International Lead Management Center

Training Workshop for Senegal Site Assessment for the ESM of ULAB Recycling

Environmental Aspects of Blast and pRotary Furnace Operations

Le Méridien President Hotel and Conference Center Pointe des Almadies

Dakar

12 – 13 July, 201012 13 July, 2010

Brian WilsonILMC

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Blast Furnace

ULAB are recycled in Lead Smelters and in Senegal there are likely to be twopyrometallurgical technologies employed, namely Blast and Rotary Furnaces.Whatever pyrometallurgical technology is used in the Lead Recycling process thereare potentially major sources of lead fume emissions. Lead Smelting involves thereduction of the lead oxides in the battery paste into metallic lead in the furnace.Each furnace has certain characteristics that need to be understood in order toEach furnace has certain characteristics that need to be understood in order tomaintain good environmental, health and safety practices.For many years the blast furnace was the major pyrometallurgical technology forlead recycling. Not only can it recycle ULAB material, also drosses and otherleaded wastes such as soil.A Blast furnace requires metallurgical coke, silica or sand and lime to produce LeadBullion and remove most of the impurities in the feed material. The majorp jadvantage of this technology is that it produces and inert and stable solid residue orslag that can be used for road building or ballast in building projects.Normally Blast Furnaces use Oxygen Enrichment to reach temperatures of over1000 Celcius, but oxygen is not used here in Senegal.The disadvantages of the large scale blast Furnace are that it produces largevolumes of gas that needs to pass through an afterburner to remove the carbondioxide and then the off gas must be filtered to remove lead fume and dust

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dioxide and then the off-gas must be filtered to remove lead fume and dust.The other disadvantage is that it is not suitable for processing caustic skims fromthe refining process.

Blast Furnace – Basic Reactions

The main chemical reaction producing the lead bullion is the reduction of the LeadOxide by carbon dioxide:Blast air blown into the furnace reacts with the carbon in the coke to producecarbon monoxide and heat.

C + O2 = COCO2 + C = 2CO

The carbon monoxide then reacts with the Lead Oxides to produce molten Leadand carbon dioxide.

PbO + CO = Pb + CO2Hot carbon dioxide, un-reacted carbon monoxide, and nitrogen from the air passup through the furnace as fresh feed material travels down into the reaction zone.As the material travels downward, the counter-current gases both preheat the feedcharge, decompose the limestone to calcium oxide and carbon dioxide, and beginto reduce the Lead Oxides and Sulfates in the solid state.

CaCO3 = CaO + CO2The calcium oxide formed by this decomposition reacts with various impurities inthe feedstock and the silica to form a Calcium Silicate slag.

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SiO2 + CaO = CaSiO3

Occupational Exposure Risks

Blast furnace tapping operations involve removing the slag and then tapping moltenlead from the furnace into moulds or pots. Some smelters tap metal directly into aholding kettle which keeps the metal molten for refining. The other smelters cast thefurnace metal into blocks and allow the blocks to solidify.Potential Sources of Exposure:

• Lead oxide dusts can become airborne during furnace charging, especiallyis the charge material is dry.

• Lead fumes may be emitted at the lead or slag tapping holes during removalof the tapping plug and when the lead or slag is being poured into themould or pot via the tapping launder.

• Slag or lead Spillage may emit lead fumes, if disturbed during operations.H i k b i i i d bHowever, exposure risks can be minimized by:

• Adopting strict operating conditions that minimize the generation of dusts• Providing local exhaust ventilation above the Tapping holes• Ensuring that any spillage is removed and returned to the processimmediately

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Population Exposure Risks

Potential Sources of Exposure:Potential Sources of Exposure:Unfortunately no pyrometallugical process can retain all the Lead in the Furnace and some willvapourize and become entrained in the Off-Gas that can potentially contaminate the localpopulation if released to the atmosphere.Incomplete reduction reactions will result in carbon monoxide in the Off-Gas.Carbon dioxide is produced by the reduction of Lead oxides to produce the Lead Bullion andfrom the decomposition of the lime, essential for slag formation.Sulfur dioxide is produced by the reduction of Lead sulfates present in the Battery paste and fromthe decomposition of any residual sulfuric acid in the paste.Dioxins and Furans are toxic gases that can be formed during furnace operations if Chlorine andorganics, such as plastics in the case of ULAB, are present in the feedstock and there isincomplete combustion.Population exposure risks can be minimized by:• Installing a long flue to condense the Lead vapours to dust and capturing the dust in aproperly maintained Baghouse Filter plant.

• Adjusting the Blast to the Furnace to ensure complete combustion.• Optimise coke and lime additions to minimize excess carbon dioxide production• Drain as much sulfuric acid from the feedstock as possible and add iron to the flux toremove sulpfur from the Lead Sulfate paste - PbSO4 +Fe0↔ Pb0 +FeSO4

• Eliminate any PVC in the Feedstock - PVC separators were phased out many years ago,but are present in some ULAB from Zimbabwe. So remove all separators. Certain smallbut are present in some ULAB from Zimbabwe. So remove all separators. Certain smallsecurity batteries have PVC cases and these must be removed from the feedstock anddiscarded. Dioxins and Furans cannot be formed in the absence of Chlorine. It is alsoimportant to minimize any organics in the feed material such as the polypropylene casematerial, although polypropylene cannot produce a furan or dioxin without Chlorine. Hightemperatures associated with Blast Furnace Technologies also prevent Furan and Dioxinformation.

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Flue and Baghouse Systems

Inevitably there will be dusts and toxic leaded vapour emissions from the furnaceand the particulates must be captured by the Baghouse Filtration Unit.However, the Furnace Off-Gas is usually over 1000 Celcius and the Filter bags canonly tolerate temperatures up to about 135 Celcius. So it is vital that the Off-Gas iscooled and that is why you will see a concertina of a Flue from the Furnace to theBaghouse in order to give the gases time to cool before reaching the Filter bags.Th L d d V i i f th f ill d i t f d d tThe Leaded Vapour emissions from the furnace will condense into fume and dustparticles throughout the length of the Flue and so drop out chambers are providedat intervals along the ducting to collect the fume and dust.To prevent any red hot particles from reaching the filter bags and burning holesthrough the polyester material, spark arresters have been installed at the entry tothe baghouse.The Filter bags are arranged in Zones and when one Zone is operational, the otherg g p ,Zone will be on a pulsed air cleaning cycle to remove the captured dusts from theinside of the Bags and deposit the dust at the base of the Baghouse to be removedthrough a sealed rotary valve.The Baghouse must be sealed and this is especially important at the top of the unitwhere there are a number of inspection doors. They are sealed to ensure that anycarbon monoxide and sulfur dioxide emissions do not leak to the atmosphere in thevicinity of the plant, but are carried to the stack for high level dispersion.

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vicinity of the plant, but are carried to the stack for high level dispersion.The top of the Baghouse must NOT be inspected during furnace operations.

Blast Furnace Slag

Blast Furnace slag produced using Lime Flux (CaCO3 ) can be inert and classifiedas non-hazardous provided it passes the EPA Toxicity Characteristic LeachingProcedure (TCLP)..Blast Furnace slag is also a very hard material and if granulated can be used inRoad Construction as a base material.Blast furnace slag, produced using CaCO3 as flux, has been investigated for its useas an admixture and/or aggregate in the production of concrete blocks. The resultsshowed that the oxide components of the slag were similar to those of ordinaryPortland cement (OPC). Philippine Recyclers dispose of their Blast Furnace Slagby selling it to the local Cement Works.The Pagrik Senegal methodology is different from the traditional process andBhupendra Singh will explain later how they operate the furnace here in Senegal.p g p y p g

Utilization of secondary lead slag as construction material Metta Penpolcharoen,

Department of Civil Engineering, Mahanakorn University of Technology, 51 Cheum-p g g, y gy,Sampan Road, Nong Chok, Bangkok 10530, Thailand

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Rotary Furnaces

In most of the world other than the USA, rotary furnaces have replaced the blastfurnaces as the primary smelting vessels for recycling of ULAB. Rotary furnaces arehighly versatile and can accept almost any type of lead-bearing feed material,including battery paste, grids, drosses, and baghouse dust.The rotary furnace consists of a horizontal cylindrical vessel, in which the charge isheated by a burner located at one end of the furnace The flue-gases leave theheated by a burner located at one end of the furnace. The flue gases leave theoven through the opposite side or the same end, depending on the design. Togenerate the required heat, fuel or natural gas is used combined with air or pureoxygen.During the smelting process the furnace is rotated slowly to allow the heat transferand distribution. The furnace atmosphere is controlled by the air (oxygen)/fuel ratio.Once the smelting process is complete, a tap hole is opened and the metal andl di h d i t t ibl U lik t diti l Bl t F th tslag are discharged into pots or crucibles. Unlike traditional Blast Furnaces thatoperate on a continuous basis, Rotary furnaces are a batch process, and hencecan be operated in stages to produce low-impurity bullion for refining to pure lead,or they can entirely reduce the charge to recover all metal values for production oflead-antimony alloys. Rotary furnaces typically use soda ash, or sodium carbonate(Na2CO3)and iron as fluxes that produce a fluid, low-melting slag.A rotary furnace can use any carbon source, like coal, coke, or ebonite as reducing

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agent, and they can use various types of fuels, such as used oil, coal, or naturalgas.

Occupational Exposure Risks

Rotary furnace tapping operations involve removing the slag and then tappingmolten lead from the furnace into moulds or pots. Some smelters tap metal directlyinto a holding kettle which keeps the metal molten for refining. The other smelterscast the furnace metal into blocks and allow the blocks to solidify. As we shall seeshortly Furnace design will affect the potential occupational exposure.Potential Sources of Exposure:

• Lead oxide dusts can become airborne during furnace charging, especiallyis the charge material is dry.

• The Rotary furnace is very energy efficient, but if the flue and the furnaceare not balanced correctly, lead fume will escape to the atmosphere.

• Lead fumes may be emitted at the lead or slag tapping holes during removalof the tapping plug and when the lead or slag is being poured into the mouldor pot via the tap hole.

• Slag or lead Spillage may emit lead fumes, if disturbed during operations.However, exposure risks can be minimized by:

• Adopting strict operating conditions that minimize the generation of dusts• Balance the exhaust flue and burner systems• Adopt the best design for controlled tapping and provide local exhaustventilation above the Tapping holesE i h ill i d d d h• Ensuring that any spillage is removed and returned to the processimmediately

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Rotary Hygiene Control

Whilst the Rotary furnace technology is suited to the smelters in the region some ofthe furnaces in use have the tap holes in the center of the furnace drum. Thisdesign does enable the furnace to be completely drained, but at the lead tappingtemperature, fugitive emissions are very difficult to control as the fume rises aroundthe whole furnace. Employees working in such conditions will be exposed to highlead in air levels and without respiratory protection could be poisoned Furthermorelead in air levels and without respiratory protection could be poisoned. Furthermorelead fumes that are vented to atmosphere will exposure any populations living orworking close to the smelter to lead contamination.The fumes can be contained if the furnace is completely enclosed or a large hood isused to completely encapsulate the furnace. However, such a large hood willrestrict the tapping operation and be very expensive to operate because theelectrical demand required to a power the extraction fans necessary to produce anair flow of at least 1m/s will be expensiveair flow of at least 1m/s will be expensive.

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Rotary Hygiene ControlThe solution is to reposition the tap holes,180 degrees apart to the front of thefurnace.This configuration will allow a ventilation hood located just over the front of thefurnace to efficiently capture any emissions during the tapping operations at afraction of the cost of a center tapping hood configuration.

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Rotary – Energy Efficient Improvement

A further development would to seal the back end of the furnace with refractorybricks, install an oxygen enriched burner and a new door with an integrated duct forthe exhaust gases which would be expelled directly into a ventilation hoodpositioned above the furnace door.This configuration not only improves hygiene control by eliminating emission fromthe rear of the furnace but also increases energy efficiency by forcing the heat tothe rear of the furnace, but also increases energy efficiency by forcing the heat topass through the furnace twice.

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Tilting Rotary FurnaceThe front tapping and hygiene hood configuration has since led to the introductionof the tilting rotary furnace.The tilting rotary furnace permits:

• Quicker charging• Faster tapping as the furnace can be tilted to accelerate the flow of furnace

materials• Reduced cycle times, not only because of quicker charging and tapping,

but also due to the fact that the burner can now be angled directly into thecharge material during the smelt

• The new burner configuration provides improved thermal efficiency• This in turn reduces noise levels as the burner does not need some much

compressed air• Maintenance levels and refractory wear and tear are reduced• And there is a further improvement in hygiene control as the drum can be

angled towards the ventilation.

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Rotary Furnace Slag

Soda-Iron is the most common slag system used in Rotary Furnace Operations tosmelt used lead acid battery paste. It's low melting point, ability to capture 95% ofsulphur and remove the vast majority of unwanted impurities will ensure itscontinued use.But this ease hides a very complex system with many hundreds of differentchemical reactions possible during smelting and others that take place duringchemical reactions possible during smelting and others that take place duringcooling and disposal including some that can render the slag combustible.The sodium salts present in the slag will tend to be soluble and will degrade onexposure to the atmosphere and leach toxins.The slag is therefore not inert, but toxic and classified as hazardous.The slag is also so alkaline that it is corrosive and can burn skin and eyes.

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Rotary Furnace Slag

An inert rotary slag, sometimes known as “Green Slag” has been developed byLead Metal Technologies of Monterrey, Mexico. When first tapped from thenfurnace the slag is green, hence the name “Green Slag”, but after conditioningduring the stabilization phase it turns to brown and then black.At first, the fluxing reagents seem to be the normal mix of Lead pastes, Soda Ash(sodium carbonate) Iron and a carbon source But they are added in carefully(sodium carbonate), Iron and a carbon source. But they are added in carefullycontrolled amounts and the smelting operation is monitored and controlled. Theslag produced under these controlled conditions is based on the sulfide iron matrixformed as the Lead Sulfates are reduced to Lead metal as shown in the followingabridged reactions;

PbSO4 + C→ PbS + CO2Then: PbS + Fe→ Pb + FeSIron sulfide also forms compounds with sodium sulfide, formed when the soda ashalso reacts with the Lead sulfates.

PbSO4 + Na2CO3 + Fe + C→ Pb + “FeS.Na2S ”+ CO/CO2This slag removes 99.5% of the sulfur and the trace impurities, but unlike thetraditional soda slag it is stable and will not leach and therefore it does not have atoxic effect enabling it to be classified as non-hazardous.

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Personal Protective Equipment

Finally, it is very important that employees are protected against any spillages,metal splashes, fugitive emissions and so the best dressed operators should bewearing hard hats, overalls, eye protection, dust masks, safety footwear and leathergloves and aprons.

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