CHEMICAL PROCESS FOR RECOVERY OF METALS CONTAINED IN INDUSTRIAL STEELWORKS WASTE Field of the Invention

This invention regards the chemical process for treatment of industrial steelworks waste, specially those of electric arch kiln, (EAK), called "steelworks powder", to make possible the subsequent recovery of zinc, iron and other interesting metals there contained. The main purposes of the process are: (i) the opening of the crystalline webs called zinc ferrite originally formed in the steelworks powder, with the simultaneous production of sulfates soluble in aqueous medium; and (ii) the destruction of the possible organic chins containing chlorine, the so called dioxins without its subsequent regeneration.

Generically, the process may be summarized as the production of salts or sulfates of the metals constituting the steelworks powder, utilizing concentrated hydrogen sulfate and reaction reactants or catalysts by dry medium, at normal pressure and moderate temperatures. It consists in promoting the direct reaction of hydrogen sulfate concentrated with the elements constituting the steelworks powder keeping a homogenous mixture without the presence of water, and preventing there is agglomeration of the particles through the addition of secondary reactants and catalysts, in the specific case, the ground solid potassium chloride. Background state of the invention

The metallic zinc has always been employed as coating for protection of ferrous metals against corrosion, generated the so called "galvanized metal" substituting, in many cases, stainless steel in items like plates, tiles, screws and tubes.

Most of the ferrous metals or metal alloy including common iron, carbon steel, alloy steel and cast iron, are recycled several times, closing a

recovery cycle of the metal through the reutilization of the scrap as raw material source. Thus, all zinc applied in the galvanization of ferrous metals follow the same way. But, when these scraps are recycled, the zinc there contained is separated from the iron, as with the high temperatures of the steelworks furnaces, and due to its lower melting point regarding the iron, the zinc is volatilized and pulled with the other powders from the furnace, being captured in special electrostatic or sleeve filters, becoming the known residue called steelworks powder. As it is generated in steelworks that recycle scrap employing electric furnaces called electric arch furnace, it is also called electric arch furnace powder.

This residue contains average percent values of 20% for the zinc, considered here in the elementary form (Zn), and 28% for the iron, also considered in the elementary form (Fe), combined in a chemical structure know as "zinc ferrite", whose formula is oxides combined in the ZnO-Fe ₂ O ₃ form. Besides these, are also found in its composition lead (1,5%), chrome (0,25%), cadmium (0,05%) and tin (0,15%), besides other elements in less contents, such like sulfur, manganese, copper, calcium, magnesium and nickel, generally as oxides. Additionally, are also found in the steelworks powder significant amounts of fluorine (F) and chlorine (Cl), this last coming from the contaminating plastics that constitute the scrap and that have great possibilities of being combined in carbonic structures called dioxins. Such substances, added to the heavy metals lead, chromium and cadmium, classify the steelworks powder as "dangerous", being compulsory its disposition in controlled fillings. This restriction brings high costs to the steelworks, which generates significant amounts of rejects per produced steel ton without there being definitive and environmental correct solutions. Many attempts of reutilization have been presented, but few with economical viability, as

by conventional methods, the steelworks powder present several technical barriers to be processed. Currently, few processes have been consolidated in an efficient way regarding the economic and technical point of view for recycling this material. One of the most known and utilized, currently, is the Waelz process. But, this process requires heavy investments, large production scale, besides generating new residues, being in some countries considered unsustainable from the environmental adequateness point of view. The process proposed in this document treats the technical, economical and environmental conditions, not needing large production scales, reducing costs, valuating liabilities and minimizing impacts to the environment. In the current status of hydrometallurgical technique for the extraction of metals from ores and industrial residues, are utilized, normally, acid or alkaline liquid means for dissolution of the oxides, hydroxides, carbonates, silicates and sulfides that contain the interest metal. The traditional applications of the hydrometallurgy include the production of alumina, gold, uranium,, zinc, nickel, copper, molybdenum, titanium and rare earth, among others. In the first phase of the hydrometallurgy, are adjusted the physical- chemical properties of the solid, such like granulometric strip, composition, content, chemical nature and porosity, for the next phase, the leaching. It consists in an extraction process of a substance of a solid medium by means of its dissolution in an acid or alkaline liquid that shall react chemically with the elements present in the ore or residue, originating a new soluble substance. The hydrometallurgy is very much employed in several fields of science, such like geology, metallurgy and chemistry. Its preparation comprises classical ore treatment operations, such like the comminuting or grinding, classification, concentration and separation solid-liquid.

After the preparation of the ore, we have the leaching phase. Both constitute the most characteristic phases of the hydrometallurgical flowchart. The leaching makes the selective dissolution of mineral containing the metal or interest metals, promoting the contact of the solid formed by the concentrate ore or the industrial residue with an aqueous phase containing acids, most of the time the hydrogen sulfate or muriatic, or bases, like ammonium hydroxide or sodium, or complex agents, like sodium cyanate and ammonium hydroxide, in varied pressure and temperature conditions, usually from 25 ⁰ C to 95°C. This phase is followed by the separation operations solid-liquid, employing processes such like cycloning, thickening and filtering, whose purpose is the obtainment of the aqueous or liquor phase that contains the interest metal. The efficacy of this phase is determined for the minimizing of the losses of soluble metal in the pulp, which shall constitute the reject, and consumption of new water in the process.

The characteristics of the solids to be discarded shall also be determined in the costs of disposition of the reject and the potential risk of environmental impacts. The treatment phase of the liquor produced in the leaching aims the purification of the solution through the separation of elements coming from the dissolution of the ore or residue, and that may affect the subsequent phase of recovery of the metal, and the concentration of the solution containing the dissolved metal until the adequate levels to the following recovery phase. Eventually, this phase may lead to the obtainment of byproducts. The treatment of the liquor envelopes processes such like: precipitation, adsorption in activated coal or in polymeric resins of ionic change and extraction by solvents. It is important stress that the processes employed in this phase may be applied to the treatment of effluents, aiming the

concentration and removal of contaminants.

The last phase of the hydrometric flowchart has as objective the recovery of the metal. This may be obtained in salt or metallic hydroxide form, like Al ₂ C^nH ₂ O and CuSO ₄ , through precipitation and crystallization processes, or as metallic form. In the second case, are used reduction reactions in aqueous phase, as the cementation, that is the reduction by oxidation of a less noble metal, the reduction by hydrogen or the electro recovery, that is the main process used in the production of metals of high purity directly from aqueous solutions. The process comprises the application of a difference of potential between cathodes and anodes immersed in aqueous solution and is used to obtain copper, zinc, nickel and gold, among others. For metals of very negative redox potential, like aluminum, the electro recovery is made in molten salt bath. However, some difficulties are found for the viability of this process. Several ores or residues present significant resistance to leaching, that is, they are not attacked by the acids or bases even when exposed to high temperatures and high concentrations of leaching agents. For these cases the status of the current technique suggests pretreatment like: (i) the reduction, that consists in the heating of ores or residues in reducer environment with temperatures above 1.000 ⁰ C with coal, coke or products containing carbon, that shall be of use for seizure of the oxygen present in the ores, that are found as Oxides; (ii) the ustulation, that is resumed as the heating of ores containing sulfides at temperatures above 600 ⁰ C for oxidation of sulfide ores and liberation of sulfur dioxide; (iii) the hydrometallurgy under pressure, that is the oxidation under pressure and high temperature, reaching up to 250 ⁰ C; and (iv) the biohydrometalurgy, that is the biological oxidation of refractory ores utilizing micro-organisms. This additional treatment makes that the costs with equipment and reactants

increase significantly, bringing, in some cases, the technical unfeasibility. As the steelworks powder is very refractory to the acid attack and the alkaline attack, it is also necessary processes previous to the proper leaching. The most known and worldly used nowadays is the "Waelz" process that is made in two different phases. The first phase or pyrometallurgic phase, is made in direct flame rotating kiln. The steelworks powder containing iron and zinc is previously mixed and briquetting coke or mineral coal and reduced at temperatures around 1.200 ⁰ C. It happens then the distillation of the zinc that is taken by the exhaust gases and later oxidized, being captured in electrostatic filter, originating the so called "Oxiwaelz". The second phase comprises the removal of the filtered material that follows to hydrometallurgical separation and concentration phases until the obtainment of the metallic zinc purified in electrolytic process. The other metals form a mass or slag that concentrates specially iron and the heavy metals that go to fillings or immobilization in Portland cement industries. This process has a zinc recovery index above 93%, but is applied only in large scale due to the heavy implantation and production costs and the complex number of hydrometallurgical phases, excepting possible dioxins, removed by the temperature of the pyrometallurgic phase, that can be regenerated in the cooling, keeping the original problem.

The processes that only use hydrometallurgy for processing the steelworks powder normally make it with alkaline leaching using sodium hydroxide or ammonia. Nevertheless, the recovery rate of these methods is too low, being in some cases under 50%. Additionally, we have to consider the liquid effluents generated in the process, which necessarily shall have to bear treatments before being discarded.

In the specific case of this invention, it is attained the extraction of the zinc in percentages of up to 98% and about 95% for iron, with average mass

reduction of 89% on the original mass. Besides, all the carbonic chains linked to chlorine, and eventually to fluorine, are also broken by the energetic reaction the concentrated hydrogen sulfate promotes at temperatures above 100 ⁰ C, chemically depriving the possible dioxins, with the fluorine and chlorine being recovered and economically valorized. Additionally, the investment costs are much reduced, being possible to construct local and small scale recycling units. Analysis of the technique status The patent US5538532 describes a method for separation and recovery of metals consisting of iron, cadmium, zinc and lead, from raw material comprising a mixture of metals, comprising the phases of (i) heating of the raw material at a temperature able to substantially vaporize cadmium, zinc and lead; (ii) separation of secondary powder and vapor, produced during the first phase, of the residual sintered mass, that comprises iron; (iii) condensing of the secondary powder in aqueous solution of ammoniac carbonate to dissolve zinc and cadmium; (iv) separation of a zinc/cadmium leach liquor substantially insoluble particulate by filtering; (v) treatment of zinc/cadmium leach liquor for recovery of cadmium by the addition of metallic zinc to the leaching to produce a cement containing cadmium; (vi) separation of the cement of the leach liquor; and (vii) removal of the ammonia leach liquor to precipitate zinc carbonate. The method regards a process non-similar to the present invention, as it employs high temperatures and, mostly, pyrometallurgy principles. The document US4614543 exposes a process for the hydrometallurgical treatment of steelworks powder containing finely divided iron, zinc, lead and others. The process is conducted by the formation of an aqueous condensation of the fine powder with a mixed leach comprising HCl and H ₂ SO ₄ , where the concentration of sulfate ion presents excess regarding the

concentration of the chloride ion and in stoichiometric excess of the required to substantially sulfate all the lead and calcium present. The amount of ion present as HCl must be enough to keep the pH between 1 and 4. The leach is conducted at a temperature varying from the room temperature to a temperature under boiling point for a time at least enough to make the dissolution at least of zinc and other value metals and form a residue containing iron oxide, calcium sulfate and lead sulfate. The method regards a process characterized by the se of hydrogen sulfate and some hydrometallurgical operation not much similar to the present invention. The Patent US4915730 regards a process and apparatus for the recovery of metals, such like silver, from fine phosphate powder. The process includes the steps of mixture of a salt chloride and the fine powder to produce a mixed material, oxidizing the material mixed in an oxygen atmosphere to oxidize the carbon in the mixed material producing a gas and to react the chloride salt with the metal in the mixed material, producing a metallic salt soluble in water, dissolving the metallic salt in water to produce a solution, filtering the solution to remove solids and precipitating metals of the filtered solution with the ready precipitate for conventional fusion. The preferential configuration of the apparatus includes a hopper and a Mill for salt, for feeding the powder and salt to a tubes drier, and a tubular oxidizer to mixture and oxidize the materials, besides a spray chamber at the oxidizing exit for separation of solids and gases, where a certain amount of solids come into suspension. The apparatus also includes a filter for removal of non-dissolved solids, a zinc feeder to add zinc to precipitate the dissolved silver and a filter to remove the zinc-silver precipitate that is ready for foundry. The methods regards a slightly similar process, as it happens at substantially higher temperatures, employing partial fusion and reactants different from those proposed in this invention.

The document WO/ 1994/019501 regards a process for treatment of electric arc furnace, comprising a mixture of zinc oxides, iron and lead, that comprises the steps of (i) leaching of the powder with a mixture of leaching containing ferric chloride to produce a condensate containing a hydrous iron oxide; (ii) conversion of the hydrous iron oxide to a filtering hematite by means of thermal treatment of the concentrate at high temperature and pressure. The thermal treatment is preferably performed at temperatures of at least 140 ⁰ C. The process may also comprise the steps of (i) dissolution of the zinc present in the powder during such leaching, with such leach solution to provide a zinc chloride solution; and (ii) separation of the solution by extraction of solvent using a salvation extractor. The method regards a not much similar process, employing hydrometallurgical means, making its operations in liquid medium and with reactants different form those of the present invention. The patent US4355009 describes a hydrometallurgical process for the separation treatment of fine steelworks powder containing zinc and significant amounts of lead, chloride and iron. The process is specially adapted for extraction of zinc sulfate from the blast furnace white powder, resulting in the casting of secondary copper. According to the process, the fine powder is leached in a hydrogen sulfate for a substantially complete dissolution of the soluble constituents, specially the zinc, leaving an insoluble residue consisting specially of lead oxide. At the conclusion of the leaching, the pH is selectable adjusted, corresponding to the desired extension of subsequent removal of chloride, where the concentration of chloride is substantially and selectable reduced by precipitation of cuprous chloride, with the cuprous ions being promoted by the reduction regulated by cuprous ions pH. The non-chlorinated leach solution is treated by cementation to regulated pH with zinc for removal of residual cuprous

copper of the previous phase, together with other impurities of metals less noble than zinc. The iron is precipitated from the acidified leach solution of ferrous ions soluble in acid, for the relatively insoluble ferric status. Finally, the purified leach solution is subject to evaporative crystallization to recover zinc sulfate at commercial grade. The method regards a not much similar process, being distinguished only by the fact that it also proposes the formation of soluble salts or sulfates, substantially different from the proposition of the present invention. The document US5961691 describes a process to extract and recover zinc or its byproducts with high purity of several materials and or residues like copper steelworks powder. The process allows also the substantial or complete recovery of noble metals, otherwise lost in the rejects. The method regards a process non-similar to the present invention. The document US3983218 describes a process using steelworks powder discarded in the basic process of manufacture of oxygenized steel, that has some values of iron and zinc, as absorption and pollution control of the sulfur dioxide for steelworks powder of industrial and public utility furnaces, whose materials, under continuous dry injection in a chemical reaction zone of industrial furnaces, that results in the dry removal of the sulfur dioxide. The products of the reaction are cleaned by a conventional gas cleaning instrument. The method regards, then, a process non-similar to the present invention.

The patent US4119455 regards a method for recovery of steelworks powder collected as a by-product in mud or dry form from the steelworks process for recycling. The steelworks powder humidity collected is adjusted to a level in which the wet powder (mud) gets a plastic consistency, as extruded in agglomerate (generally humidity index of 8% to 16%). If the powder is collected in a dry status, the humidity is added; if

collected in a wet status, the humidity index is adjusted by the addition of a complementing dry material. The hydraulic cement is added to the mixture in the scale of about 4% to 15% in weight and the mixture is extruded in agglomerates and then cured to be taken in steelworks furnaces. The method regards a process non-similar to the present invention. Detailed description of the invention

The treatment process that makes possible the recovery of metals contained in the steelworks powder consists in promoting a slow and homogeneous mixture of the powder as concentrate hydrogen sulfate, at 98%, providing a direct chemical reaction of the acid with the steelworks powder, without the presence of water and without having agglomeration of the material. It is known that the finely divided materials, under shaking and with progressive addition of liquids, tend to agglomerate forming pellets or clots. This makes possible the dissipation of the temperature and the reactants by the mass, preventing the chemical reactions in dry medium, a reason that justifies so far only having made possible production processes of salts through hydrometallurgy. According to the present invention, this problem is solved making that the mixture does not acquire this characteristic and may be made in dry medium, with temperatures above 100° C. For such, it is added solid ground potassium chloride to the mass that acts as a multifunctional conditioner, directly participating in the chemical reactions and acting as non agglutinant.

The steelworks powder originated from the steelworks industries is generally presented in granulate form or partially agglomerate by the effect of transport and with humidity varying between 5% and 8% in mass.

Due to this, first of all it is necessary the drying of the steelworks powder, for example, in rotating indirect flame drier, with temperatures varying between 120 ⁰ C to 180 ⁰ C for removal of the surplus humidity, reaching 2%

in weight, to make easy the subsequent processes. Next, it is made the grinding or reduction, for example in balls mills, until reaching the average granulometric strip of 40μm. Thus done, the steelworks powder is homogenized, what may be done in an inverted blades mechanic mixer, adding the dry and previously ground potassium chloride with average granulometry of 50μm, in percentages varying between 5% and 15% in mass. It is begun then the heating cycle until reaching an average temperature of 120 ⁰ C, ideal for the beginning of the addition of concentrate hydrogen sulfate. A typical example is making the mixture in a closed reactor with central shaking, provided with forced exhaust and indirect side heating.

Prepared the load as described, it is made slowly the addition of concentrate hydrogen sulfate as mist. The hydrogen sulfate is typically feed through a ventury type dosing nozzle, provided with additional tube for simultaneous injection of compressed air for the formation of mist, and the discharges controlled by dosing pump for the hydrogen sulfate, and the pneumatic valves for the compressed air. Immediately, begin the chemical reactions of the concentrate hydrogen sulfate with the steelworks powder and the utilized reactants (potassium chloride), liberating a great amount of power as heat that heats all the mass in reaction. With the heating coming from the exothermic chemical reactions, the temperature rises to values near 150 ⁰ C. A small heating is made only to keep the temperature between 150 ⁰ C and 250 ⁰ C, temperature band proper to the chemical reactions of opening of the steelworks powder and the formation of sulfates. The rest of the necessary acid is dosed to complete the opening process and the chemical sulfating reaction. The reactions shall occur in a cyclical form according to the following generic equations: (i) Generic reaction of the mixture chlorides, including potassium

chloride and the chloride ions of the organic chains present in the steelworks powder with concentrate hydrogen sulfate forming rising hydrogen chloride.

2KCl + H ₂ SO ₄ + organic chlorides = K ₂ SO ₄ + 2HCl (gas) + heat + products of the decomposition (carbon and water)

Where the potassium chloride and the chloride ions present in the organic chains of the steelworks powder react with energy with the concentrate hydrogen chloride, forming potassium sulfate and rising hydrogen chloride, destroying composites containing chlorine. The generated hydrogen chlorine reacts immediately with the components of the steelworks powder according to the reactions thus described:

(ii) Reaction of the zinc ferrites with the rising hydrogen chloride: ZnO-Fe ₂ O ₃ + 8HCl = ZnCl ₂ + 2FeCl ₃ + 4H ₂ O(vapor) + heat Where the zinc ferrite reacts with the hydrogen chloride to form zinc chloride and iron chloride.

(iii) Reaction of the zinc and iron chloride with concentrate hydrogen sulfate regenerating the rising hydrogen chloride. ZnCl ₂ + H ₂ SO ₄ = ZnSO ₄ + 2HCl (gas) + heat 2FeCl ₃ + 3H ₂ SO ₄ = Fe ₂ (SO ₄ ) ₃ + 6HCl (gas) + heat Where the zinc and iron chlorides react with hydrogen sulfate regenerating the hydrogen chloride regenerating the hydrogen chloride that is withdrawn from the system by exhaustion together with the water vapor. The final product of all the involved reactions is a mixture of sulfates of most of the elements present in the steelworks powder. All the potassium chloride is also transformed in sulfate. The hydrogen chloride that is being distilled leaves the reactor through exhaust gases and is recovered in absorption towers by an alkaline solution, preferably of calcium hydroxide, being obtained as product from the absorption the calcium chloride

(CaCl ₂ ), that may be traded. Below is the generic reaction of absorption of hydrogen chloride in alkaline medium:

Ca(OH) ₂ + 2HCl = CaCl ₂ ^(soluble) + H ₂ O

Where the calcium hydrogen reacts with the hydrogen chloride forming soluble calcium chloride and water. Optionally may be used potassium hydroxide to obtain the potassium chloride that shall be utilized again as reactant.

For the fluorides ions present in the steelworks powder the chemical behavior is similar to that of chloride ions, with forming of the hydrogen fluoride (HF). When the absorption of the gases is made with calcium hydroxide, there is forming of the calcium fluoride that is insoluble, being easily separated from the calcium chloride that has high solubility according to the following reaction:

Ca(OH) ₂ + 2HF = CaF ₂ ^(insoluble) + 2H ₂ O Most of the sulfates formed in the process is soluble in water, excepting lead and calcium sulfates. After the end of the reactions the sulfated material is taken from the reactor and transferred to a tank with central shaking and re-pulped with water in the proportion of 300kg of sulfated material for each cubic meter of water, being the pulp kept under shaking and heat between 65 ⁰ C and 70 ⁰ C during 60min, for total extraction of the soluble sulfates. Then, the insoluble residue, normally lead and calcium sulfate, silica and carbon, is decanted and the solution of the soluble sulfates filtered in press filters. The insoluble residue is also filtered in press filter and washed several times for extraction of the residual soluble salts of the filtration cake. The final mass of these residues corresponds to about 10% of the initial mass of the steelworks powder. This residual mass has, essentially, the elements non-attacked by the hydrogen sulfate and the insoluble sulfates, like the

lead sulfate, calcium sulfate, carbon, iron in the form of magnetite (F ₃ O ₄ ), silica, manganese etc. This final residue goes to final destination and or coprocessing in structural ceramics, being mixed in the ceramics mass in proportions that vary between 10% a 20% in mass. The elements of the residue are combined with the crystalline structures of the ceramics clays formed in high temperature, encapsulating the heavy metals and preventing they are taken to the environment, closing the complete recycling circle. The sulfates solution, mostly comprising zinc and iron, is submitted to precipitation phases of the iron and other impurities. The iron is precipitated in acid pH between 2,0 and 2,5 and obtained as hydrated oxide, and may be applied as inorganic pigment. The other soluble elements as chrome, nickel, copper and manganese, are selectively precipitated in pH between 3,5 to 4,5, and are incorporated to the mass of the final residue, remaining only a solution with zinc sulfate. The diluted and purified zinc sulfate goes then to the concentration and crystallization phases in evaporators shaken with direct vapor heating. After this is done the zinc sulfate is dried, ground and goes to packing, stock and commercialization. This invention is not limited to the representation herein commented or illustrated, and must be understood in their broad scope. Many changes and other representations of the invention shall come to the mind of those versed in the technique to which this invention belongs, having the benefit of the teaching presented in the previous description and enclosed drawings. Besides, it is to be understood that the invention is not limited to the particular disclosed form, and that the changes and other forms are understood as included in the scope of the enclosed claims. Although specific terms are employed here, they are used only in a generic and descriptive form and not as limitation purpose.