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Recycling residues into metalsA combination of the PRIMUS® pyrometallurgical process and a hydrometallurgical process makes it possible to recover valuable elements from steelmaking process residues which would normally be dumped in landfill. Iron is recovered as pig iron and zinc is recovered as high purity zinc or as zinc alloy. Other elements are recovered in a stable slag suitable for road construction or as saleable by-products such as Pb-Ag concentrate.

All over the world, iron and steelmakers have to comply with ever more stringent environmental regulations.

Gas, liquid, solid wastes and effluents have to be correctly captured, made stable and innocuous. For an iron and steel company, such compliance (where technology exists to solve the problem) means high compulsory investments without any cash return. In many other cases, the compliance process does not even exist and solving the problem means a cash drain for disposing of unprocessable residues in a proper and safe way. Thus it is a burden.

In the steel industry, the number of EAF mini-mills has increased in recent decades as an alternative to the traditional blast furnace-BOF route. Mini-mills recycle steel and iron scrap arising from the steel processing industry itself as well as from end-of-life metallic products. A large proportion of the scrap is galvanised material and as a result, dusts containing 20-40% zinc, 20-40% iron and other non ferrous metals (lead, copper, manganese) generated from the EAF process. The dust also contains chlorine (over 5%) and fluorine. These dusts are collected and have to be either recycled or dumped.

Dumping costs money and creates environmental liabilities. Moreover, many countries strictly regulate the dumping of such material and even forbid the construction of new disposal sites. Therefore, iron and steelmakers are looking for better and more economic solutions. The best solution is the extraction of the contained valuable metals to generate a financial return from these elements. This approach also converts a burden into an opportunity. Much research has been undertaken in the past 20 years; some has resulted in industrial processes producing zinc oxide, but none has solved completely both the environmental and economic issues.

Zinc bEAring rESiduES: A nEw Zinc concEnTrATEAs with any other base metal, the zinc market is the result of a permanently unstable balance between mining, smelting and demand. The London Metals Exchange (LME) price and inventories fluctuate in direct relation

Authors: Patrick C Guillaume, Gaspard Devos and Jean-Luc Roth Paul Wurth SA

RAW MATERIALS AND IRONMAKING

to this production/demand imbalance. Eight years ago, on a worldwide level, too much zinc was produced, LME inventories were too high and the LME zinc price dropped below US$800/t. The zinc market suffered badly during 2001-3 and the zinc surplus had severe consequences for miners and refiners. The unfavourable economics led to new mining projects being put on hold and to the closing of less profitable mines. Some smelter-refiners also closed part or all of their production capacity.

As a consequence of these delays and closures, the zinc market became tighter, the LME inventories decreased quickly, there were some additional cutbacks resulting from a lack of concentrates and the market conditions evolved to a situation of supply deficit. This trend has been accelerated by an average growth in consumption of around 4% per year, supported by the booming Asian economy and so the LME zinc price recovered, reaching US$4,500/t. According to analysts, the price should now stabilise at around US$2,000/t for several years.

In such a context, zinc made from secondary material is becoming more important and zinc-bearing residues like EAF dust are becoming economic alternative resources.

STEELmAking rESiduES: A SourcE of pig ironZinc-bearing residues generated by the steel industry contain both zinc and iron. This iron can be considered as an additional source of raw material if it is properly recovered as pig iron. Since the steel raw material market situation is becoming tighter, and since scrap is becoming scarce and expensive, an alternative source of iron is of great interest for steel producers. This situation is not expected to change in the short term.

A combinEd ApproAch: from Zinc rESiduES To Zinc mETAL And pig ironBased on the above observations, Paul Wurth initiated a development programme to set up technologies able to recycle residues from the iron and steelmaking industries. Among others, zinc bearing residues and, more a

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particularly, EAF dusts, were investigated. The basic idea of the programme was to combine both pyrometallurgical and hydrometallurgical routes. This programme aimed to:` solve an environmental issue through a ‘zero waste’

process` properly recover the iron content as pig iron` produce SHG (special high grade zinc with minimal

purity of 99.995% zinc) or CGG (continuous galvanising grade – zinc alloyed with Al or with Al/Pb) zinc alloy

` collect the other metallic elements as marketable or easily recyclable by-products

The Paul Wurth combination comprises two main steps:` A reduction-smelting step producing pig iron,

non-ferrous metal oxides and an inert slag` A washing-leaching-purification-electro-

winning step producing SHG cathodic zinc and other marketable by-products

This combination achieves the desired targets of environment protection and metal recovery.

pYromETALLurgicAL STEp: rEducTion-SmELTing procESSThe PRIMUS® technology developed by Paul Wurth, consists of the combination of a multiple hearth furnace (MHF) and electric arc furnace (MF; see Figure 1). EAF dusts are pelletised and fed into the MHF where drying and heating

r Fig 1 PRIMUS® combined reduction-melting furnaces

r Fig 2 Industrial PRIMUS® plant, operated by PRIMOREC SA

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take place. The addition of high-volatile coal induces iron and zinc reduction, and reactive carbon for reduction, and metal carburisation is prepared. The subsequent melting step in a specially designed MF completes the iron reduction, melts the pig iron, forms the slag and completes the zinc reduction and de-zincing processes. Zinc and lead are transferred to the off-gas system and recovered as a marketable zinc oxide concentrate. The iron content of the feed material is recovered as liquid pig iron containing up to 4% C. The resulting slag, similar to blast furnace slag, can be used for road construction.

The first industrial PRIMUS plant started operation in mid 2003 at Differdange in Luxembourg and is operated by PRIMOREC SA (see Figure 2). The plant has been designed to treat 60,000t of EAF dust and 15,000t of rolling mill sludge per year from the three ArcelorMittal mini-mills located in Luxembourg. It is also possible to treat external dusts in order to fully utilise its capacity. The key process data are illustrated in Table 1.

The block diagram (see Figure 3) illustrates the general mass balance. The strengths of the PRIMUS process are the 100% recycling rate (zero waste) and the high value products produced. The process is highly flexible in terms

of feed dust analysis and is able to handle even high zinc content residues. The use of a high volatile coal as a cheap reducing agent is also an advantage. The reactors are industrially proven by the stable and excellent operating results of the PRIMOREC plant.

In May 2006, Dragon Steel Company in Taiwan decided on the installation of a 120,000t/yr PRIMUS plant, aiming to treat EAF dust, blast furnace sludge and mill scales. It plans to start the plant mid 2008.

hYdromETALLurgicAL STEpwashing-leaching-purification-electrowinning process The PRIMUS zinc oxide is an intermediate product and has to be treated in order to recover its full value. Research has helped identify and develop the best available technology for treating the zinc oxide to produce SHG zinc together with other marketable by-products.

The hydrometallurgical step consists of four main stages:1) washing (dehalogenation)2) Leaching with a second dehalogenation step3) purification (removal of other metals)4) Electrowinning (EW)

washing The washing stage consists of a multiple-step process aimed at eliminating almost completely the chlorine and most of the fluorine contained in the PRIMUS oxide (see Figure 4). Leaching The leaching of the solution obtained from the washing stage then comprises three steps: de-chlorination, acid leach and neutralisation. The first step eliminates the remaining chlorine from part of the solution flow. A major part of the solution then goes directly to the second step together with the output of the de-chlorination step where lead is eliminated. The third step treats the output of both previous steps. Here, the purpose is to precipitate by neutralisation, the iron and the remaining fluorine and to adjust the fluorine content to a level that is acceptable for EW.

mhfHearth internal diameter, m 7.7Number of hearths 8Operating temperature,°C 800-1,000Fe metallisation, % ~60Coal consumption, 300 kg/t input

mfVessel internal diameter, m 3.5Electrical power, MW 10Operating temperature,°C 1,500Electricity consumption, 900 kWh/t input

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r Fig 3 General mass balance r Fig 4 Washing route schematic

r Table 1 Key process data

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aqueous phase is separated from the zinc-loaded organic compound in a settler. The zinc contained in the organic compound is stripped out of the organic phase by mixing it together with spent electrolyte from the EW step of the combined process.

After separation of the organic and aqueous phases, the zinc-loaded spent electrolyte (also called purified solution) is sent to the EW stage. Comparison of the outputs from the two routes is shown in Table 2.

The main saleable by-products removed either through the washing or the SX processes are shown in Table 3. Uses of these products are as follows:` The Pb-Ag concentrates are marketable to Pb

smelters` The hydroxides produced at the effluent treatment,

can be recycled into the MHF or to the cement

purification The purification stage aims to clean the solution from the leaching (also called neutral solution) and to eliminate remaining elements which could be detrimental to efficient operation of the EW plant, such as copper, cadmium, nickel and cobalt.

This efficient washing makes it possible to implement well-known, robust and cheap leaching and purification processes. Nevertheless, solvent extraction (SX) can be considered as an alternative solution under certain circumstances. In such cases, the process flow sheet would be as shown in Figure 5.

SX involves smoothly mixing together the pregnant leach solution (PLS) and an organic phase made of a cationic extractant di(2-ethylhexyl) phosphoric acid (D2EHPA) diluted in kerosene. The organic phase selectively absorbs the zinc out of the PLS, then the

Element Zn cu cd co ni fe mg mn cl f g/l mg/l mg/l mg/l mg/l mg/l g/l g/l mg/l mg/lWashing route 160 <0.2 <0.2 <0.2 <0.01 <5 6 3.5 <400 <10SX route 105-150 <0.2 <0.2 <0.1 <0.01 <5 0.025 2.5 <200 <5

pb/Ag hydroxides neutralisation gypsum Salty cu/cd concentrate residue effluent cement

Washing route Pb, Ag No Recycled in the No Cl, F, Mg Cu, Cd MHF(Fe, Zn) SX route Fe, Pb, Ag, F Ca, Mn, Mg Ca, F Cl Cu, Cd

r Fig 5 SX route schematic

r Table 2 Typical analysis of the purified solution produced by the two routes

r Table 3 Main by-products removed through the washing or the SX routes

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for production of CGG alloy as ingots (1 or 2 tonnes) for the galvanising industry or cast as 25kg ingots. Paul Wurth has developed a proprietary concept of EW based on new stripping machines, anode flattening machines and plant layout. Back-up experience via Canadian Electrolytic Zinc at Valleyfield, QC, Canada, for continuous melting and alloying has been obtained and a cooperation agreement has been established between the two companies.

fLExibiLiTY of ThE SoLuTionThe combination of a pyrometallurgical and hydro-metallurgical process gives a wide range of possible solutions which can be customised for taking care of the local conditions and of specific requirements of the concerned industries.

At the upstream feeding level, the reduction-smelting process through PRIMUS furnaces can treat residues such as blast furnace sludge, oily sludge, mill scales, dross from galvanising, or from the zinc industry such as leach residues from a classical zinc route such as jarosite, goethite or hematite.

The feeding of the hydrometallurgical step can also be adapted to specific situations. The zinc oxide treated at this stage can come not only from a PRIMUS process but from other pyrometallurgical processes. Waelz oxide can be mixed with PRIMUS oxide or can be treated on its own.

The pyro-hydro combination also provides a wide modularity which makes it possible to achieve full synergy with existing plants. The PRIMUS pyrometallurgical stage can be implemented alone at a steelmaking site for specifically tackling an environmental issue or for processing BOF or EAF feed sources. The same pyrometallurgical stage, completed with the hydrometallurgical one, can also make sense for complementing the feeding of an existing EW plant. The hydrometallurgical stage can also be implemented alone, close to an existing source of zinc oxide.

concLuSionSThe pyro-hydro combination offers both a standalone solution for removing a steelmaking process burden and a source of iron, zinc and other re-useable products, generating value from previously discarded materials. It offers a whole set of process stages which can be designed, assembled and implemented in full accordance with local market and industrial situations. MS

Patrick C Guillaume is Vice President – Sales & Project Management, Gaspard Devos is Senior Project Manager and Jean-Luc Roth is Senior Technology Manager, all at Paul Wurth SA, Luxembourg, GD Luxembourg.

conTAcT: patrick.guillaume@paulwurth.com

industry depending on the impurity level. It mostly contains Ca, Mn and Mg

` The neutralisation residue is recyclable into the MHF in order to recover the Zn and Fe

` Clean gypsum is obtained out of the gypsum precipitation stage. The clean gypsum is recyclable by the cement industry

` The salty solution does not contain metals. Therefore, and depending on the local conditions and environmental regulations, it could be pumped into the sea or even a river. If such disposal is not permitted, the salt can be crystallised by evaporation. The crystallised salt contains Na and K chlorides and sulphates which are recyclable in the aluminium industry. In some cases, lime producers are interested in this salt in order to limit the hydrophilic characteristic of the lime and to help its mechanical cohesion.

` Cu/Cd cement produced at the zinc dust cementation plant/step is marketable through metals traders.

Reagents and utility consumptions for the two process variants are given in Table 4. For the leaching and SX, Paul Wurth collaborates with another partner, having developed an internationally recognised expertise in this domain. The overall processes, with washing or with SX, have been extensively piloted, achieving excellent results in terms of total zinc recovery, quality of by-products and operation parameters.Electrowinning The zinc contained in the purified zinc solution generated by the purification stage is extracted by an electrolysis process (EW). This process consists of depositing the zinc on aluminium cathodes under strictly defined operational conditions. SHG zinc is then stripped by automated machines, melted and continuously alloyed

reagent/utility washing route Sx routeElectricity for process, 3,140 3,140 kWh/t zinc Electricity for utilities, 440 500-1,020 kWh/t zinc Steam, kg/t zinc 2.5 45Natural gas, m3/t zinc – 35-74Water (all qualities), 7.5 15.7-19.3 m3/t zincH2 S04 (all qualities), 240 360-430 kg/t zincHCl, kg/t zinc – 25Limestone, kg/t zinc – 250-285Lime, kg/t zinc – 21-98Zinc powder, kg/t zinc 7-12 7-12Kerosene, kg/t zinc – 3D2EHPA kg/t zinc – 1

r Table 4 Reagents and utility consumptions

RAW MATERIALS AND IRONMAKING