global slag july 2006

8/12/2019 Global Slag July 2006

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2+2/3+3-TechnologyLoesche vertical roller mills are char-acterised by

-a flat grinding table with a verti-

cal axis;- 2, 3, 4 or 6 grinding rollers of

conical shape equally spaced on thetable;

- individual fixed rollers;- modular design;- hydro pneumatic spring system;- air swept operation with integrat-

ed drying and classification process.As clinker and blastfurnace slag

are ground to higher fineness than

cement raw material a higher specificgrinding pressure is necessary whichis achieved by controlled formation ofthe grinding bed.

Figure 1 shows a view inside anLM 56.3+3 installed in India mill withthe principle of roller arrangement forgrinding of cement and slag. The so-called 2+2 or 3+3 technology is char-acterised by pairwise arranged rollerswhich form a unit. Each pair of rollersis composed of a small support (S)-roller and a large master (M)-roller.The S-roller de-aerates, compactsand prepares the feed material for theM-roller, where underneath the com-

minution takes place1. For over 10years this principle has been appliedsuccessfully.

ProcessLoesche slag and cement mills offera very simple and compact layout.All the unit operations with drying,grinding and separation occur in themill itself and therefore the number ofauxiliaries are limited to a minimum.

The feed is transported to the millthrough a rotary star feeder to avoidfalse air in the mill which otherwisewould load the fan and cause powerloss. The wear parts in the star feeder

SLAGGRINDINGWITHLOESCHEVERTICALROLLERMILLS

Vertical roller mills by Loesche for grinding slag and cement have been in operation formore than 12 years. More than 90 mills with the 2+2/3+3 - technology have been soldworldwide by June 2006. Here Caroline Hacklnder-Woywadt shows the high energy effi-ciency and availabilty of vertical roller mills.

July 2006 25 GLOBALSLAG

lobal lobalGSLAGS L A G G R I N D I N G

BYDR. CAROLINEHACKLNDER-WOYWADT

LOESCHEGMBH

Figure 1: View inside an LM 56.3+3, M-roller and S-roller


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can easily be changed. For dry and powdery materials likefly ash an individual feeding system will be installed. Thecomponents are dried, finish-ground and are finally pass-ing through the classifier which is mounted on top of the

mill. All mills are equipped with a reject transport system.Material falling down to the gas ducts through the nozzlering after leaving the grinding table rim is withdrawn fromthe mill by scrapers.

In the case of slag grinding the table has verticalholes for the recharge of the iron particles from the tablebecause the main wear is caused by the pig iron. Figure2 shows the principle ofthese recharge holes.

In the case of slag,the reject mainly con-sists of iron particleswhich are present inthe agglomerates of thegranulated slag andseparated during thegrinding process. Thoseiron particles present inthe reject are removedby a magnetic separatorto prevent its accumula-tion on the table.

The grinding sys-tem is less sensitive tochanges of feed materi-

als with a wide rangeof properties and par-

tial high moisture contents. The hydro-pneumaticspring system makes it easy to adjust the mill whenthe type of product is changed. Moisture contents of15% or even more are acceptable with the excellentdrying capacity. Changing from one type of cementto another or GGBS can be done within a very short

period. The amount of intermediate products is verylittle.The heat required for the drying is supplied by a

hot gas generator (Loesche LOMA furnace) if otherprocess gases are not available. The use of differ-ent fuels is possible. If the grinding unit is near ablastfurnace, low calorific blastfurnace gas can alsobe used. In cement plants cooler gases or preheatergases can also be used. The design of the hot gasgenerator for slag grinding is made for a moisture of12 to 15%. Most of the grinding heat remains in thesystem, the hot gas is recirculated after the producthas been collected in the filter.

Wear ratesAs granulated blastfurnace slag has very abrasiveproperties the grinding parts (M-rollers and tableliners) have to be hardfaced. The wear occurs mainlyat the outer diameter of the M-roller. The S-rollersusually do not wear. Extent of wear is influencedby the quality and type of grinding elements, thefineness of products and the moisture content of thefeed materials. Hardfaced grinding elements with

high carbide content have a lower wear rate than thatof cast metals with a high chromium content and thusextend the service lives of the M-rollers and table liners.

Table 12

compares the set up and hardfacing times of twomills. The specific wear rate for GGBS is below 5g/t in thecase of produced Blaine figures from 3500 to 5400cm/gaccording to EN 196-6. The specific wear rate based onlong-term-experience is between 4 to 7g/t of product. Thenumber of welding heads influences the required weldingtime. The hardfacing in nearly all cases is carried out

GLOBALSLAGMAGAZINE SLAGGRINDING

Figure 2: Sectional view of grinding table / discharge hole.

LM 35.2+2 LM 46.2+2 LM 46.2+2

Product GGBS GGBS GGBS, CEM I, CEM II/ -V

Spec. surface area (Blaine) acc.

to EN 196-6

3500cm/g 5400cm/g 3000 - 4000cm/g

Throughput, dry 60t/h 75t/h avg.119t/h

Set-up time (preparation forhardfacing)

15h 12h 40h

Time for hardfacing 36h 36h 60h

Weld

Master-Roller 1 231kg

Master-Roller 2 231kg

Grinding table 270kg

total 480kg 732kg 517kg

Net wear rate, dry 4.97g/t 4.35g/t 3.16g/t

Table 1: Wear rates.


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inside the mill (in-situ hardfacing, Figure 3). If carried out

regularly the tyres can be hardfaced many times. There aretyres in operation which have been hardfaced more than20 times and have ground more than 2Mt of GGBS.2

Specific surface area acc. to BlaineDIFFERENCES BETWEEN EN AND ASTM For comparisonand discussion of properties of ground slag the specificsurface area acc. to Blaine is amongst others an importantfigure. The standards EN 196-6 and ASTM C 204 for thedetermination of the fineness are the same regarding theuse of the Blaine air-permeability apparatus, calibration,

sample preparation, andpermeability test. The differencesare in the calculation of specificsurface area.

The fineness of hydrauliccement is determined with the

Blaine air-permeability apparatusthat measures the surface areain square centimetres per gram,

or square metres per kilogram. With this method rela-tive rather than absolute fineness values are obtained.For determination of the fineness of other materialse.g. ground granulated blastfurnace slag, the methodof resp. calculation has to be suited in case ASTM isapplied.3

For calculation the following are taken into consid-eration:

- the surface area of a standard cement (SS) used forcalibration (in general 114p or 114q)

-the measured time interval for standard sample (TS)

and test sample (T),-viscosity of air during test for standard (S) and test

sample (),- density of standard (S) and test sample (),- porosity of prepared bed of standard sample (eS) and

test sample (e) as well as- constant for the standard sample (bS) and a constant

specifically appropriate for the test sample (b).Table 2 shows the comparison for the values of the con-

stants acc. to EN and ASTM. According to EN 196-6 the


ASTM C 204 EN 196-6

Standard sample(114p, 114q)

bS= 0.9 bS= 1.0

Test sample

b = 0.9for hydraulic cement;

b has to be determined for materi-

als other than Portland cement

b = 1.0for tested samples regardless

of material

Table 2: Constants for standard and test sample acc. to ENand ASTM.

EN [cm/g] ASTM [cm/g]

Sample 1 4080 4120

Sample 2 3830 3950

Table 3: Results for specific surface area for two Portlandcement samples.

Figure 3: View inside the mill during in-situ

hardfacing


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value for the constant b is equal for both standard sampleand tested samples regardless of materials.4

Equation 1 shows the calculation for the specificsurface area. For samples other than cement the constantb has to be specifically appropriate for the test sample. Inaccordance to ASTM C 204 the values of b are determinedon no less than three samples of the material in question.Each sample should be tested at a minimum of fourdifferent porosities over a certain porosity range. For thecorrelation coefficients is also given a limit.3

Table 3 shows a comparison for a Portlandcement sample (porosity e of test sampleunequal 0.5) measured in Loesche test centre.The specific surface area was calculated inaccordance to EN and ASTM. The discrepancyis modest.

For ground granulated blastfurnace slag thediscrepancy is higher. Table 4 shows results ofBlaine-tests carried out at the Loesche test cen-tre. The customer had confirmed these resultsfor ASTM that differ not more than 60cm/g.

Figure 4 shows the comparison of Blaine-figures calculated acc. to EN 196-6 and ASTMC 204 for blastfurnace slag samples of differ-ent finenesses with a constant b determined to0.9676. The Blaine figure of 6000cm/g calcu-

lated acc. to EN has an equivalent of appr. 5300cm/gcalculated acc. to ASTM. For the determined range of 3500to 6500cm/g a difference of 600 to 700cm/g between EN

and ASTM results is noted.

Operating experienceTable 5 shows operating results for grinding blastfurnaceslag in Loesche mills. These figures are different dependingon the different grindabilities of the slag which can varystrongly with the provenance and age of slag. Intergrindingof slag and Portland cement clinker is easily possible withthe properties of the mill for various feed materials buta lot of customers are grinding slag and OPC separately.With separate grinding a higher flexibility is given for mix-ing different products with targeted properties.

Final remarksThe Loesche vertical roller mill has been in operation formore than 12 years for grinding blastfurnace slag andcement. By June 2006 more than 90 mills with the 2+2/3+3 - technology have been sold worldwide. By showinghigh availability and remarkable energy efficiency the ver-tical roller mill for grinding slag is a worldwide success.

References[1] Brundiek, H.: Die Loesche-Mhle fr die Zerkleinerung vonZementklinker und Zumahlstoffen in der Praxis. ZKG 47 (1994)Nr. 4.[2] Hacklnder-Woywadt, C.: Granulated blastfurnace slag

grinding with Loeschemills. ZKG 58 (2005) Nr. 3,44 - 51.[3] ASTM C 204-05:Standard Test Methodsfor Fineness of HydraulicCement by Air-PermeabilityApparatus.[4] EN 196-6: Methodsof testing cement;Determination of fineness.

GS


Figure 4: Comparison of Blaine-figures for one GGBSsample acc. to EN 196-6 and ASTM C 204.

EN [cm/g] ASTM [cm/g]Sample A 4860 4240

Sample B 7010 6150

Table 4: Results for specific surface area for two GGBSsamples.

56.2+2 46.2+2 35.2+2 46.2+2 56.3+3

Output [t/h] 136 92 55 103 140

Blaine acc. EN 196 - 6[cm/g]

4550 3800 3520 4065 4500

Spec. energy consump-tion mill

21.8 -- 20.2 24.2 --

Spec. energy consump-tion total*

-- 30.2 -- -- 41.0

* (mill, mill fan, classifier, aux. drive)

Table 5: Operating results for different mills when grinding slag.

Equation 1.

global slag july 2006

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