Smelter Flue Dust Research Articles

A novel process for extracting bismuth from high iron content copper smelting dust by magnetic separation and leaching process

In this study, an efficient method for extracting bismuth from copper smelter flue dust with high content of iron was proposed. In this method recovery of bismuth from copper flue dust was carried out after copper and iron separation. To separate copper from dust, this material was leached with water and dilute sulfuric acid (0.6 M). Magnetic separation was performed with field intensities of 0.1 T and 0.4 T to separate iron in the leaching residue. 70 % of bismuth is placed in the non-magnetic fraction. The non-magnetic fraction of this separation was leached with (1.4 M) H2SO4 + (1 M) NaCl solution to recover bismuth. In optimal conditions, more than 98 % of bismuth in the non-magnetic fraction is leached. Bismuth precipitation from this solution was done by adjusting the pH to 2.5 with the ammonia solution. Impurities such as; Pb, Fe, As, and Cu were precipitated with bismuth in this condition. Lead was removed as a PbCl2 compound by leaching the precipitate with 6 M hydrochloric acid. Other elements were leached in this chloride environment and entered the solution phase. Bismuth was separated from this leaching solution by hydrolysis in the form of BiOCl. The purity of the BiOCl product was about 95 %. This substance can be used in photocatalyst and solar cell industries.

Treatment of high-arsenic copper smelting flue dust with high copper sulfate: Arsenic separation by low temperature roasting

Arsenic is the most important hazardous trace element in copper concentrates. In order to separate arsenic from the smelting processes, a pyrometallurgical treatment with additives was developed to deal with high-arsenic copper smelter flue dust high in copper sulfate. Under the optimized conditions of mixing 30 wt% pyrite and roasting at 550 °C under an inert atmosphere for 30 min, more than 92% arsenic was volatilized as a high purity arsenic trioxide by-product while most copper was retained in the calcine to be returned to smelting as a high-copper feed. The high separation of arsenic is due to the transformation from the limited vaporization of arsenate to volatile arsenic trioxide by pyrite at above 460 °C. Two reaction paths of pyrite were identified as follows: one followed the pattern FeS2 → Fe2O3 → FeAsO4 → Fe3O4/Fe2O3 and the other was FeS2 → FeS + S2 (g) → Fe3O4/Fe2O3.

The potential wildfire effects on mercury remobilization from topsoils and biomass in a smelter-polluted semi-arid area

Wildfires can be responsible for significant mercury (Hg) emissions especially in contaminated areas. Here, we investigated the Hg distribution in topsoils and vegetation samples and temperature-dependent Hg mobilization from biomass-rich topsoils collected near a copper (Cu) smelter in Tsumeb (semi-arid Namibia), where Hg-rich Cu concentrates are processed. The thermo-desorption (TD) experiments conducted on representative biomass-rich topsoils (3.9–7.7 mg Hg/kg) indicated that more than 91% of the Hg was released at ∼340 °C, which corresponds to the predominant grassland-fire conditions. The mineralogical investigation indicated that the Hg comes mainly from the deposited smelter emissions because no distinct Hg-rich microparticles corresponding to the windblown dust from the nearby disposal sites of the technological materials (concentrates, slags, tailings) were found. A comparison with the TD curves of the Hg reference compounds confirmed that the Hg in the biomass-rich topsoils occurs as a mixture of Hg bound to the organic matter and metacinnabar (black HgS), which exhibits similarities with the TD pattern of smelter flue dust residue. Despite the installation of a sulfuric acid plant in the smelter in 2015 and a calculated drop in the estimated Hg emissions (from 1301 ± 457 kg/y for the period 2004–2015 to 67 ± 5 kg/y after 2015), the Hg legacy pool in the smelter surroundings can potentially be re-emitted back to the atmosphere by wildfire. Using the Hg spatial distribution data in the area (184 km2), the estimates indicate that up to 303 kg and 1.3 kg can be remobilized from the topsoils and vegetation, respectively.

Selective Separation of Arsenic from Lead Smelter Flue Dust by Alkaline Pressure Oxidative Leaching

This study investigated the feasibility of using an alkaline pressure oxidative leaching process to treat lead smelter flue dust containing extremely high levels of arsenic with the aim of achieving the selective separation of arsenic. The effects of different parameters including NaOH concentration, oxygen partial pressure, liquid-to-solid ratio, temperature, and time for the extraction of arsenic were investigated based on thermodynamic calculation. The results indicated that the leaching efficiency of arsenic reached 95.6% under the optimized leaching conditions: 80 g/L of NaOH concentration, 1.0 MPa of oxygen partial pressure, 8 mL/g of liquid-to-solid ratio, 120 °C of temperature, 2.0 h of time. Meanwhile, the leaching efficiencies of antimony, cadmium, indium and lead were less than 4.0%, basically achieving the selective separation of arsenic from lead smelter flue dust. More than 99.0% of arsenic was converted into calcium arsenate product and thus separated from the leach solution by a causticization process with CaO after other metal impurities were removed from the solution with the addition of Na2S. The optimized causticization conditions were established as: 4.0 of the mole ratio of calcium to arsenic, temperature of 80 °C, reaction time of 2.0 h. The resulting product of calcium arsenate may be used for producing metallic arsenic.

Recovery of zinc and copper from copper smelter flue dust. Optimisation of sulphuric acid leaching

ABSTRACTCopper smelter flue dust is a residue, rich in zinc, originating from the pyrometallurgical process of copper, and constitutes a potential raw material for zinc production. In order to valorise this waste by hydrometallurgical processes, its leaching with sulphuric acid at constant pH is studied. Experimental conditions (temperature, pH, and pulp density) are optimised by design of experiments in order to maximise Zn and Cu extractions while minimising the leaching of Fe. The experimental results are fitted to a full-quadratic second-order equation model by applying multiple regression analysis for extractions of Zn, Cu, and Fe. In order to determine the optimum conditions, the response surfaces are overlaid and the intersection of the plot of the three responses identifies conditions of pH and pulp density that satisfy recoveries of Zn higher than 95%, recoveries of Cu around 70%, and Fe extractions lower than 35%. Under the best conditions, that is, pH = 2, 10% of pulp density, and at 25°C, a liquor rich in zinc and copper, and a lead concentrate are obtained.

Transformation of arsenic-rich copper smelter flue dust in contrasting soils: A 2-year field experiment

Dust emissions from copper smelters processing arsenic-bearing ores represent a risk to soil environments due to the high levels of As and other inorganic contaminants. Using an in situ experiment in four different forest and grassland soils (pH 3.2–8.0) we studied the transformation of As-rich (>50 wt% As) copper smelter dust over 24 months. Double polyamide bags with 1 g of flue dust were buried at different depths in soil pits and in 6-month intervals; then those bags, surrounding soil columns, and soil pore waters were collected and analysed. Dust dissolution was relatively fast during the first 6 months (5-34%), and mass losses attained 52% after 24 months. The key driving forces affecting dust dissolution were not only pH, but also the water percolation/retention in individual soils. Primary arsenolite (As2O3) dissolution was responsible for high As release from the dust (to 72%) and substantial increase of As in the soil (to a 56 × increase; to 1500 mg kg−1). Despite high arsenolite solubility, this phase persisted in the dust after 2 years of exposure. Mineralogical investigation indicated that mimetite [Pb5(AsO4)3(Cl,OH)], unidentified complex Ca-Pb-Fe-Zn arsenates, and Fe oxyhydroxides partly controlled the mobility of As and other metal(loid)s. Compared to As, other less abundant contaminants (Bi, Cu, Pb, Sb, Zn) were released into the soil to a lesser extent (8-40% of total). The relatively high mobility of As in the soil can be seen from decreases of bulk As concentrations after spring snowmelt, high water-extractable fractions with up to ∼50% of As(III) in extracts, and high As concentrations in soil pore waters. Results indicate that efficient controls of emissions from copper smelters and flue dust disposal sites are needed to prevent extensive contamination of nearby soils by persistent As.

Characterization and pH-dependent environmental stability of arsenic trioxide-containing copper smelter flue dust

Increasing amounts of impurities (especially As) in Cu ores have aggravated the problem of flue dust generation in recent years. As an example from a smelter processing As-rich Cu ores, we characterized a flue dust particularly rich in As (>50 wt%) to understand its mineralogy and pH-dependent leaching behavior, with special emphasis on binding, release and solubility controls of inorganic contaminants (As, Bi, Cd, Cu, Pb, Sb, Zn). Whereas arsenolite (As2O3) was the major host for As and Sb, other contaminants were bound in sulfides, arsenates, alloys and slag-like particles. The EU regulatory leaching test (EN 12457-2) indicated that leached As, Cd, Sb and Zn significantly exceeded the limit values for landfills accepting hazardous waste. The pH-dependent leaching test (CEN/TS 14997) revealed that As, Sb and Pb exhibited the greatest leaching at pH 11–12, whereas Cd, Cu and Zn were leached most under acidic condition (pH 3) and Bi leaching was pH-independent. Mineralogical investigation of leached residue coupled with geochemical modeling confirmed that newly formed Ca, Pb and Ca-Pb arsenates (mimetite, Pb5(AsO4)3Cl) partly control the release of As and other contaminants under circumneutral and alkaline conditions and will be of key importance for the fate of smelter-derived contamination in soils or when stabilization technology is employed.

Hot Corrosion Mechanism of Steels Exposed to Heavy Metal Chlorides and Sulphates in SO2 Environment

Copper flash smelting produces flue dust containing SO2-rich exhaust gas, causing corrosion problems in the heat recovery boiler of the gas train. In order to understand the corrosion behaviour of boiler steels, conditions of the boiler were simulated in the laboratory. Corrosion damage occurred as chlorine reacted with steel surfaces forming chlorides which deplete the steels from their alloying metals. At the scale/dust deposit interface, where the highest sulphur partial pressures prevail, a sulphate layer covered the mixed oxide and chloride scale. Molten sulphate deposit reduced the metal loss of AISI 304 steel by preventing chromium chloride diffusion away from the steel surface. The Cr2O3 scales were quite stable under molten sulphates. The dust deposit melted partially due to the presence of ZnCl2 in the deposit which initiated corrosion damage by producing a molten salt layer on the steel surfaces bringing it in contact with aggressive compounds in the copper smelter flue dust.

Influence of nickel alloying on hot corrosion of stainless steels in SO2-environment

Flue dust produced by copper flash smelting causes problems by forming corrosive melts on the walls of heat recovery boiler used to recover the process heat value and to separate flue dust from SO2-rich gas, derived from smelting sulphidic copper ore, before manufacture of sulphuric acid. The corrosion phenomena of molybdenum containing low and high nickel stainless steels AISI 316 and Sanicro 28 were studied by simulating the conditions prevailing in the heat recovery boiler in laboratory in the temperature range of 250–350°C. ZnCl2 in copper smelter flue dust resulted in partial melting of the deposit containing sulphates and oxides of copper, zinc, arsenic, iron and lead, which increased the rate of corrosion dramatically. Chlorination of the steel was the dominant corrosion mechanism. The high nickel steel corroded more than low nickel steel due to reactivity of nickel under molten sulphates.

Leaching of metals from copper smelter flue dust (Mufulira, Zambian Copperbelt)

The leaching behaviour of electrostatic precipitator dust from the Mufulira Cu smelter (Copperbelt, Zambia) was studied using a 48-h pH-static leaching experiment (CEN/TS 14997). The release of metals (Cd, Co, Cu, Ni, Pb and Zn) and changes in mineralogical composition using X-ray diffraction and PHREEQC-2 modelling were investigated in the pH range of 3–7. The highest concentrations of metals were released at pH 3–4.5, which encompasses the natural pH of the dust suspension (4.3). About 40% of the total Cu was leached at pH 3, yielding 107 g/kg. Chalcanthite (CuSO45H2O), magnetite (Fe3O4) and delafossite (CuFeO2) represented the principal phases of the studied dust. In contact with water, chalcanthite was dissolved and hydrated Cu sulphates precipitated at pH 4–7. Gypsum (CaSO42H2O) and secondary Fe or Al phases were observed in the leached residues. Serious environmental impact due to leaching may occur in dust-contaminated soil systems in the vicinity of the smelting plants.

Treatment of copper flash smelter flue dusts for copper and zinc extraction and arsenic stabilization

A flue dust sample of copper from flash smelting furnace has been extensively characterized. The main phases of the dusts are magnetite, cuprospinel, chalcocyanite, zincosite, dolerophanite, and claudetite. As minor phases, tenorite, arsenolite, delafossite and enargite were detected. In the sample studied, particles were mainly spherical and less than 60 μm. The leaching of flue dust sample with water dissolved copper sulfates and zinc sulfates and, partially, arsenic oxides, in an exothermic process. The leaching of the residue after water leaching in several acid media increased the dissolution of copper and arsenic oxides and small amounts of zinc. A preliminary study by hydrocyclone classification was carried out with the objective of to obtain two fractions: a coarse fraction rich in copper and a fine fraction rich in arsenic. However, results obtained in this sample were not very successful. In the stabilization study, the residue obtained after leaching the copper flash smelter flue dust with sulfuric acid was mixed with sludge from a paper factory (co-inertization process). The smaller size fraction (> 12 μm) obtained in the hydrocyclone was also mixed with this sludge. Results obtained derived of the application of a standard leaching test over these residues indicate that they were stabilized. Thus, the resulting mixtures should be dumped without generating major environmental problems.

Transfer of cadmium, lead, and zinc from industrially contaminated soil to crop plants: A field study

The documeneed adverse health effects of soil Cd and Pb have led to public concern over soil contamination with metals. A 4-year field experiment was conducted to study the transfer of Cd, Pb, and Zn from soil contaminated by smelter flue-dust to crop plants grown in a rotation. The soil was amended with PbZn smelter flue-dust (2–66.8 kg per 10 m 2 plot) to simulate the long-term effect that the smelting of non-ferrous metal ore has on arable soils. The treated soil became strongly contaminated with metals (Cd 3.2–106 mg/kg, Pb 146–3452 mg/kg, Zn 465–11 375 mg/kg). Concentrations of Cd, Pb, and Zn in barley grain, barley straw meadow bluegrass, red clover, and potatoes were generally low. The highest metal concentrations were found in potato tubers (intact), meadow bluegrass, and barley straw. The observed reduction in crop yield was probably the result of possible nutrient imbalances rather than of metal (Zn, Cu) phytotoxicities. Zn and Cd uptake by the plants can be described by the saturation (plateau) model (y = ax b, b < 1). The relationship between Pb in the soil and plants was linear with an extremely low slope (0.0001–0.0003). No excessive dietary intake of Cd is expected when Cd concentrations in barley grain and potato tubers grown on the contaminated soil are not higher than 0.6 and 1.0 mg/kg, respectively. Based on the risk analysis and taking into account the saturation model of the soil-plant metal relationship, it was concluded that, under the conditions of this experiment (neutral soil pH), soil with Cd concentrations of up to 30 mg/kg is still safe for production of these crop plants.

Predicting the environmental stability of treated copper smelter flue dust

Column tests were conducted to determine the leachability of As, Cd, Cu, Fe and Pb from copper smelter flue dust that had been treated by the Cashman Process in an effort to recover metals while rendering the residue inert. Between 100 and 300 μg/l As [predominantly As (V)] leached from the residue from a batch-reactor test, while between 1000 and 1400 μg/l As leached from residue from a continuous-reactor test. Electron microprobe analyses of the two materials identified scorodite (FeAsO 4·2H 2O) as the principal As-bearing phase. The pHs of leachate from the batch-and continuous-reactor residues were approximately 5.0 and 4.0, respectively. Cadmium and Cu leachate concentrations decreased through the tests, while Pb equilibrated at approximately 200 and 1300 μg/l for the batch and continuous residues, respectively. The measured Eh did not agree with the Eh calculated from either the As(III)/As(V) or Fe(II)/Fe(III) couple. Modeling of system chemistry using MINTEQA2 indicated that scorodite controlled the As concentration of the leachate. The variability of leachate As and metal concentrations between the batch-and continuous-reactor residues indicates that the process conditions failed to produce residues of satisfactory stability.

Electrogalvanizing using zinc recovered from nonferrous smelter dusts

An objective of the U.S. Bureau of Mines recycling research is to devise technology that enables the recovery of metals, minerals, and other values from waste products and thus promote the wise and efficient use of the resources of the U.S.A. In line with this objective, Bureau researchers are investigating the use of waste products containing zinc as a source of zinc for electrogalvanizing. Industrials wastes used in this work were lead smelter flue dust, wastewater treatment sludge, copper smelter flue dust, and brass smelter flue dust. After zinc extraction with sulfuric acid, the resultant solutions were partially purified and used to electrogalvanize 0.234-cm (0.092-in.) diameter 1070 alloy steel wire, a common electrogalvanized product. The corrosion resistance of the electrogalvanized wire from waste electrolyte was compared with electrogalvanized wire from relatively pure electrolyte prepared in the laboratory, and with electrogalvanized wire from an industrial electrogalvanizing plant. Corrosion rate measurements indicated that electrogalvanized wire obtained from waste electrolyte prepared from brass smelter flue dusts compared favorably with industrial electrogalvanized wire and electrogalvanized wire from the relatively pure electrolyte. The brass smelter flue dusts were chosen for more detailed studies. Impurities in the waste electrolyte caused some problems. Copper was undesirable because it passivated the soluble zinc anode. Other impurities such as cobalt and nickel were more tolerable but also slowly coated the anode. Owing to the ease of removal of copper by cementation, it posed no processing problem for electrogalvanizing. Successful completion of bench-scale work with brass smelter flue dusts led to large-scale trials at an industrial electrogalvanizing plant.

Polarization and Corrosion of Electrogalvanized Steel—Evaluation of Zinc Coatings Obtained from Waste‐Derived Zinc Electrolytes

The corrosion of electrogalvanized 1070 steel wire has been investigated in molar quiescent ammonium chloride and ammonium sulfate under near‐neutral conditions. Zinc coatings obtained from waste‐derived electrolytes were evaluatedvs. coatings from relatively pure zinc electrolytes. The waste source of zinc was brass smelter flue dust. Corrosion rates were measured by Tafel line extrapolation and the polarization resistance technique. Values of the Tafel slopes and the corrosion currents were also compared with those for pure zinc (99.999%). Corrosion rates were found to be affected by the medium employed, pH, and bimetal diameter reduction (drawing). Drawn electrogalvanized steel displays higher values of the Tafel slopes than do the as‐plated samples. The Tafel slopes are different from those obtained on pure zinc. This is assumed to be due to inhomogeneous surface features obtained from additive adsorption (during plating) and residual lubricants used in the drawing process as well as surface structure. The corrosion rates of electrogalvanized samples plated in the waste‐derived zinc electrolytes were similar to the corrosion rates of samples plated in relatively pure zinc electrolytes. Therefore, wastes are a potential source of zinc for electrogalvanizing.

Recovery of copper, silver and zinc from Huelva (Spain) copper smelter flue dust by a chloride leach process

Copper smelter flue dust is a serious problem in terms of pollution and storage because of its high content of metal in water-soluble compounds and its low apparent density (450 kg/m 3). This paper describes a laboratory-scale study of a method for processing this material based on leaching in- hydrochloric acid-sodium chloride medium, cyclone separation of matte particles, and precipitation of iron, arsenic, antimony and bismuth in the presence of calcium ions at a pH of 3.5, leaving a residue of these metals together with lead. The solution from this operation is treated with zinc to cement copper and silver for recycling to the converter. Then the zinc and cadmium contained in the dust, as well as that used in cementing, are precipitated with sodium carbonate. The leaching solution can be recycled. This process makes it also possible to reduce the residue volume to one-sixth of that of the flue dust, and to minimize pollution problems.

Recycling of zinc waste for electrogalvanizing

An objective of Bureau of Mines recycling research is to devise technology that enables the recovery of metals, minerals, and other values from waste products and thus promote the wise and efficient use of our nation's resources. In line with this objective, Bureau researchers are investigating the use of waste products containing zinc as a source of zinc for electrogalvanizing. Industrial wastes used in this work were lead smelter flue dust, wastewater treatment sludge, copper smelter flue dust, and brass smelter flue dust. After zinc extraction with sulfuric acid, the resultant solutions were partially purified and used to electrogalvanize 0.234-cm (0.092-in.) diameter 1070 alloy steel wire, a common electrogalvanized product. The corrosion resistance of the electrogalvanized wire from waste electrolyte was compared with electrogalvanized wire from relatively pure electrolyte prepared in the laboratory, and electrogalvanized wire from an industrial electrogalvanizing plant. Corrosion rate measurements indicated that electrogalvanized wire obtained from waste electrolyte prepared from brass smelter flue dusts compared favorably with industrial electrogalvanized wire and electrogalvanized wire from the relatively pure electrolyte. The brass smelter flue dusts were chosen for more detailed studies. Impurities in the waste electrolyte caused some problems. Copper was undesirable because it passivated the soluble zinc anode. Other impurities such as cobalt and nickel were more tolerable but also slowly coated the anode. Owing to the ease of removal of copper by cementation, it posed no processing problem for electrogalvanizing. Successful completion of bench-scale work with brass smelter flue dusts led to large-scale trials at an industrial electrogalvanizing plant.

Disposal of arsenic bearing copper smelter flue dust

A study of arsenic fixation was conducted to investigate the stabilization of arsenic in copper smelter flue dust by dissolution in slag matrices, and by encapsulation in cements, mortars, clays, and concrete. The success or failure of the stabilization was determined by immersion leach tests. Results of the study demonstrated that arsenic can be effectively stabilized by first converting it to an arsenate form, then dissolving it in a slag matrix. Further kinetic limitations can be imposed on arsenic release by encapsulating the arsenic bearing slag in cements, concretes, and mortars. Aqueous extraction results indicate minimal release.

Thiourea Leaching of Gold and Silver — Technology Update and Additional Applications

The hydrometallurgical application of thiourea in processing precious metal ores and concentrates has reached the development stage. This paper reviews recent technical literature covering the process development of thiourea leaching. In addition, recent fundamental contributions are discussed. Most of the interest in thiourea leaching has involved treating high grade concentrates that have sufficient value to justify the use of thiourea as a reagent. An important development in thiourea technology is the addition of SO2 to the leaching solution. This procedure effectively reduces the decomposition of thiourea and eliminates the problems of passive surface layers that limit the leaching efficiency. Results for the thiourea leaching of a bulk sulfide concentrate and a residue from the autoclave leaching of a smelter flue dust are also reported. As determined by a standard bottle roll cyanidation test, the bulk concentrate was extremely resistive to cyanide leaching yielding only 16% gold and 2% silver extraction in 24 hours. Leaching with a relatively concentrated thiourea solution (1MCS (NH2)2) resulted in 83% gold and 75% silver extraction in four hours. With the treated flue dust residue, gold extraction was about 60%. Gold and silver extraction with cyanide was sensitive to both cyanide concentration and oxidizing conditions. Cyanidation resulted in 59% gold extraction and 77% silver extraction in 24 hours.

Recovering accessory minerals from lead and zinc process wastes

This research is part of a continuing effort to reduce environmental conflicts and occupational hazards associated with mineral processing and to recover valuable constituents in wastes. A laboratory process for lead smelter flue dust (sulfation roasting followed by water leaching) extracts over 950 of the Cd and Zn. Ninety-nine percent of the Cd is recovered from solution by sponging with zinc dust, and Zn is electrowon from the resulting solution after purification. The Cd and Zn leach residue consists largely of PbSO 4, which is converted to PbCO 3 and leached with H 2SiF 6 to produce PbSiF 6 electrolyte for lead electrolysis. Accessory minerals (such as Co and Ni) are separated into purification residues for subsequent recovery. A copper-cake residue treatment process consists of leaching with sulfuric acid to extract Cd and Zn, which are recovered as in the flue dust processing. The insoluble residue, containing Cu, As, Co, and Ni, is leached with manganese dioxide and sulfuric acid to solubilize those metals while leaving a residue of PbSO 4. The solubilized metals are recovered from the leach solution by caustic purification (As and Fe removal) and sequential sulfide precipitations. The products are copper sulfide and cobalt-nickel sulfide. The manganese is reclaimed from the leach liquor as manganese dioxide for recycle.