Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
  • C22B7/00—Working up raw materials other than ores, e.g. scrap, to produce non-ferrous metals and compounds thereof; Methods of a general interest or applied to the winning of more than two metals
  • C22B7/02—Working-up flue dust
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
  • C22B19/00—Obtaining zinc or zinc oxide
  • C22B19/30—Obtaining zinc or zinc oxide from metallic residues or scraps
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
  • C22B7/00—Working up raw materials other than ores, e.g. scrap, to produce non-ferrous metals and compounds thereof; Methods of a general interest or applied to the winning of more than two metals
  • C22B7/04—Working-up slag
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
  • Y02P10/00—Technologies related to metal processing
  • Y02P10/20—Recycling

Definitions

• the object of the present finding is a pyrometallurgical treatment aimed to treat slag resulting from metallurgical processes.
• the object of the present invention is a treatment for purifying slag containing non-ferrous metals such as lead, zinc and cadmium or oxides thereof or chemical compounds thereof.
• the present invention relates to the recovery of lead, zinc, and any cadmium contained in the slag generated by a waelz treatment.
• Electric arc steel and iron furnaces are used for producing steel and iron products (laminates, section bars, rods for reinforced concrete,...) which are fed with ferrous scrap which, according to the source, may be galvanised or coated with zinc- and lead-based products.
• the zinc and lead compounds leave the charge, exiting the furnace in the form of very fine dust, mainly consisting of iron, zinc and lead oxides, with smaller amounts of other oxides and chlorides, among which cadmium compounds.
• powdery residues are known as steel plant fumes or by the English term Electric Arc Furnace Dust (hereinafter referred to as EAF dusts).
• EAF powders are classified as special hazardous waste and were once delivered to dump.
• waelz process has experienced a very quick growth. This has allowed the recovery of the zinc and lead contained therein in significant amounts (up to 35% by weight).
• EAF dusts are currently treated with the waelz process, corresponding to over one million tons a year.
• the waelz is a metallurgical technology aimed to recover non-ferrous metals, especially zinc and lead, from natural raw materials or from scrap and residues of other iron and steel or metallurgical operations, such as in particular EAF powders.
• the raw materials admixed with reducing agents (such as coke dust, anthracite, petroleum coke) and basic ratio correctors, constituting as a whole the charge, continuously feed a rotary furnace (generally 50 - 70 metre long and with a diameter of about 3 - 4 metres) where they are thermally treated at the solid state at a temperature that exceeds 1000 °C.
• Such process generally takes place as a “basic process” and with material at the solid state (by “basic process” or “acid process” it is meant a process carried out in a basic or acid environment, respectively).
• basic ratio correctors among which calcium oxide (CaO), are added in such amounts as to not only ensure the basic process but also raise the melting point of the charge treated to such levels as to prevent the local melting thereof, or the furnace would lock.
• Zinc, lead as well as other compounds are thus extracted from the charge but the extraction is not total also because the process temperature is not sufficiently high to allow all that part of such metal oxides that is inside the same particles to migrate towards the surface of the EAF dust particles.
• the charge exits from the furnace in the form of a granular mass and constitutes the so-called "waelz slag" which is significantly but not sufficiently free from the non-ferrous metal content.
• Lead in particular, is generally present in amounts higher than 0.5 %; a limit value, as will soon be seen, for reusing the slag which must therefore be disposed of in suitable dumps.
• the main object of the present finding is to provide a treatment process for slag still containing moderate amounts of zinc, lead and optionally cadmium, capable of purifying the same slag to a sufficient point to allow the classification thereof as a product and not as waste, thus obtaining reasonably low energy costs.
• FIG. 1 shows the layout of a known waelz process plant
• FIG. 2 shows the same waelz plant but equipped with the waelz treatment plants according to the invention
• FIG. 3 shows, in a ternary diagram CaO, Si0 2 , FeO the possible chemical- physical states of the charge in a waelz furnace and of the waelz slag during the treatment thereof according to the invention
• FIG. 4 shows, in a diagram, the pattern of the melting temperatures of waelz slag as a function of the ratio between lime (CaO) and silica (Si0 2 ) present in the slag charge to be treated.
• non-ferrous metals will be used to indicate metals such as zinc, lead and optionally cadmium too, object of the extraction process according to the present invention, or also other metals capable of being extracted according to the same process.
• mass it is meant the material, generally slag from previous metallurgical treatments, from which said non-ferrous metals are to be extracted with the process according to the invention.
• charge it is meant the material consisting of said mass with the addition of the additives needed for said process.
• the charge composition coincides with that of the mass.
• a mass from which said non-ferrous metals are to be extracted, contained therein in the form of oxides (in particular lead and zinc) is brought to melting through heating by combustion in a static tank furnace where it is kept in a reducing environment and at a sufficient temperature to obtain the reduction to metal of at least a part of said non-ferrous metal oxides, with the consequent escape of said non-ferrous metals by evaporation.
• oxides in particular lead and zinc
• additives are added to the mass, apt to form a charge the melting point whereof is sufficiently low to make the same melting easier through heating by combustion.
• the mass to be treated does not per se includes reducing substances in a sufficient amount to reduce to metal substantially all the amount of oxides of the non-ferrous metals contained therein, at least the additional amount thereof needed to achieve such object is added.
• chlorides may also form; they undergo the same treatment and the same use purpose as said oxides and shall not be mentioned again anymore considering them similar to oxides for the purposes of the invention.
• the molten charge and the slag thus purified are continuously or intermittently tapped off, left to cool and prepared for a reuse thereof.
• the elimination of most of the zinc present in the molten charge and substantially of all the lead is obtained with such process, or at least the content thereof is reduced to values well below 0.5% that the ECHA has set as a limit to classify such material as a product.
• the temperature of the treatment chosen for the molten charge is just that sufficient to reduce to metal such non-ferrous metal oxides and is substantially a little higher than the melting temperature.
• additives or “scorifiers” apt to decrease the melting point of the charge shall also be, generically, called “low melting additives” or “low melting scorifiers”.
• the above reducing substances, the low melting scorifiers and any other additives suitable or needed for a correct execution of the process according to the invention may be added to the mass wholly or partly before the introduction into the furnace, or in the same furnace.
• methane burners are used for the combustion; even more preferably, the burners are of the oxy-fuel type (burners where the combustible is pure oxygen instead of air).
• the oxy-fuel burners have reduced sensitive heat dispersions through the combustion gases since these have a significantly lower mass if compared with combustion gases obtained using air as combustible.
• the burners may be arranged in such a way as to be directed on the liquid mass, with the dual advantage of reducing energy consumptions, since the heat exchange efficiency is improved, and of producing a strong remixing that increases the reaction kinetics.
• Methane or carbon fed under-slag nozzles may also be provided; the fuel thus introduced also validly contributes to form the reducing environment required in the molten charge.
• a slag treatment plant 1 according to the waelz process comprises the waelz rotary furnace 1.1, the loading station 1.2 of the incoming materials (EAF dusts, carbon, basic correctors), the air inlet 1.3, the waelz slag outlet 1.4, the powdery oxide outlet 1.5 and the collection unit 1.6 of said dusts, a product known on the market as waelz oxide.
• the incoming materials EAF dusts, carbon, basic correctors
• the dashed area represents the working field of the waelz furnace where, with a predetermined ratio between CaO and Si0 2 (equal to 2 in the example of fig. 3), by the effect of the reducing action of carbon, the iron oxides present in the fumes at the higher level of valence are progressively transformed into others at a lower valence up to having also finely dispersed metal iron in a small part.
• the temperature in the furnace increases so the reduction of the zinc and lead oxides and their elimination from the mass becomes prevalent.
• Air is blown in the end part of the furnace which partly re-oxidises the iron, with an important energy recovery, moving the slag composition to the right of the area, thus towards higher FeO values, in the proximity of the area of the ternary diagram that shows the wording wustite which has low melting points.
• the melting of the slag into the furnace must be absolutely avoided or it would lock the operation of the same furnace for a quick closing thereof, thus the operating temperatures are kept relatively low reducing the efficacy of the lead and zinc elimination.
• the slag composition exiting from the waelz furnace with suitable additions of scorifiers apt to lower the melting point of the charge thus obtained so that bringing it to melting in a fuel-fed tank furnace is easier and energetically less expensive.
• the addition of the low melting scorifier allows the treatment range of the charge obtained from the waelz slag to be brought in the dashed area indicated with F in the ternary diagram of the CaO - FeO - Si0 2 system of fig. 3 (where the low melting scorifier is Si0 2 ) without substantially changing the nature of the same waelz slag, so as to keep the features as per REACH definition.
• the Si0 2 percentage is 22%, that of CaO is 27% and the CaO/Si0 2 ratio is about 1.2.
• fig. 4 shows the pattern of the melting point of the charge as the ratio between the amounts of CaO and Si0 2 present in the charge varies, where with a predetermined total amount of calcium oxide present in the waelz slag, a reduction of such ratio is obtained by increasing the amount of Si0 2 . It can be seen that melting temperatures can be reached which are absolutely feasible in a tank furnace with combustion heating.
• said values are not binding; it is only preferred that also with the addition of silicon oxide Si0 2 or of other substances having a similar effect, a basic process is in any case ensured.
• the temperature pattern is as shown in fig. 4, operating in a zone with the lowest possible melting temperature but compatible with such preference of carrying out the basic process treatment implies a good process control if it is deemed appropriate to add additives that alter the chemical composition; a good uniformity of the molten charge is also required.
• a tank furnace ensures uniformity of the molten charge and said possible strong mixing that may be obtained with the burners suitably arranged as mentioned above may also help in obtaining said uniformity (which is impossible in a rotary furnace).
• the low melting scorifier added contains silicon oxide Si0 2 , it may comprise or consist of siliceous sands or preferably, exhaust foundry sands. In this latter case, there is the further advantage of also recovering the foundry scrap, per se unusable, besides the waelz slag. Glass scraps may also be suitable for the purpose.
• clay and/or bentonite may also advantageously be used as low melting scorifiers, as they are substantially composed of silica (Si0 2 ) and alumina (aluminium oxide; A1 2 0 3 ); the latter has the capability of affecting the melting temperature not really by reducing the minimum value thereof but enlarging the low melting area indicated with F in the ternary diagram of fig. 3; more exactly, the presence of alumina in moderate amounts (2 ⁇ 4 %) in the charge ensures the stay of the charge in the low melting area for wider variations of the weight ratio between CaO and low boiling substance (e.g. Si0 2 ) than in the absence of the same alumina (an effect simplified in a qualitative manner by the dashed line of fig. 4).
• silica Si0 2
• alumina aluminium oxide
• Bauxite may be used as scorifier for the addition of alumina, besides said clay and/or bentonite.
• the reduction and evaporation of the non-ferrous metals is only possible in a reducing environment, a condition ensured by the fact that the waelz slag, very advantageously for the purposes of the process according to the present invention, always contains significant residues of unburnt carbon.
• An advantage of the process is that the calcium oxide CaO present in the slag is neutralised forming, with silica and iron (at least in the waelz slag, always present), chemically stable compounds such as calcium silicate and calcium iron silicate so that the resulting material is a stable ceramic compound usable at least as inert for road beds or concrete mixes.
• FIG. 2 shows, in the essential components thereof, a plant that implements the process according to the invention applied to the waelz slag treatment.
• Reference numeral 2 indicates the plant as a whole.
• Reference numeral 2.7 indicates the tank furnace with combustion heating.
• Reference numeral 2.1 indicates the loading section of any additives (such as scorifiers apt to lower the melting point of the charge and reducing agents) whereas inlet 1.4 of the slag to be treated coincides with the outlet of the waelz slag 1.4 from which said slag can advantageously be directly taken.
• Furnace 2.7 is shown uncovered only for schematic simplicity but it is clear that it is closed at the top by a dome wherein the evaporated metals, encountering an oxidising environment generated by a suitable excess of the comburent air of the burners or by the administration of an oxidising agent suitably introduced, are transformed into oxides and are carried over by the combustion fumes along conduit 2.3 which takes them to the collecting unit 1.6.
• the latter can advantageously be the existing unit of the waelz plant 1.
• Arrow 2.2 indicates the inlet of fuel and combustible.
• the purified mass can be easily tapped continuously through conduit 2.5, preferably at the surface of the molten charge; the possibility of periodically tapping said material also from the furnace bottom through conduit 2.4 is also provided for maintenance operations or others.
• the purified mass is fed to a granulator 2.6 from which it exits as reusable product 2.7 in a more suitable size.
• the waelz slag treatment plant 2 according to the invention can constitute an appendix of the waelz plant 1 of which it becomes an integral part where the final step of the EAF foundry dust treatment is carried out.
• Plants according to the invention for treating slag other than waelz slag have a structure equivalent to that just described.

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• Organic Chemistry (AREA)
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• General Life Sciences & Earth Sciences (AREA)
• Environmental & Geological Engineering (AREA)
• Life Sciences & Earth Sciences (AREA)
• Manufacture And Refinement Of Metals (AREA)
• Curing Cements, Concrete, And Artificial Stone (AREA)
• Processing Of Solid Wastes (AREA)

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• 2012
  • 2012-04-17 IT IT000043A patent/ITAN20120043A1/it unknown
• 2013
  • 2013-04-10 WO PCT/IB2013/000641 patent/WO2013156832A1/en active Application Filing
  • 2013-04-10 EP EP13720545.6A patent/EP2839044A1/en not_active Withdrawn
  • 2013-04-10 RU RU2014145782A patent/RU2014145782A/ru not_active Application Discontinuation
  • 2013-04-10 CN CN201380031566.4A patent/CN104379780A/zh active Pending
• 2015
  • 2015-04-28 HK HK15104105.7A patent/HK1203570A1/xx unknown

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