Loesche Mills for

Metal Recovery and Filler Production from Steel Slag

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This dry process developed by Loesche for recovering metals from

steel slag combines all the advantages of conventional processes:

• Efficientandcost-effectivemetalrecoveryinthecoarserange

• Virtually 100% metal recovery down to the finest fractions

range by means of optimised dry grinding, classification and

sorting technology

• Production of a high-quality mineral filler product with pre-

ciselydefinablegrainsizedistribution

Theprocessisparticularlysuitablefor

• Slagfromtheproductionofstainlesssteels

• LDslag

• Metallurgicallymodifiedslag

Followingcoarseslagprocessing (recoveryofmetale.g.+20mm,

crushingofthemineralcomponentsto-20mm)bymeansofcrush-

ing,screeningandsortingtechnologiessuchashandpicking,sen-

sor sorting, magnetic separation and/or dry density separation, a

pre-crushedslagfractionisfedintotheLoeschemill.

Selectivegrindingandclassificationof themineral fraction to the

desiredfineness,andtreatmentandpre-concentrationofthemetal

contentareperformedsimultaneouslyinthemill.Therangeoffine-

nessofthemineralfractioncanbevaried,therebyallowingfineness

levelsof-2mmaswellas5,000Blaineormoretobeadjusted.The

millsoperatingprinciplepermitstheutilisationofboththedifferent

levelsofgrindabilityandthedifferentspecificdensitiesofthesub-

stancescontainedinthegrindingstockforseparation.

While a defined filler product exits the mill through the classifier

situatedat the topof themill in thedirectionof the filterandsilo

plant,ahighgrademetalfractionisdischargedviaachuteoutletin

downwarddirection.

Iftheflowofmetaldoesnotyetachievethedesiredpurityof+90%

metal, furtherconcentrationstagesbymeansofsensor,magnetic

and/ordensityseparationareinstalleddownstream.Inthatcasethe

non-metallicproductof thesortingprocess is recycledto themill.

Thisway, it is possible to recover virtually 100%of themetal as

metalproductand100%ofthemineralproductaspreciselydefined

andmetal-freefiller.

TheLoescheprocessthereforeoffersthepossibilityofcreatingtwo

marketableproductsinasingleunitandadditionallyhastheadvan-

tage of avoiding the necessity to treat the residual slurry that is

otherwisecustomaryinwetprocesses.

Atypicalprocessvariationforspecialsteelslagreductionisdepict-

edinthefollowingflowdiagram.

FeaturesoftheLoescheprocessforthecomplete and dry processing of steel slag

Loesche Mill type LM 15.200 under construction

Loesche Mill type LM 15.200, Wolica, Poland, 2009

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Themetalliferousslagtobecomminuted ①isintroducedcentrally

orfromthesideintothemill.

Onthegrindingtable ②thematerialmovesundereffectofcentrif-

ugal force towards theedgeof thegrinding tableand in thisway

passes under the hydropneumatically spring-loaded grinding

rollers ③.

Themineralfractionisselectivelygroundbythemechanicalstrain

duringthecomminutionprocessunderthegrindingrollers,whilethe

metalparticlesareonlysuperficiallyliberatedfromadherentmineral

particles and their particle shape is retained to the greatest possible

extent.Theheaviermetalparticles ④ concentrate on the grinding

table,whilethemineralfraction ⑤concentratesabove.

Thegroundparticlesaretranportedoverthedamringintothegas

flow.

Thelightmineralfractioniscarriedbythegasflow ⑥ to the classi-

fier ⑦. Themetal particles – on account of their higher specific

density–arenotinterceptedbythegasflowanddescendagainst

thegasflowintothedischargesystem ⑧.

Material of the mineral fraction to which metal still adheres is

rejectedbytheclassifierandreturnedviatheso-calledgritcone ⑨

tothegrindingprocess.

Small amountsof themineral fraction still naturally adhere to the

metal fractiondischarged in thedownwarddirection. Ifnecessary

these mineral particles can be separated in subsequent sorting

steps.

Withthisoperatingprincipleitispossibletorecovernearly100%of

themetals.

Operating principle of metal recovery in a Loesche mill

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Example of a process flow diagram for the complete and dry processing of slag with the Loesche vertical mill 1 Wheel loader 2 Conveyor belt 3 Jaw crusher 4 Impact crusher or cone crusher 5 Conveyor belt 6 Screen 7 Handpicking, sensor sorting, magnetic

separation, dry density separation 7a Collecting container, coarse metal 8 Conveyor belt with metal detector 8a Collecting container, medium metal 9 Intermediate silo 10 Proportioning conveyor 11 Rotary star gate feeder 12 Loesche mill 13 Filter 14 Volumetric flow measurement 15 Mill fan 16 Stack with flap 17 Recirculation gas line with flap 18 Hot-gas generator 19 Rotary star gate feeder 20 Conveyor belt 21 Bucket elevator 22 Product silo, filler 23 Filler transportation 24 Discharge system, metal fraction 25 Conveyor belt, metal fraction 26 Magnetic separation 27 Sensor sorting 28 Dry density separation 29 Conveyor belt 30 Metal fraction 31 Conveyor belt, returning mineral

fraction to grinding circuit

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Coarse grain processing CG Metalrecoveryonlyinthecoarsesizerange(~+20mm)(metalrecovery:50–70%) bymeansofsimplesortingprocesses Products: Coarsemetal,pre-crushedmineralmatrixwithresidualmetal

andhardlydefinedgrainsizedistribution

Wet grinding WG Completewetfinegrindingofthemineralmatrix,(metalrecovery:>95%) separationofthemetalbymeansofclassification,

density separation and/or magnetic separation Products: Metal,de-wateredslimeswithhardlydefinedgrainsizedistribution

Process with wet pre-concentration WPC Concentrationofthemetalinpre-concentrates(metalrecovery:65–85%) (e.g.byjigging,risingcurrentsorters);

wetgrindingofthepre-concentratesonly Products: Metal,washedandclassifiedmineralaggregates(e.g.0/2,2/8,8/16…),

proportionoffinelyground,de-wateredslimeswithhardlydefinedgrainsizedistribution

Process with dry pre-concentration DPC Concentrationofthemetalinpre-concentrates(metalrecovery:60–85%) (e.g.byairjigs,fluidisedbedseparators);

drygrindingofthepre-concentratesonly Products: Metal,classifiedmineralaggregates(e.g.0/2,2/8,8/16…),

proportionoffinelygrounddustasdefinedfillerproduct

Dry grinding Loesche DGL Completedrygrindingofthemineralmatrix,(metalrecovery:>95%) separationofthemetalbymeansofclassification,

density sorting and/or magnetic separation Products: Metal,variablefillerproduct(e.g.fillerquality,5,000Blaine…)

EconomicComparison

Precondition:High-valueapplicationforthefillerproductcomparedtolow-valueapplicationofthedewateredslimesproductfrom wetprocessing.ExtensiveapplicationanalyseshaveshownthatdrygrindingbytheLoeschemillproducesahigh-qualityandversatilemineralfiller.

conventionalwetprocessing

dry processing

Loesche process

CG WG WPC DPC DGL

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LM 24.2M

LM 21.2M

LM 19.2M

LM 17.2M

LM 15.2M

LM 12.2M

Productrate[t/h]asafunctionofLMsize

Finenessfine coarse

heavy lightGrindability

0 10 20 30 40 50 60 70 80 90 100

Product rate [t/h]

Model series and capacities

Gearboxes ready for shipment

Stainless steel slag with metal inclusions

Reflected light microscopy with typical phases of a stainless steel slag

Heavy product (metal) & light product after dry density separation (- 2 mm)

Loesche GmbHSales Ores & SlagsHansaallee 24340549 DüsseldorfTel. +49 - 211 - 53 53 - 777Fax +49 - 211 - 53 53 - 500E-mail: ores@loesche.dewww.loesche.com/ores

Typical element distribution in a stainless steel slag

Filler product

Raw slag