Zinc Oxide EAFD : Electric Arc Furnace Dust part 4 Zinc Oxide EAFD : Electric Arc Furnace Dust part 4 Zinc Oxide EAFD : Electric Arc Furnace Dust part 4 Zinc Oxide EAFD : Electric Arc Furnace Dust part 4 Zinc Oxide EAFD : Electric Arc Furnace Dust part 4
Caterina Benigni 1, Christoph Pichler 2, Jürgen Antrekowitsch 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 1.5-2.5 million tons of EAF dust produced in 2015 This amount corresponds to a zinc amount of approximately 0.33-0.55 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 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 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 2012 The calculation with 1.8 % is based on surveys from several steel mills [1] Christian Doppler Laboratory for Optimization and Biomass Utilization in heavy metal recycling, Montanuniversitaet Leoben, Austria Chair of non-ferrous metallurgy, Montanuniversitaet Leoben, Austria Tab 1: Overview of the production- and recycling amounts of EAFD [1] Year 2010 2011 Geographical Region [mmt of steel] [mmt of steel] Asia 185.3 201.7 European Union 71.0 75.8 North America 67.7 71.8 Other Europe 23.6 28.4 CIS 23.0 23.8 Middle East 17.8 20.7 South America 15.1 16.8 Africa 11.1 10.6 Oceania 1.5 1.5 Total 416.2 451.1 [mmt] Million metric tons * Estimated dust production in kmt 2012 [mmt of steel] 203.6 (3,666)* 70.1 (1,261) 72.5 (1,306) 29.6 (532) 24.3 (437) 22.5 (405) 18.2 (291) 10.3 (186) 1.4 (25) 450.5 (8,109) 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 15-20 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 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 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 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 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: Ca2+ + 2F- 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 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