vertical roller mill state of the art raw grinding

8/12/2019 Vertical Roller Mill State of the Art Raw Grinding

1/4

State o the art raw grindingBy J. M. Brugan Bethlehem, USA (English text provided by the author)

Summary - The vertical roller mill ha s been th e common choice for a large percentage Complete German versionof large expansions. This is because it has been proven for low power consumption, ea seof operalion, large capacity range, and competitive costs over a wide range of duferent in ZKG 111992, pp. 9-13)raw n~aterial s. he use of the hydraulic roll crusher, although more energy efficient thana hall mill, does not ojfer a clear advantage over a vertical roller mill in terms ofpower,cost, and proven reliability. nor in terrns ofcapaci ty and moisture range. The applicatiolzof t h e lzydraulic roll crusher is morefe asible in the augmentat ion of existing ball mill sys -terns, o which there are ntany still operating. The circumstances for justification areinstall ation specific and are difJicult togene ral ize. The ball mill is the only choice for wettnilling sysletns. Otherwise, new dry ball mill sy stem s are very rare. The large popula-tion of existing ball mills can be upgraded with the high efliciency separator and I fRCoften at, a nzuch lower cost t han a new VRMsystem.

1. IntroductionThe methods and machines used for the preparation ofcement raw meal have unde rgone significant changes in thepast several years, especially when the changes are com-pared against the past several decades. This paper willdescribe the rece nt evolution of dry raw milling to the pre-sent. and sh ed some light on future trends.

Raw milling technology has been, and is being, driven bycapital costs, maintenance and power consumption. Theraw meal preparation area of a cement producing facilityconsumes 15-25 of the require d power consu mption of90-110 kWhlt of cem ent. Therefore, it is not s urprisin g thatraw milling processes have basically focused o n the lowestpower consumption possible, eve n if the capital expendi-ture is higher than alternatives w ith lower capital but higherspecific power require ments . The following discus sions willmake reference to the capitaYpower equation and to thedependability of the machinelprocess combination, withoutwhich the economics of ceme nt production can be quicklydegraded.

The ball mill (BM) an d vertical roller mill (VRM) processesrepres ent 99' , of all raw mill sys tem s. High efficiency

separators (HES) and hydrau lic roll crus hers*) (HRC) haveinvigorated these s ystems in t he pa st five years, especially inthe case of ball mill processes. Additionally, th e applicationof a hydraulic roll crus her makes it possible to eliminate theball mill and make it a stand -alone alternati ve to the verticalroller mill.

2 Vertical roller millThe vertical roller mill is clearly the d omi nan t choice for rawmilling as evidenced by a 90-95 mark et share for new mil-ling systems. The market is defined as North and LatinAmerica, Pacific Rim, So uth Asia, Middle Eas t, North Africaand Europe. The largest, recent market growth has occurredin the Pacific Rim countries, where approximately 40 mil-

lion TPA of new capacity ha s been installed or purch ased inthe last four years. All of these p roduce rs a re installi ng verti-cal roller mills for raw m eal production.

The vertical roller mill Fig. 1) has been the obvious and pre-ferred tool of choice for raw milling. Its depend ability, eco-nomy and versatility a re hardly m atch ed i n a wide variety ofapplications. The capacity ran ge of thes e mills has increasedto 600 TIH, making it possible to m atc h a single kiln produc-tion of 7500 TPD w ith only o ne roller mill for op tim um eco-nomy. In other cases, two roller mills are being us ed to su p-ply single kiln capaci ty of 10,000 TP D or 3 million TPA.

*)Also known as high-pressure grinding rolls or roller presses

The vertical roller mill was basically introduced for largecapacities > 100 TPH) in the early 1970's. The mill and pro-cess were initially c halleng ed in certain applications, such asabrasive feed m aterials , hard feed materials, large capacity,and high moi sture materials. Sinc e then, the vertical rollermill has prove n itself in all of these areas th at were beforeconsid ered or pred icted t o be borderline or marginal. What

was said to be impossible before has been proven true in thelight of a growing list of references. The advantages aremany:

The vertical roller mill offers simplicity by combining thefunctions of secondaryltertiary crushing, drying, grind-ing and classifying in one c ompact machine.Power con sumption is the lowest or is as low as any pre-sen t alternative process.

Capital expe ndit ure is typically lower than other proces-ses.

Abrasive raw m aterials do not exclude this application.

Availabilityldependability rnaintenance osts are com-parable to t he other alternatives.

Vertical roller mills can accept up t o 22 moisture in the

mill feed.Single machine capacities are available from 50 to 600TPH.

The previous discus sion lead s one to ask if there is any casewhere t he roller mill is unattra ctive. Quite frankly, there arevery few instances, if any, where the roller mill is not the

MATERIAL FLOPATH

GRINDING TABLE / \

FIGURE : Sectional diagram of a Fullcr-Loesche vertical rollermill with grinding rollers


2/4

BYP SS

MILL 1RECIRCUL TION

FIGURE 2: Process arrangement of a vertical roller mill with arotary kiln plant using 2 fans

most economical/low risk choice for raw milling particu-larly with reference to new expansion projects. Alternativesto the roller mill generally rest on very special cir-cumstances.

The use of high-efficiency classifiers to repla ce static o r con-ventional dynamic VRM classifiers has further improvedthe VRM performance. The coarse particle residues havebeen reduced for better raw meal burning characteristicsand higher VRM capacity at a lower mill power c onsump-tion. Additionally the use of mill-exter nal material recircu-lation has lowered the mill pressure loss which in turn low-ers the system fan power consumption. These and otherVRM improvements will be discussed in greater detail in afollowing paper.

There are two basic VRM process arra ngem ents as shown inFigs 2 and 3 They are common ly referred t o as 2-fan and 3-fan systems. The 2-fan system is lower in cost but the dustfilter must be operated un der 800-1000mm wg under pres-sure making air infiltration a significa nt factor. The 3-fansystem has a much lower du st filter pressure but is 7 - 1 0

more expensive to install. It is mo re su itable for high mois-ture feed materials where operational isolation from t he kilnsystem is more necessary or when the kiln waste gas volumeexceeds the VRM requirements and this excess gas must bebypassed around t he VRM.

The VRM has recently gained additiona l attraction becauseof its ability to easily acce pt all kiln exh aus t gas and serve asan SO scrubber due to the intimate gaslmaterial contact inthe mill and by passing the sam e kiln gas through the millproduct filter cake formed i n a baghouse when one is used asthe d ust filter.

In the past three years Fuller has supp lied over thirty rollermills to the cement industry. Table outlines a few of these

TABLE 1: Technical and technological data for selected vertical rol-ler mills supplied by the F uller Company during the last 5 years

Table Capacity Feed moisture Mill motorDiameter m) t h X kW

recent applicat ions. The capacity and size range including

the moi sture are se en to be inclusive of practically any con-ceivable situation.

Table 2 outlines the hardness and abrasive character ofVRMs in ope ratio n. The SiOz cont ent of most raw meals isbetween 12 4 . The mi x quartz conten t commonly rangesbetween 3 and 8 , or up to X of the necessary 14 X)SiOz nthe mix.

The mills are economically processing these mixes withcomparatively high quart z conten ts. It is important to notethat t he qu artz co ntent itself is not an indication of abrasivecharacter however the nat ural grain size is critical relativeto t he VRM produc t size. This is conveniently evaluated bythe m easu rem ents of the VRM residual quartz found on the

DUST COLLECTOR

COLLECTORFAN

ah

FIGURE 3: Process arrangemcnt of a vertical roller ]nil1 with arotary kiln plant using 3 fans


3/4

TABLE 2: Data for characterizing material abrasiveness

Table Hard- Abrasion Feed MillDiameter (m) ness*) Index Quartz Residual

( YO) Quartz70

2.7 1.6 10 9 16

5.0 0.6 19 6 10

* 1.0 is average hardness. Less th an 1.0 ndicates increasing hardness.

grinding table. Also of inter est is tha t some mix es with lowquartz content and small natural quartz size can

demonstrate an abrasive character a nd vice versa.

3. Ball millThe ball mill has .not been a c ommon choice in the recentpast, mostly because of higher system cos t, complexity andpower consumption. The ball mill was the most prevalentmachine before the adv ent and subsequ ent acceptance ofthe vertical roller mill. Nevertheless, there may be specialcircumstances where a ball mill system is preferred, nor canwe neglect the huge population of existing systems whichwill be discussed in anoth er paper.

The discussions in this paper center on dry milling. Occa-sionally, wet ball mills are employe d in new syst ems wherethe raw materials are excessively wet and/or sticky and it

becomes prudent to process a slurry in the raw milling andkiln process. Wet ball milling con sum es approxim ately 30

less powe r than dry milling a nd is therefore competitive ona power consumption basis, however, this is more thanoffset by the higher kiln fuel consumption necessary toevapo rate th e slurry m oisture conten t. Wet milling systemsare also sometim es empl oyed in countries that have low fuelcost or in developing countrie s where smaller capacity rangeand system simplicity balance th e higher operating cost fac-tors.

The ball mill system is depe ndable , but it normally cannotcompete on power consumption and capital investment.The power consumption is normally 1 5 - 2 5 higher than avertical roller mill system. The use of high efficiencysepara tors an d hydrau lic roll crushers has slightly reducedthe power gap to 10-20 ) higher than a VR system, butthey i ncrease t he ca pital gap, m aking it difficult to justify fornew systems.

The ball mill alternative can be employed for capacityexpansions where the use of hydraulic roll crushers andhigh- efficiency separat ors are used to augm ent existing ballmill capacity. These instan ces are depe ndent o n the installa-tion, and various specifics require careful comparison toother alternates, making generalizations very difficult.Upgrading existing ball mill facilities can sometimes bemore cost effective than a new mill facility, provided wastekiln gas is utilized for drying.

4. ydraulic roll crusherIt is possible to use a hydraulic roll crusher to produce rawmeal without a ball mill or vertical roller mill Fig. 4 . Theparticle size distributio n characteristics of HRC products isnot a limitati on in th e case of raw meal like it is for finishcem ent wh ere serious quality p roblems can occur. The recir-culation rates through the rolls must be sufficiently high6-7 times) for a given production. This limits the capacity

potential for this proc ess as the machine size becomes toolarge for ove r 2000 TPD kiln systems (ba re roll weight = 30tonn es each). Higher kiln ca pacities would necessitate dualHRC system s makin g this option less attractive.

Analysis indicates that although hydraulic roll crusherapplication for raw meal processing is feasible, it does not

show a clear adva ntage over the vertical roller mill in systemcost, power consumption or simplicity. Added to, at best,

UST OLLE TOR

N

FIGURE 4: Hydraulic roll crusher in close d circuit with a high effi-

ciency separator and a deagglomcrator impact mill)


4/4

equal cost/power cons umption, is the risk of the applicationitself because of the h ighe r risk of the mec hanical difficul-ties sometimes encou ntered. Raw materials present a higherrisk of roll surface damage th an clin ker du e to the fact tha tthe feed is more incompressible. Th erefore, more care need sto be taken in as suming an a ppropriate top size control of thenew feed. That is, unlike th e use of clinkerlcem ent millingwhere power cons umption savings are significant, there isno present advan tage to justify t he risk of using a hydraulicroll crusher over a proven application like the vertical rollmill.

In most instances, the application of an HRC will be only

attractive when it is com bined in an e xisting ball mill pro-cess, making the overall investmen t costs attractive com-pared to a totally new e quipm ent installation. This situationis normally found when the kiln capacity increase is anincremental percentage of th e original system rating. Thissituation needs to c onsi der the utilization of kiln waste gasand the multiplicity of equ ipme nt, that is, inability torecover heat in the kiln gases, and t he additional equipmen tto feed and recycle HRC p roduct comprom ises the feasibil-ity of using an HRC process. Th e age and m aintenance con-dition of the e xisting ball mill syste m(s) are also factors.

5 ComparisonA typical cosffpower consumption comparison betweenVRM, BM and HRC is ma de in Tables 3 an d for a 150 t/h sys-

tem.TABLE 3: Capital cost comparison in million US

Description VRM BM HRC

Second ary cru sher 0.00 0.50 0.30

Tertiary crusher

The material was assume d to contain 6 moisture and havea Bon d w ork in dex of 10.5 kWNt. Although it may not be allinclusive, t he comparisons in Table 3 do demonstrate ourexpe rienc e that t he VRM is the most c ost effective choice fornew systems. The power comparisons in Table 4 (for whichthe same assump tions apply as in Table 3 sho w the VRM tohave th e lowest specific power.

Retrofits or increm ental expa nsio ns draw in too many con-siderations t o generalize.

QU RRY

PRIM RYC R U S H E R

DRYING G S

Mill 3.20 2.00 1.90

ESPIGCT 0.93 0.70 0.75

Fans 0.22 0.18 0.20

FIGURE 5: Raw grinding alternatives

Misc. it em s 1.00 1.00 1.00

TranslInsur/ I ax 0.57 0.58 0.60

ErectionICivil wo rk s 6.00 7.24 6.83

ErectICornm 0.75 0.75 0.75

Spares 0.61 0.32 0.57

otal Est. Cost (million U S ) 14.95 16.02 16.28

TABLE 4: Comparison of the specific power consumption i n kWhlt

Description VRM BM HRC

Sec ond ary cru sher 0.00 0.60 0.30

Tertiary crusher

Mill 8.00 15.00 9.00

Classifier 0.70 0.70 0.70

Disagglomerator 0.00 0.00 1.50

B. Ele vat ors 0.20 0.80 1.50

Conveyors 0.00 0.60 0.90

Fig. 5 is a si mple flow ch art of the three basic dry millingchoices for raw meal pre paration. It is easy to visualize the

relative simplicity of th e VRM system compared to the BMand HRC. The VRM has fewer sepa rate machines because itcombines several functions in a more compact arrangement.It is imp ortan t to note the circulation factors of the BM x3)and the HRC (x6 ). These factors require conveyor systemsnot necessary for the VRM, and impose capacity limitsbecause of the p ractical size limits of the conveyors them-selves or the milling machine, and could require dual sys-tems to m atc h the higher kiln capacities necessary for scaleeconomy . The VRM has a m uch higher limitation in this crit-ical factor.

The simplicity of dryinglgas flow arrangements is alsoanot her positive factor for the VRM, where generally a singleinlout point is all tha t is nece ssary for a wide moisture range.The BM and HRC often r equir e elaborate divided gas flowsto achieve the necessary moisture reduction and dewpoint

elevation.The as pect of abras ive wear is obviously important. In gen-eral, VRM wear com pare s favorably for even abrasive rawmaterials, because abrasive materials are no less forgivingon other processes.

grarnlton productvertical roller mill 1-4grind ing balls 20-50hydr aulic cru shi ng rolls 0.08-0.12

The HRC rolls hav e a very low wear rate but the amount ofmaterial available for wear is also muc h less where 6000 to15,000 hou rs of service life is exp ected before the roll surfacerequires renewal. The VRM achieves roll lifetimes in thesam e range. Grinding balls cost 2-3 times less than equlva-ent we ight of VRM wear components, but th e ball wear rate

st total power(kWhIt)

is ten times a s high.

vertical roller mill state of the art raw grinding

Documents