example about 30-80 wt%, 40-70 wt%, or about 50-60 wt%. The remainder is primarily ZnO and FeO/Fe₂O3, with the relative ratio of these depending on the Zn concentration of the EAF dust feed.

[045] Preferably, the pellet moisture content is reduced to the point that there is substantially no free water, only the water of crystallisation, present in the pellets.

[046] The exhaust gas 12 from the evaporator/pelletiser, which typically includes a mix of CO₂ and HCI, goes to the scrubber 13 (described below) for removal and recovery of the HCI.

Pyrohydrolysis Reactor

[047] The iron chloride/iron oxide/zinc oxide pellets are fed to a pyrohydrolysis reactor, such as the fluidised bed pyrohydrolysis reactor 14, for conversion of the iron chloride to iron oxide, and liberation of HCI for recovery. [048] In this reaction, the metal chloride reacts with water and oxygen to recover the acid, producing a metal oxide as a by-product.

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

3FeCI₂ + 3H₂O + ½O₂ → FeO.Fe₂O₃ + 6HCI

[050] Pyrohydrolysis may be carried out in a suitable pyrohydrolysis reactor, for example a fluidised bed reactor 14, wherein the solid pellets 1 1 from the evaporator/pelletiser are fed into a fluidised bed pyrohydrolysis reactor, and the mass maintained at high temperature by insitu combustion of a gaseous, liquid or solid fuel or any mixture thereof.

[051 ] A variety of fuels may be suitable. For example, solid coal of up to 12mm size may be fed to the top of the pyrohydrolysis reactor, or fuel gas 5 fed to the bottom of the reactor.

[052] A large proportion, for example up to 80%, of the fuel energy, may be incorporated in the pellets themselves by way of the fine coal incorporated with the EAF dust feed to the evaporator.

[053] Pyrohydrolysis is conducted at temperatures which may range from 600°C to 1200°C but preferably in the range of 850°C to 1000°C, for example about 950°C. The fluidising gas is typically combustion gases. Sufficient fuel is added to maintain reaction temperature via combustion and to control oxygen potential to about 1 % excess O₂, to result in the formation of magnetite and limit the formation of Cl₂(g). The off-gas 15, containing products of combustion and hydrochloric acid vapour, goes to the scrubber 13 where it is treated for recovery of the hydrochloric acid component.

[054] As mentioned above, at these temperatures any ZnCI₂ in the pelletised feed will not substantially convert to ZnO and HCI, which is why it is desirable to minimise ZnCI₂ formation in the previous process step. ZnCI₂ is a stable volatile chloride at these temperatures and therefore would collect in the scrubbing system.

[055] Solids residence times in the fluid bed pyrohydrolysis reactor are approximately 0.5 to 2 hours, optionally about 0.75 to 1 .25 hours, for example about 1 hour.

[056] In one embodiment, the residence time is sufficient for the pelletised solid feed to be broken down for intimate contact of the iron chloride with the feed gases, and re- pelletisation of the resultant iron oxide with zinc oxide from the EAF dust, to form a pellet size of about 1 - 5 mm, for example about 2-3 mm.

Gas Scrubbing

[057] The gas scrubbing to recover HCI is shown generically in Figs 1 and 2.

[058] A mixture of hot combustion gases and HCI gas leaves the pyrohydrolysis reactor and goes to a two-stage venturi scrubber (not shown), where the hot gas is initially quenched and then scrubbed of particulate matter using SPL. It then passes to a three stage absorption column (not shown) where the HCI gas is absorbed into water counter-currently to form aqueous HCI, for example a concentration of 30%w/w as illustrated. This aqueous HCI 16 is a valuable by-product of the process, and can be sold for use in industry. Non-condensable gases 17 are vented to atmosphere.

[059] HCI typically comprises more than 50% by mass of the condensable components, so super-azeotropic strength acid can be obtained directly by cooling. The azeotrope for HCI is 20.4% by mass, while commercial acid strength is about 32%, so due to excessive vapour pressure above this value, the acid from pyrohydrolysis can be diluted down. Weaker, sub-azeotropic strength HCI streams from down-stream pyrohydrolysis off-gas and evaporation off-gas scrubbing can be used for this duty. The condensable components in the off-gas from evaporation contain an excess of water vapour and produce sub- azeotropic HCI, therefore the off-gas from pyrohydrolysis is used to make commercially saleable acid.

Pre-Reduction Reactor

[060] The iron oxide/zinc oxide pellets 18 from the pyrohydrolysis reactor 14 are fed to a pre-reduction reactor 19, preferably of the fluidised bed type. An example reactor type is a bubbling fluidised bed reactor, fluidised with pre-heated air.

[061 ] In this reactor the pellets are contacted with coke or coal 4 (example particle size less than 15mm) and a fuel gas such as natural gas 5 (optional), to reduce a portion of the iron oxide in the pellets to elemental iron via insitu gasification to form syngas 20 to provide H₂ and CO for reduction. By way of example, the pre-reduction reactor may reduce 30-60%, or 40-50%, of the iron oxide feed to iron, and substantially the remainder to FeO. [062] The ZnO is not reduced in this step, as the partial pressure of O₂ is too high to favour formation of Zn metal.

[063] Example solid feed proportions are about 30% by weight coal. An example reactor temperature range is about 800 to 1000^°C, and solids residence times about 30 to 90 min.

[064] Within the pre-reduction reactor, the iron oxide is reduced by direct reduction of the pellet. The solid carbon remains free / separate rather than being incorporated in the pellets.

[065] The gas offtake from the pre-reduction reactor is primarily a syngas, which is fed to the combustion chamber which feeds the evaporator/pelletiser.

[066] The solids output is a pelletised mix of iron, iron oxide and zinc oxide, plus residual free solid carbon from excess coal/coke introduced in the pre-reduction step.

[067] Typical pellet size may be similar to that fed from the pyrohydrolysis reactor, for example an average pellet size of about 1 - 5 mm, for example about 2-3 mm.

[068] Typically the carbon content of the pre-reduction reactor 19 output 21 is targeted to about 5% to 10% carbon for the next stage smelting operation, depending upon the amount of residual FeO to be metallised.

[069] The proportion of iron/iron oxide to zinc oxide is again determined by the zinc content of the EAF dust in the original process feed, and by the ratio of EAF dust to spent pickle liquor. The process is believed to be tolerant of quite a wide range of Fe/Zn ratios.

Coreless Electric Induction Furnace

[070] The solids output 21 from the pre-reduction reactor 19 is fed to a coreless electric induction furnace (CEIF) 22 for production of iron, for example pig iron 23 at an iron concentration of about 94-96 wt%. The pig iron may also contain about 3.5-4.5 wt% carbon, for example about 4.0%.

[071 ] The CEIF may be of any suitable type, but may preferably be a low frequency induction furnace operating at about 50-150 Hz, for example about 100 Hz.

[072] In the CEIF, the pelletised input is melted by induction heating to form a melt in which the residual iron oxide is reduced by the carbon in the pelletised feed, to form elemental Fe. [073] An advantage of the low frequency CEIF type is deeper active stirring of bath, which assists the incorporation of the pelletised pre-reduced feed into the melt.

[074] Operation of the EIF is in itself known.

[075] The zinc in the EIF feed pellets is reduced to elemental zinc. As zinc has a relatively low boiling point (907^QC), which is well below the temperature of the molten pig iron bath in the furnace (>1250^QC), the zinc volatilises and converts to ZnO in the vapour phase, forming a ZnO dust 24 which is extracted. As the EIF does not generate sparks, there is less contamination of the ZnO vapour phase with Fe.

Zinc Extraction and Reduction

[076] The dust extracted from the induction furnace - which is primarily ZnO with other impurities - is captured from the gas stream by scrubbing, preferably with aqueous H₂SO₄ to dissolve the dust and to form an impure aqueous ZnSO₄ for further processing.

[077] Some of the H₂SO₄ is recycled from a later stage of the zinc purification process, while some make up H₂SO₄ will be required to make up for loss of sulphates from the circuit in the next process step.

[078] The scrubbed gases 34 may be vented to atmosphere.

[079] Any suitable scrubber apparatus may be used, however a venturi scrubber design may be advantageous.

[080] Alternatively the dust may be collected, for example by dry baghouse filtration (not shown), and sold as a saleable commodity in its own right, for example to a zinc refinery.

[081 ] The economic value of the process may be further enhanced by further processing of the ZnSO₄ to zinc metal, which is itself known.

[082] The impure ZnSO₄ solution 26 from the venturi scrubber is conditioned in one or more mixing vessels 28 where metallic zinc dust 29 and other additives such as compounds of antimony and arsenic are added in amounts depending upon the nature and extent of contaminants. The mixing vessels may be heated or cooled again depending upon the nature and extent of the contaminants. [083] In this step, impurities 30 such as Pb, Cd, Sb, and Co precipitate out of solution. These precipitates are removed for disposal or further recovery, and the purified ZnSO₄ solution 31 passes to the electrolysis cell 32 for reduction of the zinc 33.

[084] An example of a suitable electrolysis cell type is a Unicell™ from Corrosion Technology International, Australia, which may be fitted with floor mounted electrolyte distribution manifolds as well as cathodes made from aluminium and anodes made from lead and where the electrodes are connected such that the current flows either through one or both hanger bars.

[085] The outputs from the electrolysis cell are metallic zinc 33, which is extracted for sale, and aqueous H₂SO₄ 34, which is recycled to the venturi scrubber.

[086] Thus, the process extracts the iron and zinc from the waste EAF dust. And while other components such as the Pb, Cd and As in the EAF dust may still require disposal, these are concentrated into a smaller volume than in the EAF dust and can thus be more economically dealt with.

[087] Where the furnace dust used as feed for the process has a lower zinc content, for example comprising wholly or mostly of blast furnace dust or BOS dust with little or no EAF dust, economics may not favour inclusion of the zinc reduction step. In that case, as shown in Fig 2, there may be no separation of the zinc oxide and impurities, and the impure zinc oxide may be removed for sale or for disposal, with the economic benefit of the process deriving primarily from recovery of the iron component of the SPL and furnace dust and from reducing the volume of material for disposal.

[088] The initial stages of the process of Fig 2 are generally similar to those described above in relation to Fig. 1 , with reference numerals and description of Fig. 1 applicable also to Fig. 2.

[089] As shown in Fig. 2, the gas stream and dust 24 from the furnace may be contacted with water 35 rather than H₂SO₄, and filtering out 36 and recovering the combined zinc oxide and impurities as a filter cake 37.

[090] Fig. 3 illustrates a variant of the process of Fig. 1 or Fig. 2, in which the EAF dust 1 and fine coal 3 feedstock is contacted with a hydrophobic fluid, for example a lubricating oil 40, in mixer 7 to form a slurry which is fed to the evaporator/pelletiser 8. [091 ] The oil forms a surface coating on the EAF dust particles which limits reaction of the EAF with the spent pickle liquor 6 in the evaporator/pelletiser 8, thus further minimising production of ZnC^.

[092] Use of a relatively heavy oil, such as a lubricating oil, optionally a waste lubricating oil, is preferred. An oil having viscosity of at least SAE 20, preferably about SAE 30 or higher, is preferred.

[093] The use of oil to slurry the EAF dust and fine coal instead of water as in Figs. 1 and 2 reduces the energy requirements of the evaporator 8, as less water needs to be evaporated.

[094] Further, the oil becomes incorporated in the iron chloride/iron oxide/zinc oxide pellets 1 1 from the evaporator/pelletiser 8 and provides a part of the fuel requirement for the pyrohydrolysis step 14.

[095] The remainder of the process of Fig. 3 is similar to that described above for Fig. 2.

[096] Where ever it is used, 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.

[097] It will be understood that the invention disclosed and defined herein extends to all alternative combinations of two or more of the individual features mentioned or evident from the text. All of these different combinations constitute various alternative aspects of the invention.

[098] While particular embodiments of this invention have been described, it will be evident to those skilled in the art that the present invention may be embodied in other specific forms without departing from the essential characteristics thereof. The present embodiments and examples are therefore to be considered in all respects as illustrative and not restrictive, and all modifications which would be obvious to those skilled in the art are therefore intended to be embraced therein.

Claims

1 . A process for treatment of zinc-containing waste materials, including the steps of:

(c) reducing the iron oxide to iron, and liberating zinc and impurities in a gas stream, and

(d) capturing the zinc and impurities from the gas stream.

2. A process according to claim 1 wherein the zinc-containing waste material comprises a waste dust from a steelmaking furnace.

3. A process according to claim 2, wherein the zinc-containing waste material comprises one or more of electric arc furnace (EAF) dust, basic oxygen steelmaking (BOS) dust, or blast furnace dust.

4. A process according to claim 2, wherein the zinc-containing waste material comprises EAF dust.

5. A process according to claim 2, wherein the zinc-containing waste material comprises EAF dust mixed with one or more of blast furnace dust and BOS dust.

6. A process according to claim 1 , wherein step (a) comprises combining the zinc- containing waste material with spent pickle liquor and evaporating water to form solid pellets.

7. A process according to claim 6 wherein the zinc-containing waste material and spent pickle liquor are fed separately into an evaporator to reduce formation of zinc chloride.

8. A process according to claim 7 wherein the evaporator is a fluid bed evaporator.

9. A process according to any of claims 1 to 8, wherein formation of the solid, pelletised form in step (a) comprises forming a multi-layered pellet having

predominantly iron chloride layers, and combined iron oxide/zinc oxide layers.

10. A process according to any of claims 1 to 9, further comprising the steps of:

(e) separating a zinc compound from the impurities in the gas stream, and

(f) reducing the zinc compound to metallic zinc.

1 1 . A process according to claim 10, wherein the zinc in the gas stream is in the form of zinc oxide, and wherein step (e) comprises contacting the gas stream with aqueous H₂SO₄ to form an impure ZnSO₄ solution, and precipitating out the impurities prior to electrolysis in step (f).

12. A process according to any of claims 1 to 1 1 wherein step (b) comprises pyrohydrolysis of the iron chloride in the pellets to form iron oxide and HCI.

13. A process according to any of claims 1 to 12, wherein step (c) comprises prereduction of iron oxide, followed by smelting.

14. A process according to claim 13 wherein the smelting is conducted in an electric induction furnace, to form pig iron.

15. A process according to any of claims 1 to 14 wherein the zinc-containing waste material is contacted with a hydrophobic agent to reduce formation of zinc chloride in step (a).

16. A process according to claim 15 wherein the hydrophobic agent is an oil.

17. A process according to claim 16 wherein the zinc-containing waste material is slurried in the oil for feeding to step (a).