A hydrometallurgical method for extracting metals and gypsum from any type of dust of a steelworks electric arc furnace, wherein it recovers, by simultaneously extracting, zinc, lead, silver, iron and calcium, in the form of industrial products;
- zinc as zinc sulphate or zinc cathodes;
- lead and silver as a concentrate of lead and silver;
- iron as reduced elemental iron for return to the electric arc furnace;
- and, lastly, calcium as gypsum, the method including the following steps:
- 1) conditioning the electric arc dust by homogenizing and identifying the chemical composition of the dust in terms of the content of Zn, He, Cl, Pb, Ca, Ag, Cd and Hg, to know the grade of valuable metals present and of the elements of risk for metallurgical control and balance in the method, this action being performed in an encapsulated mixture which produces the homogeneous mixture, constituting the homogeneous electric arc dust stock which is stored in encapsulated hoppers, which will enter the method after the respective chemical analysis has been performed;
- 2) scrubbing the dust from step 1) with water, where the dust contains halogens and, consequently, alkalis and alkalines, elements which must be removed from the oxide complex because they are detrimental to the following steps of the method, for which the homogenizsed dust is transported from the encapsulated hoppers to the scrubbing tanks, the water comes from the condensed water feed system having an absence of dissolved gases and salts and enters at the temperature of 80° C. with a mass ratio of 3 to 1;
- the scrubbing tanks maintain a constant suspension by mechanical agitation for 15 minutes in the first part of the scrubbing, producing a liquid loaded with soluble complexes of Cl, Na, K, Mg, Ca, Ba, which is sent to the liquid purification unit of the method by the liquid/pulp decanting system;
- the pulp enters a second scrubbing time of 10 minutes, with water of the same characteristics of the method so as to force a second dissolution of halogens, alkalis, alkalines, and, similarly, the decanting system sends the liquid to the liquid purification system, and the previously filtered solids pass to the following step;
- 3) extensive sulphation by adding sulphuric acid to the solids of the scrubbing phase for the purpose of changing the chemical nature of the electric arc furnace dust from complex oxide compounds to a group of simple and complex reaction sulphates;
- to generate this reaction, one part dust to 0.87 parts pure acid is mixed by weight in a continuous mixing reactor specially designed for this phase, generating a paste, the reaction is exothermic and carried out in a range of 120° C. to 150° C., to which there is added water at 80° C. from the condensed water generation system, in order to replace the water losses due to evaporation in the reaction and maintain the plastic condition of the paste;
- 4) drying by drying the sulphated mixture of electric arc furnace dust in the same sulphation reactor, which continues to operate under continuous mixing, with the temperature of the reactor increasing to 200° C., the paste proceeds to dry, losing all the water by evaporation, forming dark grey agglomerated crystals;
- 5) thermolysis or thermal decomposition, where one part of the complex is converted to a soluble part (essentially zinc) and one part is converted to an insoluble part (essentially iron), and after the drying phase, the temperature of the reactor increasing to a working range of 680° C. to 720° C., depending on the concentration of Zn, Fe, Pb, in the paste, and the mechanical mixing is kept constant such that the crystals formed in the drying phase can increase the temperature homogeneously, the reaction time being 2 hours.
- 6) aqueous leaching, where two new complexes are obtained in this step, one being liquid with a large amount of zinc and the other one being insoluble and essentially made up of hematite, gypsum and lead oxides plus other minor oxides, being carried out at 260 RPM and 70° C. in encapsulated tanks with an evaporation water vapour extraction system and in a solid/liquid ratio of 1 to 3.6, the leaching time being between 1 hour and 1 hour and 30 minutes;
- 7) zinc recovery, where zinc is obtained in this step according to two preferred forms to be obtained, the first in the form of zinc sulphate by evaporation and crystallisation, and the second in the form of zinc cathodes by electrodeposition;
- 8) flotation, where in this step hematite is separated from lead oxides and various metal oxides in a very small amount, such as, for example, cadmium, nickel, arsenic oxides;
- the solid coming from the leaching phase is insoluble under normal conditions, with a majority composition of hematite and with the minority presence of lead oxides, calcium sulphate (gypsum) and a presence of silver;
- 9) magnetic separation, where in this step the hematite still contains gypsum (CaSO4.2H2O), where the gypsum must be separated from the hematite due to the need to use reduced hematite (FeO) as electric arc furnace feed material, in order to remove the gypsum, for which magnetic separation is used, taking advantage of the wide delta of magnetic susceptibility existing between hematite (35×103 SI) and gypsum (−0.11×103 SI), at low speeds between 0.7 feet/min and 0.8 feet/min of the magnetic separator and at a magnetic flux density between 19,000 gauss and 21,000 gauss;
- 10) a gas recovery system, where the method comprises in this step a conventional catalytic system for SO3/SO2 recovery, capturing gases in diluted solutions of H2SO4 (at 70%) in the presence of catalysts, the capture solution is added to the sulphuric acid stock of the method, the conventional catalytic system having forced air heat sinks and being encapsulated.
- 11) gas scrubbing system, where the system gases are essentially air, O2 and CO2;
- however, given the ultrafine nature of the dust, a small portion of said dust is entrained by the streams of said dust, forcing the gas and particles to pass through water mist and sprays under countercurrent (1.2 m3 per m3 of gas), repeatedly, with several scrubbing columns and coils, with the gas passing through ultrafine filters before leaving the method so as to ensure that there is no loss of solids;
- 12) recovery of sludge from the collector, where the loss of dust because of its ultrafine nature is prevented in this step, by handling sludge from these particles from all the reactors and flow systems which are encapsulated and capture fugitive particles which, because the various systems are in a closed circuit, finally produce sludge, as well as of water vapour which also entrains a minimum portion of particles, which are captured in the vacuum evaporation system and sent to the sludge collector;
- this sludge is filtered by two twin pumps with phenolic resin membranes at a pressure of 3.5 bar, the first with a 5-micron membrane and the second with a 2.5-micron membrane;
- and
- 13) liquid purification with a water decontamination unit which receives liquid flows containing anions, cations, suspension of ultrafine solid particles from the dust scrubbing method, gas scrubbing method, sludge collector and liquid flows of the flotation phase (concentrate and tailings) and eventually from the cleaning and maintenance method for any reactor, and includes a chemical ion separation system, high-pressure membrane filtration, vacuum evaporation, returning the clean water to the industrial condensed water method, crystallising the contaminants;
- the solids are integrated (precipitates, sludge and crystals), homogenised and added to the concentrate of lead/silver for the respective marketing thereof.