ZINC OXIDE FROM STEEL MILL DUST A WIDE RANGE OF OPPORTUNITIES

Transcription

1 Caterina Benigni 1, Christoph Pichler 2, Jürgen Antrekowitsch 1 ZINC OXIDE FROM STEEL MILL DUST A WIDE RANGE OF OPPORTUNITIES Abstract Caused by an increasing amount of zinc coated steel products, zinc enters the steel production through galvanized steel, which is used in the basic oxygen furnace (BOF) and the electric arc furnace (EAF). Based on the process temperatures, the zinc evaporates as metallic zinc, reoxidizes to zinc oxide in the offgas and gets collected in the bag house filter. Beside zinc, which represents the main component, elements like lead, iron, cadmium, sodium, potassium, fluorine and chlorine form part of the dust. In the European Union, this dust is declared as hazardous waste, with million tons of EAF dust produced in This amount corresponds to a zinc amount of approximately million tons. Due to the zinc content, the EAF dust displays a usable by-product. This paper deals with the different opportunities to produce products for the zinc market, using BOF dust as a raw material. A special focus is set on the production of Waelz oxide (WOX) followed by strategies to form different zinc products such as oxides, sulphates, carbonates as well as a metallic shape. Furthermore, the paper deals with the different product qualities which are generated depending on the process steps. Keywords Steel mill dust, zinc, zinc oxide, recycling, Waelz oxide 1. Introduction The rising production of steel and nonferrous metals leads to an increase of residues which contain a significant amount of valuable elements. Slags, sludges and other types of dust represent such remaining materials. The target is represented by the recycling of these residues in an environmentally friendly way by avoiding the production of residues for landfilling. Steel mill dust displays a recyclable residue which contains zinc as main component due to the input of galvanized steel scrap into the electric arc furnace (EAF) or the basic oxygen furnace (BOF). Around 55 % of the produced electric arc furnace dust is globally sent to landfill, although the recycling rate in Europe reaches approximately 90 %. Tab. 1 gives an overview of the electric arc furnace steel production of 2010, 2011 and 2012 and the estimated amount of electric arc furnace dust (EAFD). The production of 100 kg EAF-steel results in 1.8 kg EAFD and leads to 8,1 million tons in The calculation with 1.8 % is based on surveys from several steel mills. [1] 1 Christian Doppler Laboratory for Optimization and Biomass Utilization in heavy metal recycling, Montanuniversitaet Leoben, Austria 2 Chair of non-ferrous metallurgy, Montanuniversitaet Leoben, Austria

2 Tab. 1: Overview of the production- and recycling amounts of EAFD [1] Year Geographical Region [mmt of steel] [mmt of steel] [mmt of steel] Asia (3,666)* European Union (1,261) North America (1,306) Other Europe (532) CIS (437) Middle East (405) South America (291) Africa (186) Oceania (25) Total (8,109) [mmt] Million metric tons * Estimated dust production in kmt In 2015, a global crude steel production of 1,617.3 million tons [2] lead to a dust amount of approximately 32.2 million tons. The electric steel route generates 25.1 % of the whole steel production. This corresponds to a dust amount of 8.1 million tons which results from the consideration that kg dust/t of steel are generated. The amount of zinc depends on the fed scrap during the production of steel. Normally, the zinc content in EAFD fluctuates around 22 % which represents a total zinc amount of 1.78 million tons. This corresponds to a sum of 12.8 % based on the zinc production of 13.9 million in 2015 [3]. Beside zinc, elements like iron, lead, manganese, sodium, potassium, fluorine and chlorine also form part of the dust. Especially iron and the halides cause problems during the production of various zinc products. 2. Currently used recycling technologies for treating steel mill dust All of the currently applied pyrometallurgical processes utilize the low evaporation temperature of zinc which gets concentrated in the gas stream and is separated as zinc oxide. During the hydrometallurgical processes, a dissolution of zinc displays the main target. Afterwards, different ways are possible to extract the zinc from the solution. Tab. 2 summarizes the available pyro- and hydrometallurgical technologies to recycle steel mill dust. Tab. 2: Available technologies to recycle steel mill dust Pyrometallurgical process Waelz Process Rotary Hearth Furnace Shaft furnace Multiple hearth furnace Hydrometallurgical process H2SO4 leaching NaOH leaching NH4Cl leaching The Waelz process represents the dominating process. 80 % of the dust recycling runs along this route in Europe [4]. The first step is represented by the mixing and pelletizing of dust, coke and slag additives after which the pellets enter a 40 m long rotary kiln. At approximately

3 1100 C, carbon and carbon dioxide reduce the zinc oxide and the produced zinc volatilizes. Based on the temperature also non-ferrous metal compounds like lead oxide, zinc chloride and sodium-potassium chlorides volatilize and enter the off-gas stream. Iron and calcium form part of the product due to carry-over. The reoxidized zinc steam can be collected in the dust chamber as zinc oxide. [5] The advantages of a Waelz kiln include the wide range of feeding materials which are treatable, a simple process control, a low energy consumption and low investment costs. The high CO2 emission, the low product quality, the minimum amount of zinc in the residues and the high slag amount are negative aspects of this process type. Nowadays, the Waelz oxide is sold to the primary industry to produce metallic zinc. Due to the impurities iron, fluorine and chlorine a direct leaching of the Waelz oxide is not possible. The halides are especially critical because of their damaging effect to the electrodes in the zinc electrowinning. To remove these elements, the secondary zinc oxide is washed three times by applying a counter-current process. Fig. 1 displays the water solubility of different chlorides and fluorides which displays the fundamental characteristic to remove halides from the Waelz oxide. In addition, the dissolution of some of these compounds leads to a metal loss when considering zinc chloride among other. In order to avoid this, the addition of soda results in an insoluble metal carbonate and a soluble sodium salt (see equation 1). MeF2/MeCl2 + Na2CO3 MeCO3 + 2NaF/NaCl Eq. [1] Fig. 1: Solution behaviour of different chlorides and fluorides [6] The removal of chlorides is easily possible due to the high water solubility. Regarding fluorides, only sodium-, potassium- and zinc fluoride are rarely soluble. Lead fluoride is slightly soluble in comparison to calcium fluoride which is not soluble at all. An alkaline ph value improves the solubility of fluorides, however, not for a complete removal. During the washing process, calcium oxide shows a negative effect on the removal of fluorine. It forms part of secondary zinc oxides due to carry over. [7] During the washing process, a reaction between the calcium oxide and the fluorine ions is possible and leads to a reduced fluorine yield. In general, two precipitation reactions can be taken into account, one with calcium ions in the solution while the other reaction takes place at the surface of the hydrated calcium oxide. [7,8,9] The reaction with calcium and fluorine ions is explained in equation 2: Ca F - CaF2 Eq. [2] The condition for this reaction is represented by the fact, that calcium oxide dissolves and forms calcium ions. The product - calcium fluoride - is not soluble in water which causes a

4 negative effect on the removal of fluorine. Therefore, calcium fluoride still remains in the Waelz oxide and gets dissolved with sulfuric acid in the following leaching stage. [7,8,9] Furthermore, an adsorption mechanism also leads to the formation of calcium fluoride. This is a two-step process, where the calcium oxide hydrates to calcium hydroxide at first (equation 3). The aqueous fluorine adsorbs on the surface of calcium hydroxide and results in a calcium fluoride precipitate (equation 4). Based on the low solubility of calcium oxide in water, the main mechanism for the removal of fluorine is represented by the adsorption. CaO + H2O Ca(OH)2 Eq. [3] Ca(OH)2 + 2F - CaF2 + 2OH - Eq. [4] Due to the release of OH - ions, the ph value increases and with a rising OH - concentration which results in a balance between fluorine and hydroxide ions. The higher the ph value, the less fluorine ions can be adsorbed on the surface. [7,9] Low halide values are required (chlorine <100 ppm, fluorine <50 ppm) to use the Waelz oxide directly in the leaching stage of the primary zinc winning. These low values cannot be achieved with the washing process which is why the zinc oxide runs through the whole primary process together with the primary concentrates. The usage of Waelz oxide for other products is critical as well due to the impurities. In the next chapter different possibilities are listed to use secondary zinc oxide to generate different zinc products. 3. Zinc markets In the field of steel mill dust recycling only pyrometallurgical processes are industrially applied. Starting from secondary zinc oxides generated from steel mill dust, four different ways are possible which are illustrated in Fig. 2. The most common one is displayed by the usage in the primary industry to generate metallic zinc. Other routes include the production of a high purity oxide, a sulfate or a carbonate. Zinc oxide is a basic material for products in the medicine, cosmetics, paints, sun cream, catalysts, tyres and rubber industry. Zinc sulfate is used for agricultural applications as well as in the dye house. Zinc carbonate is used for pigments and as absorption medium for hydrogen sulfide. The requirements in relation to the product quality varies depending on the market. [10] Fig. 2: Flow Sheet of the different markets for secondary zinc oxides

5 Today, most of the Waelz oxides are sold to the primary industry. Based on the contained impurities iron, chlorine, fluorine these secondary oxides enter the process route at the roaster and run along the whole production route. A direct usage in the leaching stage would be preferable due to the reason that the zinc content is high enough and the amount of zinc ferrite is lower in comparison to the produced concentrate in the primary industry. Nevertheless, purification steps are necessary to generate a leachable zinc oxide without disturbing elements. The formation of high purity zinc oxide is complex because the tolerance of contained impurities is really low. Critical elements include iron due to its colouring effect as well as sulfur, cadmium and lead. The usage of zinc oxide in the tyre industry demands a stricter chemical composition than the ceramic industry. Standards like the ASTM D (2006) Standard Test Methods for Rubber Compounding Material-Zinc Oxide or the ISO 9298:1995 Rubber compounding ingredients -- Zinc oxide -- Test methods. describe the typical chemical and physical requirements. Important parameters for the utilization in the tyre industry include the surface area, heat loss, sieve residue, zinc oxide, sulphur, lead and cadmium content. Tab. 3 shows these requirements for zinc oxide which is used in the tyre industry. [10] Tab. 3: Guideline for zinc oxide for the tyre industry [10] Loss during drying (105 C) max. 0.5 % Ignition residue (950 C) min % Sieve residue (0.045 mm) max % Zinc oxide content min % Cadmium content max. 300 mg/kg Copper content max. 10 mg/kg Lead content max. 0.4 % Manganese content max. 10 mg/kg Sulphur content max % Nitrogen surface (BET) min. 5 m 2 /g In the ceramic industry there is no special quality for zinc oxide. In this sector zinc oxides with a lower quality are saleable. The pharmaceutical industry shows the highest standards regarding to the purity of the input materials. In this field the maximum content of impurities are as follows: 5 ppm Arsenic 10 ppm Cadmium 200 ppm Iron 50 ppm Lead Beside zinc oxide, zinc sulfate represents an important zinc compound. 51 % of the produced zinc sulfate is used in agriculture. Typically, this material is produced by dissolving zinc oxide in sulfuric acid. As alternative to the sulfate the carbonate can also be applied in this area. Critical elements like arsenic, cadmium, chromium, lead and mercury accumulate in the soil and possibly enter the food chain through crop plants and groundwater. The content of these elements is limited in the ppm range but differs between several countries. [11,12] The application of zinc oxide from the secondary industry for the production of zinc fertilizer is difficult. Due to the contained impurities more purification steps are needed to reach the low values of such elements.

6 4. Experimental In the past, various concepts were developed to treat zinc-containing filter dust or similar residues. As already mentioned above, the Waelz process is currently state-of-the-art for the recycling of high zinc containing residues together with other waste materials to reach a zinc content in the input mixture of around 23 wt.-%. At the moment it is not possible to treat materials which show a lower zinc value although they occur in the industry. Therefore, some integrated steel mills put an effort on getting independent from recyclers and develop their own technology to treat the zinc-containing dust, which mainly occurs at the Basic Oxygen Furnace. One example of such a concept is the so called Flash Reactor. Equal to the Waelz process a zinc oxide is produced, which can be collected in a filter house together with a heavy metal-free slag. The advantage of the process lies in the possible treatability of low-grade residues regarding to the zinc value without prior agglomeration. A Flash Reactor pilot testing plant is available at the Chair of Thermal Processing Technology, Montauniversitaet Leoben. Fig. 3 represents the 3D view of this plant. The process starts at the mixing cyclone followed by the reaction vessel where the separation of zinc takes place. The off-gas runs into the converter and the slag is tapped at the bottom of the vessel. Post combustion and cooling is the main part of the converter. The last part is the spray tower where the gas is quenched to 150 C which is low enough for the filter system. The crude zinc oxide gets collected in the filter and the flue gas leaves the system over the chimney. [13] During the cooling of the gas some amount of the solid zinc containing product settle at the bottom of the spray tower quenching system. This collected residue is characterized through a greater particle size, a higher iron content and a lower zinc content in comparison to the crude zinc oxide. The iron content of the crude zinc oxide was 5.9 % and the quenching residues shows concentrations of 14.5 % during the investigated trials. To utilize both zinc containing products they get mixed together which results in a total average iron content of 7.4 %. The separate treatment of the filter dust will become more attractive for the zinc industry due to the lower iron content. Fig. 3: 3D-view of the Flash Reactor pilot plant at the Chair of Thermal Processing Technology, Montauniversitaet Leoben

7 The generated mixture consists 82.4 % of the filter dust and 17.6 % of the residue from the quenching system. Due to the existing impurities in the mixture from the Flash Reactor treatment, a double washing step - similar to the processing of Waelz oxide - was performed to remove the existing halides. The target is to determine the difference in the product quality between the mixed Flash Reactor product and the Waelz oxide after a leaching step. The chemical analysis of both raw materials for the washing step are displayed in Tab. 4. Only elements which are relevant for the primary zinc industry are considered. Tab. 4: Comparison of the mixed Flash Reactor product and crude Waelz oxide Flash Reactor dust* Mixed Flash Reactor crude Waelz oxide product [wt-%] [wt-%] [wt-%] Ca Fe Pb Zn Cl F *Without the settled material from the Spray Tower By operating the Flash Reactor process with an air ratio of , the results from Tab. 4 are reproducible. In this case, the zinc content of the Flash Reactor dust is nearly equal to the crude Waelz oxide from the Waelz kiln, by using an input material for the Flash Reactor with 12.9 % zinc. Lower halide values are recognizable in the Flash Reactor products, which is an advantage due to the damaging effects of chlorine and fluorine in the zinc electolysis. Fig. 4 represents the process flow for the pilot washing trials, starting with the Flash Reactor and ending with the washed zinc oxide.

8 Fig. 4: Flow Sheet of the treatment of steel mill dust with the Flash Reactor The zinc-containing material from the Flash Reactor, designated as Secondary Zinc Oxide in Fig. 4 and the residue of the quenching system, was supplied from the Chair of Thermal Processing Technology. The concept for the washing procedure with soda was specified with the knowledge from the Chair of Nonferrous Metallurgy regarding to their experience in the product upgrade of zinc-containing materials. The used leaching tanks were provided from ARP Aufbereitung, Recycling und Prüftechnik GesmbH, located in Leoben Austria and are shown in Fig. 5. Water was added to the Flash Reactor mixed product in order to generate a suspension, which was continuously mixed in an indirect heated leaching tank. During this washing process, soda was added discontinuously to reach a defined ph value and to minimize the zinc loss in the solution. Subsequently, a solid-liquid separation took place in a filter press. The washing itself was performed twice with different solid-liquid ratios in order to reach the most possible purification.

9 Fig. 5: Pilot plant of the washing unit of the Flash Reactor mixed product 5. Results After the performed double leaching of the mixed Flash Reactor product an extensively chemical analysis was done. To verify the results, three different labs analysed the same samples. In Tab. 5 the standard Waelz oxide is contrasted to the treated mixed Flash Reactor product. Once more, only relevant elements for the primary zinc industry are described. Tab. 5: Contrast of the zinc containing products from the Flash Reactor and the Waelz kiln after a double leaching Mixed Flash Reactor product (double leached) Waelz oxide (double leached) [wt-%] [wt-%] Mg Ca Fe Pb Zn Cl F 0.06 < Regarding to the results, the mixed Flash Reactor product shows halide values in the range of the double leached Waelz oxide after the washing. The iron content of the Flash Reactor shows a higher value regarding to the Waelz oxide. This results by the existing gas flow in the Flash Reactor. Through optimizations of the burner system, this carry over problem can be solved. The zinc content is nearly the same and by reducing the carry over, the zinc content will

10 increase. The advantage of the Flash Reactor lies in the possibility to recycle low-grade zinccontaining residues. In the Waelz kiln only residues with a zinc content higher 25 % are recyclable. The lower zinc content in the Flash Reactor material can also be explained through the generally low zinc content of the used steel mill dust for these experiments. In general the composition of each zinc dust depends on the feeding material of the process. In case of the steel mill dust the amount of zinc-coated scrap is essential. Further Flash Reactor experiments show that the best product results are reached by running the process with an air ratio of 0.7. The improvement of the Flash Reactor dust results in a better product quality after the washing step. Therefore, the dust of the Flash Reactor is regarding to the zinc content comparable with the Waelz oxide although the zinc content of the feeding material of the Flash Reactor is lower. The treatment of zinc residues with the Flash Reactor is possible but does not lead to a dust which can directly be used in the leaching stage of the primary industry due to the impurities like iron, magnesium and halides. First, iron is critical due to its nobler behaviour in comparison to zinc. In the common hydrometallurgical route the iron is precipitated as jarosite. Through a weak acid leaching, the iron content in the solution can be held at a minimum amount but zinc, which is combined with iron, gets lost in the leaching residue. Today, the Waelz oxides enter the primary zinc process route at the roaster where the formation of iron-zinc-phases takes place. Hence, it is useful to treat the secondary zinc oxides separately. Secondly, halides show negative effects on the electrodes in the zinc electrolysis. Chlorine leads to a corrosion of the lead anode and the production of chlorine gas proves to be problematic for the working safety. Furthermore, the presence of fluorine causes the formation of hydrofluoric acid which dissolves the aluminium oxide layer of the cathode. Thus, the zinc precipitates on the blank aluminium and leads to problems at the stripping process. In order to avoid these problems, the chlorine content has to be lower than 100 mg/l and the fluorine content has to be lower than 50 mg/l. Additionally, magnesium is also critical for the production of zinc as it changes the electrodeposited zinc s morphology and causes the occurrence of a zinc reduction at a more negative potential. At magnesium concentrations lower 10 g/l the current efficiency increases and the energy consumption decreases, however, these positive effects get eliminated by concentrations greater or equal of 15 g/l. Also the viscosity of the electrolyte increases. Additional to the viscosity change the size of magnesium and zinc ions is similar and high concentrations of magnesium complicates the transportation of zinc ions to the electrode and their deposition. [14, 15] Nevertheless, selling the material to the zinc winning industry is possible but only the zinc gets paid and adding the secondary zinc oxide into the roaster pretreatment can lead to the formation of zinc ferrite which is negative as mentioned before. Furthermore, the addition into the roaster is limited due to cooling effects and accretion formation caused by lead and halides. Therefore, it is useful to apply a ph controlled leaching step to minimize the iron content if this element is not combined to zinc. 6. Summary To sum up, the recycling of steel mill dust is a necessity due to the declaration as hazardous waste in the EU. As valuable metals zinc plays the most important role followed by lead and iron. In the course of the experiments steel mill dust from the Basic Oxygen Furnace was recycled with a Flash Reactor in order to generate a zinc oxide which can be used for the production of different zinc products. The dust was leached with soda twice to eliminate the

11 halides which are critical. In comparison to the Waelz oxide (80 % of the steel mill dust gets recycled via this route) the mixed Flash Reactor dust shows slightly higher amounts of zinc and significant more iron, by lower amounts of halides. The remaining content of chlorine and fluorine is too high to use the dust directly in the leaching stage of the primary zinc production, therefore a purification like a washing step is needed. The iron which is nobler and a colouring component also shows significantly negative effects. However, the Flash Reactor is able to recycle low-grade zinc containing residues which reduces masses for landfill and the produced dust is comparable with the Waelz oxide. An additional advantage of the Flash Reactor concept is the remaining slag, which is low in zinc and high in iron, therefore it can be used as input material for the hot metal production.

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