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Hydrometallurgical separation process of steel mill electric arc furnace (eaf) dust and the pigments obtained by the processRelated Patent Categories: Specialized Metallurgical Processes, Compositions For Use Therein, Consolidated Metal Powder Compositions, And Loose Metal Particulate Mixtures, Processes, Consolidating Metalliferous Material (e.g., Ore, Tailings, Flue Dust, Fluxes, Etc.) By Agglomerating, Compacting, Or Heat Treating; Preparatory Process Therefor; Or Treating Consolidated Material TherefromHydrometallurgical separation process of steel mill electric arc furnace (eaf) dust and the pigments obtained by the process description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20070214912, Hydrometallurgical separation process of steel mill electric arc furnace (eaf) dust and the pigments obtained by the process. Brief Patent Description - Full Patent Description - Patent Application Claims
FIELD OF THE INVENTION [0001] The present invention relates generally to the field of steel mill dust treatment. More particularly, the invention comprises a hydrometallurgical separation process of dust produced by electric arc furnaces in steel mills. This process permits, on one hand, the decontamination of the dust and on the other hand, the production of ferrite and/or magnetite pigments useful in paints, plastics and concrete. The invention also comprises the pigments produced from this process. PRIOR ART [0002] Electric arc furnace (EAF) dust, also known under the name of (K061), is classified as a dangerous material because it contains high concentrations of soluble heavy metals such as cadmium, zinc, chromium and lead, but in particular lead. More specifically, EAF dust usually contains more than 5 ppm soluble lead and hence, does not meet the limits of lead specified by TCLP (Toxicity Characteristic Leading Prodecure). This dust also contains spinel compounds, notably magnetite (Fe.sub.3O.sub.4) and diverse ferrites (MO.sub.2Fe.sub.2O.sub.3). These spinel compounds as well as contaminants appear in the form of agglomerates and aggregates. To the naked eye, the dust is brown and an observer, even with the aid of a magnifying glass, will not notice the presence of black balls of magnetite, even if certain black balls can attain 150 .mu.m in diameter. The brown ferrite contained in the dust is ultrafine, and as a pigment, coats by adsorption the larger particles of magnetite. [0003] Table 1 shows the typical chemical composition of EAF dust coming from two distinct steel mills. These compositions show elevated concentrations of certain heavy metals. TABLE-US-00001 TABLE 1 CHEMICAL ANALYSES OF EAF DUST COMING FROM TWO DISTING STEEL MILLS IN THE PROVINCE OF QUEBEC SAMPLES Elements Code Units MILL 1 MILL 2 AL ICP90 ppm 7100 4500 Ba ICP90 ppm 157 120 Ca ICP90 ppm 107800 146000 Cd ICP90 ppm 153 200 Co ICP90 ppm 14 61 Cr ICP90 ppm 1200 1400 Cu ICP90 ppm 1720 1700 Fe ICP95 ppm >30 258000 K ICP90 ppm 17700 7400 Mg ICP90 ppm 49200 22200 Mn ICP90 ppm 15300 27200 Mo ICP90 ppm 18 41 Na ICP95 ppm 33300 9700 Ni ICP90 ppm 125 130 P ICP90 ppm 500 670 Pb ICP90 ppm 10950 9500 Si ICP95 ppm 15500 15800 Ti ICP90 ppm 700 600 V ICP90 ppm 98 n.d. Zn ICP90 ppm 93900 162000 [0004] Most EAF dust treatment processes in the prior art aim at recovering or removing the heavy metals in an "aggressive" manner, attacking the cristallographic structure of the spinels. [0005] Also known in the prior art, is EP 0 853 648 (equivalent to U.S. Pat. No. 6,022,406), which describes a hydrometallurgical process of EAF dust treatment with the aim to produce pigments. This process comprises a step of magnetic separation of the dust into two fractions, one fraction containing less magnetic elements, and the other fraction containing non magnetic elements, as well as treatment steps of these two fractions to obtain zinc ferrite pigments. The process disclosed also has as an effect to attack the cristallographic structure of spinels other than the zinc ferrite spinel, and in this sense, is also an aggressive process. [0006] Therefore, there is presently a need for a treatment process of EAF dust that permits an efficient and unagressive recuperation of the different ferrites and magnetites present in the dust, as well as permitting the decontamination of the dust. SUMMARY OF THE INVENTION [0007] One objective of the invention is to propose a treatment process of EAF dust that responds to this need. [0008] According to the present invention, that objective is accomplished with a hydrometallurgical process for the treatment of steel mill electric arc furnace (EAF) dust containing agglomerates of small ferrite particles and larger magnetite particles, the ferrite particles coating by adsorption the larger magnetite particles, the dust further containing calcium oxide, zinc oxide and a toxic amount of leachable lead together with minor elements selected from the group consisting of Mg, Cr, Cu, Cd, V, and chlorides. The process comprises the steps of: [0009] a) washing the EAF dust in water to dissolve soluble salts, metals and simple oxides contained in the dust, the washing step being performed with an alkaline pH; [0010] b) decanting the solution of step a) to obtain a supernatant liquid containing the dissolved salts, metals and simple oxides, and a slurry containing ferrites and magnetites, a non toxic amount of leachable lead and a reduced amount of calcium; [0011] c) separating the slurry and the supernatant liquid; [0012] d) adding to the slurry obtained in step c) an anionic surfactant to disperse the ferrite particles adsorbed on the magnetite particles; and [0013] e) treating the slurry from step d) to produce pigments selected from the group consisting of ferrite pigments, magnetite pigments and ferrite/magnetite pigments. [0014] Preferably, the sequence of steps a) to c) is performed more than one time before adding the anionic surfactant. [0015] The use of an anionic surfactant was found to increase the efficiency and quality of further separation steps such as screening, and ferrite/magnetite separation by a magnetic separator. Steps a) to c) also enable the decontamination of the dust by leaching salts, metals and simple oxides such as lead oxide. This selective solubilization is due to the alcaline pH solution, which is preferably greater than 12, resulting from the first washing, and optional second washing, with water. This alcalinity promotes the solubilization of PbO and, with the addition of surfactant, enables the product to pass the test set out by the TCLP, which regulates standards of dangerous materials. [0016] Advantageously, the process of the invention also enables the separation of the ferrites from the magnetites without breaking the cristallographic structure of the spinels, so as to produce magnetite and/or ferrite pigments of different grades, whose different compositions have commercial values. [0017] The process also permits the decontamination of EAF dust by hydrometallurgical means while maintaining the most stable families of spinels intact. [0018] The solution obtained is step a) described above has a positive zeta potential, and the anionic surfactant is preferably added in a concentration sufficient to reduce the zeta potential to or close to the isoelectric point, and more preferably to the isoelectric point. [0019] The anionic surfactant is preferably a phosphate or an equivalent thereof. More preferably, sodium metaphosphate is used as the surfactant. [0020] The use of sodium metaphosphate presents the following additional advantages to the process. Sodium metaphosphate converts the calcium and calcium hydroxydes present in the liquid phase into a calcium phosphate which is precipitated with the solid. Therefore, this form of calcium sequestering allows for a quicker and sharper fractionation of the slurry by, for example, a drum magnet, and in addition when the slurry is eventually separated by screening, clogging of the mesh opening is minimized, and therefore requires less cleaning. [0021] Step e) of treating the slurry preferably comprises the step of magnetically separating the slurry into a first fraction composed essentially of brownish ferrites and a second fraction composed essentially of black magnetite, the first fraction being less magnetic than the second fraction. Continue reading about Hydrometallurgical separation process of steel mill electric arc furnace (eaf) dust and the pigments obtained by the process... 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