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
  • C22B3/00—Extraction of metal compounds from ores or concentrates by wet processes
  • C22B3/04—Extraction of metal compounds from ores or concentrates by wet processes by leaching
  • C22B3/06—Extraction of metal compounds from ores or concentrates by wet processes by leaching in inorganic acid solutions, e.g. with acids generated in situ; in inorganic salt solutions other than ammonium salt solutions
  • C22B3/10—Hydrochloric acid, other halogenated acids or salts thereof
• • 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
• • 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
• • 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
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10S—TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10S75/00—Specialized metallurgical processes, compositions for use therein, consolidated metal powder compositions, and loose metal particulate mixtures
  • Y10S75/961—Treating flue dust to obtain metal other than by consolidation

Definitions

• the present invention relates to a hydrometallurgical process for treating electric arc flue dust (EAFD) to recover the valuable base metals from the flue dust while recycling the iron. More particularly, the present invention relates to a hydrometallurgical leach using a combination of calcium chloride and hydrochloric acid at controlled pH at a moderately elevated temperature and pressure to place the soluble base metals into solution with chloride for their subsequent precipitation and recovery.
• EAFD electric arc flue dust
• Electric arc processing of recycled steel is the primary method used today in the United States to produce new steel.
• Electric arc processing replaces the conventional Bessemer furnace that arose at the glory days of the steel age at the turn of the Twentieth century and continued in use throughout this century.
• the electric arc process is more efficient in terms of energy and labor than the traditional mill processes. It produces, however, an electric arc flue dust (EAFD) waste product that is depleted in iron while remaining relatively rich in other commodity base metals, like zinc.
• EAFD electric arc flue dust
• This flue dust waste plagues the industry because it is unstable and is difficult to economically process to create a stable waste or to recover the base metals.
• EAFD is classified as a hazardous waste under U.S. EPA standards, so few facilities will store or process EAFD.
• the primary process used to treat EAFD is the Horsehead Industries pyrometallurgical process that is energy intensive in recovering the base metals. This pyrometallurgical process, however, itself produces flue dust which is even more difficult to stabilize because it is depleted of both iron and other base metals. Nevertheless, electric arc furnaces pay the pyrometallurgical processor today to process the EAFD so that the electric arc furnaces can continue to recycle iron and steel into new products. Obtaining such treatment for the EAFD requires transport of the dust from the electric arc furnace to the pyrometallurgical processor, which introduces yet another cost.
• the hydrometallurgical recovery process of the present invention offers a process that is efficient, economical, and environmentally-sensitive, producing an iron-rich feedstock for an electric arc furnace, recovering the valuable base metals in the EAFD, and producing calcium sulfate (gypsum) suitable for building materials.
• the process of the present invention does all this without creating significant air, water, or solid wastes while eliminating the problems of the prior art processes and achieving the goals of today.
• the present invention is a hydrometallurgical process for treating EAFD to recover the base metals, especially lead and zinc, while recycling an iron-rich stream to the electric arc furnace and producing a salable calcium sulfate (gypsum) building material.
• the process uses a calcium chloride/hydrochloric acid leach to place the soluble base metals (i.e., copper, lead, cadmium, and zinc) into chloride solution generally without permitting a significant amount of the iron into solution.
• the chemistry of the process takes advantage of the relatively lower solubility of iron oxides (Fe 2 ⁇ 3 , Fe 3 ⁇ ) to that of the base metals in moderately acidic chloride solutions.
• the process also uses an oxidizing atmosphere to oxidize any soluble ferrous iron to ferric iron, which is essentially insoluble in aqueous solutions at a pH greater than 2.0.
• iron that does enter solution in the leach reactor oxidizes to the ferric form and precipitates out to be recovered with the iron-rich waste cake.
• the metal-rich solution that results from the leach is then treated using zinc dust to precipitate the copper, lead, and cadmium, followed by calcium hydroxide (hydrate of lime) to recover the zinc before the hydrochloric acid/calcium chloride leach mill solution is replenished or regenerated with sulfuric acid and recycled for reuse.
• a slurry of the flue dust and the leach mill solution having a solids content (i.e., pulp density) of about 15 - 30 wt. % at a pH of about 2.6, at a moderately elevated temperature of about 90 - 120 deg C, at an elevated pressure of about 90 psi in an oxygen-rich atmosphere. While we can use higher pressures, they do not offer a significant advantage in improving the speed of the reaction or the yield of metals, so we avoid incurring the costs associated with operating at higher pressures. So, too, with the temperature. We prefer to use oxygen in our process to ensure that any soluble iron is converted to the ferric state and so that all other soluble metals are in their highest oxidation states.
• This leach occurs in the oxygen-rich atmosphere at a temperature of about 90 - 120 deg C (preferably 120 deg C) and a pressure of about 50 - 90 psi (preferably 90 psi) and, if the pH is controlled at about pH 2.6, leaves an insoluble hematite complex that can be separated and that can be recycled to the electric arc furnace for recovery of the iron.
• the zinc-containing complexes readily entering solution by this process are ZnO and ZnS, if present.
• "Zinc ferrite" species do not readily dissolve under the conditions of the present invention and, consequently, this zinc remains in the iron- rich waste cake.
• the metal-rich solution is treated to precipitate the metals of interest separately or together to permit their recovery.
• the Figure shows a simplified flow diagram of the preferred process of the present invention.
• the present invention can be summarized as a simple, three-step process involving (1) oxidizing the EAFD in a leach mill solution to place the base metals in solution without placing a significant amount of the iron into solution, (2) precipitating the base metals from the solution, and (3) replenishing the leach mill solution.
• the leach mill solution is a calcium chloride and hydrochloric acid leach solution that has a negative water balance to eliminate any liquid waste streams in later processing steps.
• the process uses a concentrated, aqueous solution of calcium chloride and hydrochloric acid where the concentration of calcium chloride is generally equal on a weight basis to the weight of EAFD to be treated.
• concentration of calcium chloride is generally equal on a weight basis to the weight of EAFD to be treated.
• one ton of EAFD can be mixed with one ton of calcium chloride and sufficient water to yield a slurry having a pH of about 2.6 and a solid content (pulp density) of about 15 - 30 wt. %.
• a typical run would include about 200 gm calcium chloride, 200 gm EAFD and 650 gm water (i.e. 650 ml) .
• the slurry is prepared in a ball mill by adding the EAFD slowly to the leach mill solution.
• the EAFD apparently is acid consuming, so the pH of the leach mill solution often must be less than 2.6 in the range of pHl.5 to yield the proper leach mill solution.
• the paramount concern is avoiding making the iron soluble, which it is prone to do at a pH below 2.6. Therefore, study with each specific EAFD to prepare the appropriate leach mill solution is probably required, as will be understood by those of ordinary skill in the art.
• the leach mill solution is quite stable and consistent from run co run.
• Sodium, potassium, and magnesium do not build up in the solution over low levels because the combination of calcium chloride and hydrochloric acid coupled with the elevated temperature and pressure of the reactor cause jarosites of pseudojarosites to form between the sodium, potassium, and magnesium with iron.
• the leach mill solution becomes recyclable which greatly simplifies the process and makes it economical. Without the stabilization of these minerals, a waste stream that would be difficult to process and difficult to dispose of would be generated and it would jeopardize the utility of the process.
• the sulfuric acid addition step produces hydrochloric acid and a gypsum product that complexes water in the solid where it can be filtered from the leach mill solution to produce a negative water balance in the remainder of the reaction. Additional water is lost in the solid hematite product that results from the leach itself.
• the imbalance can be as much as 10 - 25 % (typically 10 % for the EAFD leach) . If we control the wash water streams and steam sparges to keep the amount of water involved below these losses in the solid products, we can carry out the process, recycling the leach mill solution, without producing any liquid bleed streams for difficult treatment or disposal. This consequence, then, improves the utility of the process and helps to distinguish the present invention from earlier processes which generated large liquid waste streams.
• the solid resulting from the addition of sulfuric acid is a salable gypsum that is filtered from the leach mill solution.
• the gypsum removes essentially all of the sulfur from the solution. Iron sulfides do not form in the reactor and sulfur is not introduced into the electric arc furnace when the iron byproduct is recycled. There are no sulfur emissions to the air.
• the mixer for the leach mill solution and the dry EAFD powder can be chosen from a variety of mechanical devices, I prefer to use a rubber lined ball mill with ceramic balls of uniform size having a diameter of approximately 2.5 in. While this description focuses on arc flue dust as the feed, the metal-rich reactant can be sewer sludge fly ash, soils contaminated with heavy metals, flue dust from base metal smelters, sulfide ores and concentrates, or other heavy metal products, as those skilled in the art will understand.
• the slurry of the metal-rich feed and the leach mill solution are heated in a titanium- or glass-lined reactor in
• an oxygen or compressed air atmosphere to a temperature of about 90 - 120 deg C at a pressure of about 50 - 90 psi.
• the heating can occur using a steam sparge or with a steam jacket or with a combination of both, taking care to limit the introduction of water to the process.
• the iron-rich waste cake is recovered from the reactor slurry with a pressure belt filter which gives a good displacement cake wash, although we could use other separators.
• the metal-rich solution is further treated to recover the base metals.
• Each metal can be precipitated individually or they can be recovered together.
• the small fractions individually of copper and lead, lead us to recover some metals together, especially lead, copper, and cadmium as metals in a sponge.
• zinc typically 100 - 200 % of stoichiometric
• copper is present in a significant enough concentration to justify independent recovery, we can precipitate it by adding calcium carbonate or can recover the copper with a solvent extraction. Generally, we simply precipitate it with the zinc cementation as a high quality metal sponge.
• the metastasis process for recovering zinc oxide from the solution using calcium hydroxide (hydrate of lime) or calcium oxide or both is desirable because it regenerates a calcium chloride stream that can be recycled. In this way, the EAFD is converted into a source of iron and a source of base metals without the creation of any waste products.
• the process of the present invention begins with the mixing of the EAFD in a ball mill with a calcium chloride/hydrochloric acid leach mill solution to form a slurry.
• About equal weights of the EAFD and the calcium chloride are combined to make the final reacted slurry have a pH of about 2.6 and about 15 - 30 wt. % solids. If the pH is higher, the reaction is slow or is ineffective at placing the base metals in solution. If the pH is lower, although the base metals do enter solution, iron also enters solution. We try to avoid the addition of iron to the solution because its presence complicates the recovery steps that follow.
• the iron-sodium-potassium-magnesium complex removes the sodium, potassium, and magnesium from the solution and limits the concentration of these metals in later processing steps.
• the EAFD is a fine powder finer than 100 ANSI mesh and generally finer than 200 mesh.
• Typical Chemical Composition 20-25 wt. % Fe; 20-25 wt. % Zn; 2-3 wt. % Pb; and about
• Cu Lead is about ten times the concentration of copper and zinc is about ten times the concentration of lead.
• EAFD compositions vary, however, in wide ranges. In some, the zinc concentration can be as low as 5 w . %. The actual composition depends greatly upon the feed to the furnace and to the product that the furnace produces.
• the EAFD sometimes is supplied in pellet form, which increases the time in the ball mill, but has no other serious side effect.
• the resulting slurry is charged to the titanium reactor which is usually about a 600 -5000 gallon pressure vessel fitted with mechanical impellers for keeping the slurry mixed during the reaction, internal aeration (for introducing the oxygen, actually) and steam sparging lines, and external steam coils and cooling coils.
• the aeration and steam lines permit the introduction of steam, oxygen, or compressed air to the slurry to help to stimulate the reaction. Bubbling the steam or gas through the slurry ensures its mixing, and , at lower pulp densities, we can reduce the work required from mechanical impellers.
• the external coils allow heating or cooling of the reactor, as appropriate.
• the pressure ensures that the water in the slurry does not boil. Using an overpressure allows us to use a higher temperature, but exerting the pressure, again, consumes energy, so we try to minimize this expense as well.
• 50 - 90 psi, and, preferably, 90 psi provides the best results.
• the slurry is about 15 - 30 wt. % solids (i.e. pulp density) , which makes it relatively fluid (although quite dense) and relatively easy to handle. Slurries of higher % solids would require a highly concentrated calcium chloride solution to maintain the weight ratio of EAFD to calcium chloride at approximately one. Processing become impractical because the slurry is so thick that mixing and oxidation are less efficient, thereby increasing the reaction time or reducing the yield from the near optimum conditions we have discovered and recommend. A % solids below 15 wt. % is processable, but the returns are diminished because of the dilution. The same amount of time and energy has been spent without the maximized recovery. Within the range, the competing factors are manageable.
• a concentrated slurry near the upper preferred limit of 30 wt. % offers the following advantages: high yields within a reasonable time with reasonable energy requirements to mix the slurry.
• Such a slurry poses the following disadvantages: the high pulp density makes the slurry thick so that more energy is expended mixing the solution to ensure that the oxidation reaction proceeds to completion.
• a dilute slurry near the 15 wt. % lower bound offers the following processing advantages: better mixing with slightly faster overall reaction times and completeness of reaction, but suffers the following disadvantages: relatively smaller metal recoveries with respect to the volumes of liquids processed.
• the EAFD reacts readily at low temperatures and pressures in most cases, but we have found that we can always achieve essentially complete reaction with the ranges of temperature and pressure we have described. We particularly prefer to conduct the reaction at 90 psi in oxygen at 120 deg C.
• the reaction typically takes from 15 minutes to about 2 hours to reach completion. We measure completion by monitoring the presence of iron in solution and monitoring the pH. We look for the conversion of iron to Fe 2 ⁇ 3 , the ferric, insoluble state.
• the metal-rich solution can cool to ambient temperature, and, thereby, can recover lead chloride precipitates from the solution by filtering. If we detect iron in the solution (which never occurs if the pH is monitored properly in the reactor) , we can proceed to an iron recovery stage. But, we generally prefer to proceed instead with a zinc cementation of the copper, lead, and cadmium to produce a zinc-rich chloride solution and a lead-copper-cadmium metal sponge. Because the sponge typically contains about ten times as much lead as copper, the sponge can be reprocessed to recover the lead values.
• the metal values in the sponge are recoverable by conventional methods as described at MATHEWSON, ZINC: THE SCIENCE AND TECHNOLOGY OF THE METAL AND ITS ALLOYS, Reinhold: N.Y., 1960, 551-554,632-633, which we incorporate by reference.
• the zinc-rich solution typically contains less than 5 mg/1 Pb and less than l mg/1 Cd and Cu.
• the metastasis process for the recovery of zinc involves the reaction of calcium hydroxide or calcium oxide with the metal-rich solution, now mainly zinc, at a temperature of about 150 deg C for about 1 hour at a pH of about 6 - 10, and, preferably as low as possible to conserve acid in later steps.
• To minimize the amount of lime required to adjust the pH we try to minimize the volume of the solution. Still, in processing large volumes of EAFD, che volumes are substantial.
• the metastasis process produces a zinc oxide precipitate and calcium chloride in solution. Because the pH of this solution is about 10.5, to reuse the calcium chloride solution, we need to add acid.
• the addition of sulfuric acid is advantages for several reasons. First, the addition of concentrated sulfuric acid produces a gypsum (calcium sulfate) solid that can be recovered and sold. The gypsum naturally complexes water so that the resulting leach mill solution can be deficient in water based upon the theoretical amount needed in all the steps in the process. This negative water balance ensures that we do not have an liquid waste steams to treat or to dispose of.
• the sulfuric acid generate hydrochloric acid in the solution so that the leach mill solution becomes a combined calcium chloride/hydrochloric acid leaching solution.
• this combined leach mill solution provides better yields of the base metals than using only calcium chloride or only hydrochloric acid, as we have earlier explained.
• the hydrochloric acid is generated at relatively low cost, since sulfuric acid is less expensive than hydrochloric acid.
• Table 1 illustrates the performance of the present invention in benchscale tests on two types of EAFD.
• an assay provides the composition of the EAFD feed, the iron-rich waste cake recovered from the reactor, and the fraction of soluble metals going to the metal-rich solution for subsequent processing and recovery.
• the EAFD was slurried in a sufficient volume of HCl/CaCl 2 solution (approximately l liter) to give a pulp density of 20-25%.
• the amount of acid added depends on the EAFD composition, while the CaCl 2 is typically 100-200 gm/1.
• the acidified slurry was mixed vigorously at ambient temperature and atmospheric pressure for one hour, followed by 30 minutes agitation at elevated temperature in an oxygen atmosphere (90 psi) .
• the majority of the zinc, lead, and cadmium species in the EAFD were converted to their highly soluble divalent chloride salts. Any divalent iron made soluble in the leach was oxidized to the insoluble iron (III) , ferric oxide.
• EAFD feed typically about 50-60 wt. % of the EAFD feed is recovered in the iron-rich waste cake and is recycled to the electric arc furnace.
• the metal-rich solution typically contains 2-5 gm/1 Pb, 35-40 gm/1 Zn, 2-5 gm/1 Cd, 100-200 mg/1 Cu, and less than 10 mg/1 iron, but the actual composition is dependent upon the concentration of these metal values in the EAFD feed.

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• 1995
  • 1995-01-23 US US08/376,778 patent/US5709730A/en not_active Expired - Fee Related
• 1996
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