AMERICAN INSTITUTE O F MINING AND METALLURGICAL ENGINEERS

Technical Publication No. 1096 (CLASS B. MILLING AND CONCENTRATION, NO. 89; CLA88 H, INDU~TRIAL MINERALS DIVISION, NO. 80)

Closed-circuit Grinding of Cement Raw-Materials a t Leeds

BY T. B. COUNSELMAN,* MEMBER A.I.M.E.

(New YorkMeeting, February, 1939)

AFTER several years study, the Universal Atlas Ccmcnt Co. decided to rebuild its plant a t Leeds, Alabama. The entire old plant, which was to operate during the new construction, was then to be scrapped with the exception of two kilns. The quarry was to be mechanized, new and larger kilns installed, with the most modern type of equipment all through the plant.

Raw grinding operations a t all of thc Universal Atlas plants were dry. However, the dust hazard of this operation and the growing emphasis on occupational diseases made wet grinding attractive. Coal a t Leeds is cheap, so that a slightly higher coal practice was unimportant. Power was cheap, so that waste-heat boilers were not desirable. Since new kilns were to he installed, these could be made long enough to make filters unnecessary.

Wet raw grinding could be either the old practice of open-circuit compartrncnt mills or closed-circuit grinding in accordance with metal- lurgical practice. The latter saved power and steel wear and saved initial investment. This had been demonstrated in cement plants, Also, the elimination of tramp oversize was a most important advantage. Ease of chemical control was another factor. Blending in the thickener, with elimination of slurry mixers had been found practicable. Therefore, if wet raw grinding were used a t all, the advantages were clearly in favor of closed-circuit grinding. Detailed layouts of the most modern possible closed-circuit dry-grinding plant and of a closed-circuit wet-grinding plant were prepared, and careful estimates were made of initial invest- ment and operating costs. Full consideration of all the factors involved resulted in a decision to install wet closed-circuit grinding.

The capacity of the new kilns was established a t 4000 bbl. per day. Two new kilns were installed, each with a capacity of 80 bbl. per hour. Since 625 lb. of raw material is required to make a barrel of finished cement, the capacity of the grinding plant for the new kilns had to average only 1250 tons per day. Sufficient additional grinding capacity had to be provided for the two remaining old kilns, or their possible replacement by a third new kiln. The grinding rate, therefore, was set a t 90 tons per

Manuscript received a t the office of the Institute Dec. 10, 1938. * Manager, Industrial Division, The Dorr Company, Inc., Chicago, Ill.

Copyright, 1939, by the American Institute of Mining and Metallurgical Engineers, Inc. MININO TECHNOLOGY, July, 1939. Printed in U. S. A.

2 CLOSED-CIRCUIT GRINDING OF CEMENT RAW MI\TERI.ILS A T LEEDS

hol.lr, which \\rc,ulcl permit the ecluivnlellt of t\vo shifts pcbr d:ty, or actu:tlly 14 hr., to grincl the tonnage rer11.1irecl for n. 4000-11111. procl~.~ct,ion, wit11

\ I 11s. st~fficiellt rcmnini~rg time t,o grincl tlre requirements for t,hc t\vo olcl ]-'I This grincling equipment was planned t o be cliviclecl into t\vo pnmllel units, for flexibility.

Cement p l a ~ ~ t s n.re becoming more a,nd more chc~nicnl plnnts, and must be nhle t o l~roduce several different ki~icls of cement,, ns cnllecl for b y orders. Tlicse different cenlents require varying proportions of raw materials; nt Lcecls, for exnmple, limestone, shale :tnd s:tnc:lstone, \vitli sometimes the :~ildibion of a, fourth rn\v mnt,crinl, iron ore. Tlie stnrnge capacity of raw grounc:l sl1.1rr-y in a thicl<ener is roughly sufficient for n week t o tell days of bunling. Therefore, to provicle flesibility nnrl permit s\vitclling a t will from one kincl of cellrent t,o another \vitlrout loss of burnil-~g time:, i t is clesir:rl~le to hn.ve two thickeners.

Then, if the p1:tnt were producing stnndnrcl cement, urit11 a, t~hickcner full of r:tnr slulary, nncl n run on some sl~ecial cement. were clesireil, grincling of the nenr nlis into tlie empt,y tliickencr cor~lcl be starteel rnhil~-. the Ijurn- ing of slurry from the full thickener could bo cont,i~ruecl for a few clays until ready t,o cut the kilns over t o the special mis. Therefore, en.cl~ of the thickeners shoulcl be In.rge enol~gli to linnclle tllc nraximu~n grinding ra te of the ranr grincling clepnrtment.

A'cert:~in nmorult of stora.gc cjf finislred, thick slurry ready for burning shoulrl he nonil:tlIle a t the kiltls. Therefore two slurry misers were requirecl 11niler each kiln, as kiln-feccl tanks, cncll of s t o r i ~ ~ g :t

supply of slurry for about S hr., one tank t,o I,c filling ~vhile I:)~.~rni~ig Ivns clone from the otlier. I t is 11ot:tble tha t no corrcctio~i tanks nncl no nddi- tionn.1 storngc! of t l~ick slurry is necessary. . T h e thickener t.akes the 11ln.c~ of both correction t,nnks 2nd large slurry-storage Ixtsins.

T h e Leerls plnnt is some\vI~:tt unusual in tlr:~.t t,lic r:tnr mntcrinls come from sepn.mbe clun.rries, nre hnncllecl scp:tr:tt,ely into the l~lant,, n ,~~cl can be accura.tely proportioned froin the st:trt. Tbis, of course, gre:ttly f:tcili- t:ttes cliemicnl control, nncl helps t o n.ccount for the excellent cempnt t.hntt lins d\irays I~een proclucec-1 n.t Leeds. T h e shale is st,icky \irhcn \vet, :1,11d in the limestone clun.ny there :ire occnsionn.1 seams of .sticky clny. 111

llnilcl c l ~ ~ ~ r r y i ~ i g , these clay senllls were rejected, I ~ u t i t urns recog~~izeil that in mech:tnizecl quarrying they cou1t:l not be nvoiclecl. This liniitecl crushing nhentl of the 13a11 nlills to o ~ ~ e - i n c h mnximum size. Any n.t!tempt n.t finer crushing n-o111cl give trouble in the crushers nntl 11:tnlnler mills. Therefore large snmples of each of t,he various ingredients \\.ere cr~.~shecl to one-inch size, nnd scnt to the T\Testl)ort mill of The Dorr Compn~ty, Iuc. nt West,port, Connecticut,. Crtreful tests were m:tcle on the proper proportions of these r:i\ir materin.1~ t.o tletermine grincl:zbility, cl:lssification characteristics, :tnd thickener recluirements, ancl in accordnnce wit11 these tests the flowsheet shown in Fig. 1 11~ns clr:t~i~n up.

& U A ~ ~ R ~ - I N C ~ AND CRUSHING

I11 the li~nestone clu:lrry, stnllclnrcl clunrry met,hocls of stripping by scr:tpers a~icl drags, churn clrilling anil big-hole I~last~ing are usr:cl. A 2%-ycl. electric shovel 1ont:ls tlie stone into 12-ycl. cars of n Wooclforcl haulage system, the co~ltrol to\ver for whicli is a t tlie crushef l~l~ilcli~ig. The cars are dumped from tlie control tower, I:)y electric clumpi:r, into a 42411. gyratory crusher. Tllere is n small storage hopper 1,elow this. from nrhich the stone is fecl by :t pan conveyor to n 42 hy 66-ill. Iinmmcr ~ i ~ i l l , \\.liicli is set to crush to one inch. No screens arc ~~scc l . A dclst-

5 r O N E CRUSHER

HAMMER MIL -- -Toll€

SHALE-SAND -ORE

1 BELT SCAL

\ DORR, T H l C h E N E R 5

/'

FIG. 1.-FLOWSHEET, UKIVERSAL ATLAS CEJIENT COAIPABT AT 'IIEEDS, ALAUAAIA.

collecting system ol)crn.t,es on t,hc houseel-in hnln111er mill and pan conveyor, delivering to tlic belt conveyor, ~vliich t,nkes the linmmcr-mill l~rociuct :L few feet to the stone storage. There n 100-ft., 10-ton traveling

\vit,ll 3-yd. l~ucket,, distributes t,lie mnt.erial t,i) t,he proper l.)oil~t in the rock-storage yard.

Tbe sllnle and snndstone. are clu:trrietl separately; snnclstolle I)y linlld methocls, shale with :I s~ilall electxic shovel. A No. 5 gyratory and n 1iu.miner 111ill cr11sIi t .11~ l)roilc~c.t of either clunrry to n nomi11n.1 one-ilicli size. The crushccl m:~.t,p.,rinls nre hnulecl to the plant! b y rn,ilroncl or trucks, and dumpcrl onto a conveyor, \\4lich dclivers to t l ~ e ruck storage. The briclge crane ston,s t,lie two mnt,erin.ls in sel)sr:tt,e piles, ~1.t tlie c~pposit~e end of the yarcl from the limestone. A t one sicle of Ithe y:l.rcl, the bridge crane clumps the different nlaterinls into four cliffcrellt hoppers, below

4 CLOSED-CIRCUIT GILINDINC; O F CERIENT .RAW 7I1ArrERI:\I,S ;\'P TJEEDS

cncll of wliiclr is :I. 11c:.lt scn.1~. These lrol,pc:rs can IIC set to give any clesircd proportion of vnrici~~s ranr mntcrinls, clcli\rc~.ing to n. co~ivcyor, \vIiiuh cn,rrics t,lie mist,r~re t>c:~ t-lie r:t\v-grincling I j ~ . ~ i l ~ l i ~ ~ g .

Rn,tir Mill r 7 llierc is n 25-tdJll surge I ~ ) i l ~ :ilrc:ltl of c:l.cll grinclillg unit,, nncl tlie cotire

st,rcam of r n . ~ \ ~ mn.teri:tl is clivertecl from one to t,he c~ t~ l~er , :~ ,~~t ,ol i~:~t ical ly , a t 5-min. int,crv:tls. A bin i n i l i c n t , ~ ~ tells the c)pc?rn.t,or tl~c. cor~clitic~)n of c:tch bin, :%lid 11y c~it,t,itrg out, tlir n.utomntic clc.vice! Ile C:EII f i l l (:ither birr :ts clesirccl, or stop t,he fcrcl cnt,ircly.

Const:tnt-\\;eight fceclers \\;it,l~tl~~n.\v mnt,crial frcom t,hc!sc sul.go I:)ilis tc) t,lic 11rims.r~ I1n.11 nlills, t,lli~s kcc.pil~g t,l~c:m I I ~ t,o ninsinr~.iln grilldil~g c:rpacity. This was found to be very iml~ortnnt.

T\\ro-stag(, grindil~g ~ v a s necessn.1.y t,o rec:luue l-ill. mnt~:ri:~l t,o 200- n>c.sh. I n selecting the ljnll mills! i t \\.as felt, desirn.ble t,o lrn.\~c: tlrc prinln.ry :tncl secouclnry mills of ex:tctly the same size, SO t,lrn.t 2.11 p:l.rtjs \voltItl be interchn~ngesble. After study of the grinclnl~ilit,y test's, mills \\.ere specified to be 9-ft,. tli:t~netcr insicle tlie liticrs hy S ft . long from the liencl liner to the gricl pln.t,e. Tl~~a:l.t, :~l~cl clisclt:~rgc t,rul-~nic>~ls arc 22 in. nucl 24 in. clear rlin.mctcr. Thc f(!cil scuops n.re 4-ft. 0-ill. rn.clius. The cliscliargc grates of the primnry mills I I ~ , v I : slots 76 in. witlc il~sic.lc, tapering t.o 1 in. out,sicIc, n.nc.1 t,he plnt,r:s are 1.46 in. thick. O n t l ~ c sc.co~l~:lnry mills t,he tspcrecl slots :tre : 3 i in. \viclc: ilisidc: I J ~ ?.g in. \vic.lc o~~t,sicSe, s:l,nle p1a.t~ thickness. These slot,s estcncl ili\\;:~rcl for 13 i l l . fro111 the inside of thc liriers ti~\\rn.rcl thc axis of t.hc niill, t,llc c.c!ntrnl port,ioll 1:)cing blnnkccl off. All of the mnt,erin.l p:tssing t,l~rougli tllc: mill m1.1st t l i~refore 11:1,ss tlrrougl~ t,hese slots nen.r the periphery, so t,lrn.t t,l~esc! :j.rc I~~I\\:-level mills. 111 tjor~l:tl r:tdin.l 1ift)ers insicle t,hc discknrgc.: 111:inr~l mise t,he mill ~,rocluc;t, to t.l~c.: ilischarge truiil~ion, n~icl :ire so rtrrnngccl tl1:l.t t,here is 110 spilll,n.ck t lrror~gl~ the slots. Tlie mills linve \\-ntel~-cocole~:l, l,nljl~itt,ecl bettrir~gs.

The prim:try mills rot,n.te a t I!) r.p.m. n1r11 are fed 1ic:lt-trenteclsteel balls 4 in. in clin.ket,er, of about 500 Briuell. The scconcln,ry mills i.un n.t 17 r.p.111. :mcl 2-in. bnlls arc fed as make-up. nlIised charges were put in t,hc mills a t the stn.rt. R3otors n.rc:. 350 l ~ p . , ISO-r.11.m. synchronous, 2200-volt, cnpn1:)le of 100 pcr cent. ovcrloncl n.t 40" C. temperntlue rise.

Tlie clisclinrgcs from tlrc primary irlills flo\v hy gravity nt n slope of 13iG in. 1x:r foot to S-ft. hy 25-ft.. 6-in. hen\ry-iluty Dorr Rlultizorie cl:tssifiers \\fit11 20-lip. motcjrs. T1rr:sc mnc\rines:~~~~ill c l i~" t~ \ iemse l~~es out eveti when shut cln\vn tuic:lcr full 1o:i.d. ~ c t u n l l,o\ver consumption is s l ~ o u t 12 1111. Sntrcls nre co~rvcpecl t,c) t,hc prilnnry I,nll-niill scoop boscs by vibrating l:l.unders liavi~rg i?,$-hp. ~not~ors.

Fig. 2 shc~ws t,he nrrn.ngement of t , l~e pri~nnry nil s~cotrc:lnry grillclil~g circuit,^, ~ v i t h the R311ltizone clnsiifier in the primnry circuit n.ncl t,he bowl classifier in the sccondn.ry.

T. B . COUNSELMAN 5

. Tile o\~erflon;s of thc R/Iultizone clnssificrs go Ly gravity, ill :I. In1.1nclcr l ~ n v i ~ r g n slope of 2 in. pi:r foot, tu tlre 1ie:t.vy-clut,y Dorr ljo\\,l classificrs, \vliich are I6 ft,. ~viclc Ijy :3!1 f t . long by 24 ft,. in tli:tmctcr, :~ncl ally finisliecl matcrial overllo\rs \vit,hout. getting into t,he secunclnry I~nll mills. Tlic rs.kcs of these ~ilncliilics are driven 11y 20-Ill)., c;onst,,znt-speec~ motors, :~ctunlly tnk i l~g 13 lip. \vit,li ~lorlnnl loa1.1. In most ca.ses of :I sutlcle~i ~llutclow~l 1111cli.r loncl, the rnkes hnvc heen able to ilig themselves out \vithout draining thc: rnnctii~ie. 'I'lie bo\vl is clriven by ~t 10-hp. motor with :t vnri:tble-spci:c.1 reiluccr t o 1)lnvicle ndjustmcnt for helping c,ont,rol the fincncfss. Actun.1 po\iler consumpt,ion is about 615 I~orsepo\\.er.

- - - r 7 .I. lie rnkc-: procl~~cts c)fltl-lese: cln.ssificrs are ele\ratccl by 12-ft,. clinmeter

s,zncl ~ \~ l i r r l s , rc-)t,n.t,irtg a t C3?4 r.p.m., clrivc~l1 by 10-111). motors, nntl flow clto\vn n alol)e of 116 ill. per foc~t to tlic scoop boses of thc secoaclary bnll mills clcsc.riljc~il nl~ovc?. The 11n.ll-mill clischn.rges flo\v to t , l~e 1:)oivl clnssi- ficrs, t l ~ c Ia1111dcjr SIOIICS 1:)rilig 1?6 ill. pc3r foot.

r 7 .I~lia o\rc.rfio\vs of tllc two I ~ o u ~ l clnasificrs flo~v ljy grn.vit,y to a. sump, :1.11tl from tlierc. are puml:)ed l)y one of t\vo rubljer-linccl i)iunps, ~vit,li 75-111). rnc~t~c)l.s n.t 25C)O gnl. per mill., t l~rcj r~gl~ :t 12411. litre to the distri1~11- t,ion l:)os tjet,\vcen the tivc:) Torcl tl~ickcners.

l'licsc? tliicke~iers, 200-ft,. cli:tiilctlcr 11y 10-ft. side-\\,fill tleptli n.nc1 20-ft,. cent,er dept.11, hn\rc n 1~ut~toti-l s1ol)e of 2 i n . per foot for n clinmet>er of 80 ft.,

:tncl n slope of 1 in. per foot fro111 there to the periphery. There :ire t \ \ ~ '

lollg a.rms of Tc)rcl construct,ion, \vhich raise autolnat,ichlly ill case of : ~ n overload. Tliey Iin.ve n o 1)laeIes within tlie SO-ft,. clin~netcr of thc st,ec.pcr 1:)ottom. There n.re :tlso t,\\lo short arms wit.11 I:)lncles to rake material \vithin this 80-ft. clianleter. These arms are .liingecl some\\~hat like Torcl nnus, bu t are lockecl into ~ o s i t ~ i o n so tha t they do not raise :i.iitomntic:tlly,

. - although they c:tn 11; renilily rnisccl in case of necessity. 011 the,lollg anns are mounteil rul~ljer-lined recirculrttion pl.tmlx \\.it11

10-111); motors, ~vhicll rill1 a t 100 g:d. per hi l l . :tncl pick up slurry a t . a n y clc.sirec1 level, vn.rying frc~m the clisc.ll:trge cone n.t tlie bot,tc~m, \vI~(:rr! t l ~ e s l t u ~ ~ is thickest, up t o t,lie surf:tce nt tho feed \vcll, atl~:l pump i t ou t two- tllirds 01. no re of the distance to the l~eripllery. TI](: tlischr~rge lines 2re SO :~rrnngecl th3.t the slurry can Ile clisclinrgeil n.t the I-)ottom of t,lie tlricke~ier or nt :i.ny tlesiretl ele\in.tiol~. The nljji:ctive, of course, is to c.lischnrgc? slurry, ut' \\:hnt,ever consist,ellcy, clrn\\:n into t,lle ~ .e~ ' i r~ i~ la t , i t lg pl-lrnp, i l~t ,o n z(:~ti(! of prsct,icnlly the s:tmo cql~sistency. I n this way, thicli s111rry is not r~'~1is~)erseiI alicl cloes llot have t,o bc retliickeliecl: Duc: account must be taken of t , l~e ililution of the slurry by the gleiicl \v;ttcr of the pumps, otllcr\vise tlie wntcr cc~)nt,ent of tllc thickener unclerflow will Ije incrcnsecl.

Crlnllcl water For the rc:c:ircrtl:tti~~g pumps is oljtninecl by nusilinry 1-in. \\r:tt,cr punll.)s. 'I'll(: s~.tctiot~ l i ~ ~ c s of' tlllc.:.3e p ~ m l ) s run out encli lollg a rm :1.11d terliiiti:tt,o, t l ~ r o u g l ~ :I. r ~ ~ l d ~ c r hose, in n fit,tillg f:~stetleil to n flo:tt,. r 7 .L l111s clear \v:2tj(:1. Iro111 ~ I O Y I : , to t l ~ t (:)\rerflo\\: rim is oljt:~itiet-l forglnticl \\:atel.

T h e capacity vf tlie recircul:i.ting ~ u m p s is c:clunl to the average \\:itli- clrn\\::tl requircrnc:it from t,lie thickclier. The purpose of this recircul:ttion is to give t,horo~.lgli I:)lellcling, as to clle~llicnl coml~osition nncl particle size! I J ~ tile sc~jliils i r ~ tlic t,llic.kener. Without suc;li rc:c:irc111:1.tio11 the coarser particlt!s (\\:11i1:11 re :~lso ~~s~t : t I ly l ~ i g l ~ e r ill Iitne : i t l i I sili(::t, :tncl lo\vt:r it1 :dumin:t, tllnn thc Average composit,iol~) teflcl t,cj sc.bt,le cluicltly t,o tile bottom 1ie:lr tlie ccntcr. When not g r i t~~ l ing in the: raw mill, t , l~is conrscrr fraction is withclrfl.\vn first, nncl sul)sequel~t \vitllclra\\~n.ls consist, of much finer msterinl. Tliis cn.rries more \\later, because i ~ f its fineness, :tncl n.lso c:t.rrics lower limc nncl higller n l ~ ~ m i n a . It is tlie inccased :imount of \r:tter in this finel. slurry tlint is scrio~.ts. R.ecirculnt,ion t,encla to miiiil11ize these vnrintions.

These tliiclieners rotate a t one re~rolution in 40 min. They are c:irrietl so full of slurry tha t tlie line of tlie top of the slurry is n t times only n foot or so belour tlie n7nt,er le\:el. Records slio\\: tha t e:tch thiclienc!r tllien contains about 33,000 l ~ b l . of slurry, or, rougllly, n \vec.k1s sup1:)ly for I~otli kilns. In spite of over 10,000 tons of soliils it1 en.ch of these mnaliincs, the heliocentric drive is so perfectly b:tl,znceel tha t the ammeter reacling slio~\rs aljout 2-1111. coasumption. T h e clrive units are 5-lip. vertical motors, clirect-collnectecl to speecl reducers, elriving througli :I clouble

train of spur ge:trs, all interllal ring gear to ~vhich tllc thickener mechanism is fnstenccl. This 1:trge ring gear rests on a continuous circle of l?d- i l~ . stecl I~alls, running in oil, alld forming in effect a very large Ixtll bcnring, nloimtcd on the center picr. The moutlting is such th:tt the motor floats :tncl tlelivers power eclually to each train of gears.

I n c:tsc of :tn overload, such ns starting 1111 :tfter a powcr interruption, the long Torcl arnls \\loulcl rise slightly ancl ec1u:tlize the 1 0 x 1 OII the nlotur. If the nlotor could not then pull out, the short :tnns could be raisecl maaually I)y n lifting device pi.ovicIec1 for the purpose, the arnls being hingecl in n some\\ll;tt simi1:tr Iilatlller to tllr Torcl arms. There is an overload device on each machine which, :lt n 20 pcr cent overload on t l ~ e motor, or if the mechanism stops rotating, sou~lcls a horn. If the load

FIG. 3.-I<ILN \\'ITH SLURRY hILXERd.

is not relieved (by starting tllc unclerflo\il pum11s and recircu1:tting bacli to the thickener, for esnmple) :tnd the overloacl on the lllotor increnscs to 30 per cent, the ~i lotor \\ill then kick out. The nl:trm horn nlust l ~ c r c ~ t l~efore the motor can be started.

Tlle slurry is \\rithclm\vn by gravity, through any of four 6-in. lincs from tlie clischnrgc cone, atid tllcnce througll an S-in. underflow line to :t feed sump for the slurry pumps. Flom tlirougll the ~ulclcrflom lines is controllecl by orifice plates in tlie line, and :tlso all electrically operntecl valve shuts off the flow from the thickener in case of a power intenuption. r 7 1 hcrc are tnro of thcse slurry pllmps, 4 in. in sizc, each driven by a 75-111). motor, one piunp being n spare. Piping is arr~zngecl so that they can deliver to the kiln-feecl tanks through an S-in. line, or recirculnte to the. feed-clistribution 130s bet\veen tlle thickeners.

Fig. 3 sho\vs one of the kilns, with tlie kiln-fcccl tanks. Tli~rc, nrcb four Dorr slurry misers, serving as kiln-feed tnllks, two i~llder each killl,

8 CLOSED-CIRCUIT C;RINDINC; OF CEMENT R h W RlATERIhLS AT IAEEDS

near bllc feecl end. These :%re 24-ft. clin. by 16 f t . clec-.~), tlrivcn I)y 7>5-llp. motors nt 17 r.l).rn., a.11cl c:irry sufficient slurry for S-111.. t)perat.ioll of one kiln. By t,lris :~rrn~lgernen t, a,ticl I~ccnr~se tlrcrs arc: tnw t~lrickcncrs, ~\ritll 01113' some minor piping ndclit,ions from the slurry lniscrs t o tlie feeclers, i t \vo~.~lcl be pe~f'cct~ly possiblc to h i ~ r n n clifferellt t,ype of cement in enclr kill1 simultnneously, if tlesirccl.

Slurry f rcj~ri tliese ki ln-feed tnlrks is pulnped Ijy slurry pumps to const:~nt-volume kill1 feeclcrs, a t about t\\rice the burnilrg rnt,c, n unifv1.m a ~ ~ i o u ~ - i t I~eing feil I:)y gravity to eacll of tIi6 10-ft. clin. by 300-ft. l o l ~ g kil~is, tlre escess slurry being retornccl to the kiln-feeil tank.

Thickener overfloiv is puml~ecl by eit,hcr t.)f t\vc~ 3000-gnl. pcr min. pumps, eqnipped \vith 7,5-111). motors, t l~rough :L 10-in. line to a constalit- level tank 10 ft. in cliametcr by 11 ft . cleep, loc:i.tecl 25 ft . above tlic main \ir:ttcr lilies in t,he rn\v mill. This tallk 1lns :in cmrrgelicy overflow to the sewer, nncl also u, m:tke-111> nr:\v \\::itcr linc, col~trollecl Ily n float valve, t,o keep tlrc tank nl\v:iys full. Tlrerc is n sepnrntc 10-in. lurtil~ water Irentler, colitrolletl by n single m:lster valve, for e~icli grincling unit, this \:alve beiag closecl \vIien t l ~ e unit is shut. ilo\\:n. 11-1 this way the spmy vnlvcs througliout the ullits nre al\\::tys Icft in n.pprosimat,e :idjustment.

The amount of 11ew \vs.ter fi\r:tilnl)le \\~onlrl I)e cntirely in:~ileclunte :tlone to opchrnte the I.n\v lliill \vit,l~oilt, ret,i~l.~i fro~rr tlic thickeners. I n st,nrt,ilrg UP, t l l ~ r e is 5000 g:~l. a.v:~ilnble in tlrc co~ist:~~it-level tank. Tlic system is 11s1.1nlly left full, so th:i.t as soon ns tlle llinster valves n.re openecl and tlie bowl-clnssificr overflow pumps :we started nla.t,er renclrcs tlre thickener, overflcjws, and is retuniecl by tlic 3000-gal. per min. crent,rif~.~gil iv:~ter ~ L I I ~ P S to tlie cc~llstant-lcvcl tn.nlr. Act,ually tlic nrnkc-111) wntcr need only be . sUfficient . t,o cumpensnt,e for the ~\::~tcr i n thc s l t~rry , ivl~ich is evnpor:l,terl in the kilns, for solar cvnporn.t,iori from tl~c! t,l~ic;kcrrcrs 2nd ally :~c~ciilentnl s11ills to tlic sc!\ver.

1'I:lnt operation urns stn.rtecl in . I I I ~ I ~ 19:3S n.1111 t,t~nc?cl up grncll.~nlly. Tlle r:i\v-grincling clepn.rtmol~t s t,nl.t,ecl off smoot,lrly, 0111 y rnil11)r ncljrlst,- ~llelrts I~eing recluirc:cl, nllcl \\:it,liill :t ~ C I I T (.la.y~ \v:is opcr:itil~g st,en.clily at, :ihout 85 per ccrlt o f rn.t.ec-l c:tpnc:it,y. At t , h ~ time t,liis is \vritt,c;.n (Nov1:m- I-)er 193S) t,lle feecl rate is set a t :~I,ou t 110 t,otls per Iiour, or 122 ~I(:I. (~(!llt, of ratccl c:ip:l.city, nntl tlic unit runs o1.1t of feocl I-~ccnsio~r:~lly, t,liis t o ~ ~ l l n g r

r 7 Ijei~lg tlle present top cnpn.city litnit, of tl~c: Feeclowcigl~ts. lypical r e s ~ ~ l t s \vlien grincling nt apprusimntc:ly this capncci t,y arc givc!11 in Tn.ljlc 1.

I n ndclit,ioll to more uniform grillcling, cjlle c ~ f the principal nclva.ntnges of closeil-ciri;rlit grilltling is saving in power. A t one plant having open-

circuit. wet grinding in 3, t\\~o-coml~:l.rt,~iie~~t mill, the po\vi!r recluirecl for t,he mills n l o l l e for rnw-grincling c ~ l o l l g l i rn:~.t,crinl t,c~ make n 1,n.rrcl of (:t i~ll i :~it , (t,hc relative grindd,ilit,y of t.he nlis I ~ c i ~ l g nlmoxt t.1~: snnlc :\s :i.t, Lee(]$) , a . ~ ~ l i , u n t e t l to about 7.5 k\v-111.. A t Leeds, u~hcn g r i ~ i c l i i i g in one un i t .54 tons per hour, ecluiv:tlent to 173 bbl., tllc p o \ v e r rrnilings \ \ . e r e

as s I ~ o \ v a in Table 2. The tot31 ran: grincling recluires :qjprc)simn.t,ely 4,1< k \ v - h r . per b:i.rrcl,

:it t h ~ : t i h o v e output rate, but this i l ~ c l u t l c s \::trious ccnvc.yors, piunps ancl hsndling equipment, which \vould be requirccl in t i n y event. 'l31e

+c. : I re ~*ecircul:ition pumps in t,hc thickener arc riot i l ~ c l u c l e c l , I~ecause t,l~c;:> p:lrt of the blencl i~~g system, anil re:l.lly :I. considcrnljlc saving in p o \ \ , e r

over the usual correction t n l i l i s a l i c l slurry I~asins. T l ~ e clnssifiers a ~ l c l t,hickeners :\.re incl1.1clec1 bcc:iuse t , l i c y arc propcrly

n p:trt of tlie clo~ecl-~il.cuit grinding systcm. T l l i s mcthoil of grincling,

TAB'LE 1.-Typical ~ c s l i l t s , Novrnrbcr 1'3, 1935 UNIT No. 1." 7.5-IN. WIDE SLWS IN PRIAIAKY MILL GRATES

n12,ry

Ol,e~ii~):: ~ 1 ~ " " ' O r - Row

Inch + I . . ' . . . . . . . . . . . . .

. . . . . . . . . . . . . . +$$ +?,?. . . . . . . . . . . . +?j . . . . . . . . . .

hlesli + 4 . . . . . . . . . . . . . . . . . . . . . . . + 5 + S . . . . . . . . . . . .

+ 14. . . . . . . . . . . . . + 3 S . . . . . . . . . . . + 4 S . . . . . . . . . . . + l o o . . . . . . . . . . . . . +200.. . . . . . . . . . . 4-325. . . . . . . . . . . . .

Pl'ecific surface.. . . . . . . .

Tons per hour . . . . . . . . . . H?O, per cent . . . . . . . . . .

SF2cond- a ' Mill

Bo,\.l ~ 1 ~ ~ ~ ~ -

S111sIs

I%o\vl 1 Pri- 1 secc~llt l- Ck;,"'- nlnry :w

Circu- Circu- Over- lating I: t t i~~g flow 1 Load 1 Luis1

-

2 . 6 5 .4

10 .7 3 x 0

45.7 48 .3 62 .5 74 . O S4.5 S0.0 02 .2 94 .0

a .4l~1,rosinintely 6000 Ib. less ball lo:ld t h a n No. 2.u11it.

10 .1 10 .2 - 4093

50 64.5

0 . 3

0 . 2 0 . 0

i . S S .9

18 .0 41 4 3 S9. 1 93,:i 95 .9

--

O.C

5

: 30.6 45.0 J . 71.3 76.6

-

100 32.6

Illcll + 1 . . . . . . . . . . . . . . . . +?< . . . . . . . . . . . . . . . +!.. . . . . . . . . . . . . . +?; . . . . . . . . . . . . . .

kleah + 4 . . . . . . . . . . . . . + 5 . . . . . . . . . . . . . + s . . . . . . . . . . . . . + 14 . . . . . . . . . . . . . + % , . . . . . . . . . . . . + 4s . . . . . . . . . . . . . +100.. . . . . . . . . . . . +?no.. . . . . . . . . . . . +3"5.. . . . . . . . . . . .

Specific sr~rf:tce.. . . . . . . .

TOILS Iber h u t ~ r . . . . . . . . . . H:O. per cent. . . . . . . . . .

2 .6 5 . 4

10 .7 23.0

45 .7 45.3 02 .5 i 4 . 0 54.5 S i l l 02 .3 94.0

--

50 2 :

(1 5 !Mi

loo lz 95 210

- 1 . 3

54 1 3 . 3 -3.5 I

- Ucst Avg. 100

5-1 50 .1

110 34 5

4 . 9 23.G 62.1 01 .5

-

0.1; 18.1; 3 2 . li 45.4 55 .9

- - Ucst. A\.& 203

-

3 . 7 i i . 5 U S ! 7 . 1

102 30 .5

1 . 0 8 . 0

37.4 69 .2

-

50 2l . G 55 .2 "0 I

I . l i 181; 37.4 5 2 . 1 61.0

IS; .,5.D

13.2 14 .5 3 . 9 7 . 2 ti1.2 i 5 . 0 S 1 . 6 . 85 .4

---

103 34.2

--

11;. 1 IS .2 32.A 58. 0

S(i.5 9 4 . 0 5lj.2 Yi.O

2 .1 15.1 4S.S 7

- -

157 23.4

54 SR.!!

-4~::. 203

I

--

0 .4 7 4

2Ci.l 64.U 9 .

---- - 4 ~ 6 . 84ti

-

13.0 22, 1

3792

214 llili 168 351 19s 340 202 342 210

Ilest

351

-- Ikst.

10 CLOSED-CIRCUIT GRINDING O F CEhlENT RAW RIATERIALS AT LEEDS

TDLE 1 .-(Cm~ti.uz~ed) SEDI~~ENTATION ANALYSES O F BOIVL O\'ERFLO~'S B Y WAGNER TURBIDI~IETER METHCJD

Assumi~lg Average Diameter 7.5-1nii;i.o11 Pnrt'icles to I J ~ 3.ij Microlis

Part.icle Size Unit No. I 1 Uni t No. 2

Test Data. Tolls per

Hr.

Belt scnlt. settings: Limestone.. . . . . Hl~aie. . . . . . . . . . . . . . . . . . . . . . . . . . ~:,l,datoll‘:. . . . . . . . . . . . . . . . . . . . .

Primary b:dl mills. . . . Secondary ball mill.-. Priiilary cluasi6ei-3.. . . . Bo\vl-classi6er rakes. . . Bowl-classifier bu\vls. . . Tliickener, rotatiot~. . . . I iw-hr . per barrel . . . . .

-

390 3 10 305

10 9 .5 3 .,S

1 . .5 fo r both I I I L I ~ ~ :i..5s I 4 . 0

f,llc:reforc, shows :I direct p o ~ \ ~ c r snvilrg uf :3.92 k\v-lir. per bn.rrcl.1 cd cement,, or., on n 4000-bIjl.'l~:tsis, n snv i r~g of 1..5,700 li\~-111.. per day.

Tllc power consumption of 3.5s kw-lir. pcr Ijnrrel, for rccluction from 1 in. to 200 ~ i ~ e s h , is eclui\7nlent to 11.45 k\v-hr. per ton. This c:tn be 1-)loken ilo\\~n to :t consumption of 5..55 li~v-11r. per tou, for redt~ct,ion from 1 in. to 14 mesli, an(:\ 5.90 k\\:-l11.. per ton for reclr~ct,ion from 14 t o 200

r , mcsh. 1 Ilia compnrcs very fnvorfi~ljly \\;it11 ~net:tll~~rgicn.l prncticc:..

Ball co~~sumpt.ii)n hns n o t yet. Ijeen cleterminecl \\:it,h sufficic-:nt fi.ccur:Lcy to be of vnlue.

7 7 ABLE 2.-Pn~oo per Bcwrcl

Pri~tl:xry 111il1.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Secollclory mi l l . .

P~ . i~~~i t ry c1:issifier.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13owl classifier: r:rkes. :

Bowl . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Thiclie~ler . . . . . . . . . . . . . . . . ., . . . . . . . . . . . . . . . . . . . . . . . . . . .

Tot:tl power

Tlie striking feature of tlie Lecils plant,, to the cclnent manufacturer, is the complete : thence of correction t:~nks or la.rge slurry-storage basins. Close cheniicnl colltrol on the Insis of CaCO, colltellt or "holcling point" is cnwiecl out right froni the s t w t of grinding. T h e bo~i.1-clnssificr overflow is samplecl automst.icslly a t the tl~ickener, a t 4-min. i l~tervals, nncl the accumula.ted snmple is nn:~lyzed every 2 hr. T h e variation from the holding point is cleterminecl, :incl the 1:)elt scales adjusted accortlingly. Tllese iniliviclunl snmplcs of l,o~\~l-classifier ovcrflo\\~ slio\v considerable cllcli~icnl \-:tri:ttion, Imt i f tlie salnples for n 24-hr. or 48-llr. periocl auera.ge a t or near thc hnlclilig point, t81ie recircula.tion in tlie t,liickener will blentl tllc material so t1ln.t eac:h kiln tank \\:ill he within the permissible limit of 0.5 pcr ccnt v:trinlivn fro111 t,he hcjlcling point.

FIG. ~ . - C H E A I I ~ I A L BLENDlKC; B Y R E C I R C U L A T I O N I N T H I C K E N E R .

Fig. 4 slio\\:s, for 3 t,ypicn.l l)el.iocl, the vnrintioli froni the liolclillg point of the feeil to t,li(:. t,llickerler, n.ncl of the s l~lr ry ~\,it.l~clra\vn from t,lle thick- ener (luring t,lic: sumc periocl. The smuothirrg out effect of the recirculn- tion is obvious.

Tlie iml~ortiince of rccircultition in t,lle thickener, in orcler to get uniform chemical l,lencling, cannot be minimizccl., Obviously, if there is no rec,ircl~lntiun, the coarser part,iclcs, wliicl~ are t,lle high-lime and tlie high-silica. particles, tcnd t o settle out near t l ~ e center, while t.he slinlc p:trticles, carrying tlie nlurninn, grind re:tclily t,o t,be fine micron sizes, :incl telicl to clistribute fn.rt,Iier out in tllc thiclienc~r. If grinding nrere con- tilu.~ous, n mliform misture in the t,liiokener unclerflow ~ v o ~ ~ l c l necessarily I : I ~ obtained. On :I five-clay-week grinding scheclule, (luring nungrincling periods, if there is no rccirc~lnt~ion, \vitlldr:iwal a t first t e ~ i d s to he lligli i l l

lime :tncl silica, coarse in particle sizc, and lo\\. in moisture. I t is lliglily clcsirable t o keep tlie slurry as uniform as possible. Blending by recircu- I:~tion in the tliickener has been foul~tl elltirely prnct,icnl from the cllenli- c:il st,nnclpoint.

Fig. 5 which is n c1i:igr:lmmntic cross section of tlie thickener, sho\vs t l ~ c principle of t,llis Bleuding. Essentia.lly, thickened or part,ially thick-

12 CLOSED-CIRCUIT GRINDING O F CEMENT RAW BIATERTALS A T L E E D S

clied slurry is \vitllclrn\vn from t,lie centcr of the t , l~ickel~er nllcl l~umpecl ol~t\varclly for soluc ~list~nllcc. Such rc!cirr:uln.tecl slurry sholllcl be cleliverccl in n zone of nl~c~l.it tllc satlie ~vn tc r cont,cnt ns the zone frcorn \\:hich i t is ~\: i thtlrawl~, to nvoicl imposing cxtra cluty on the thickclier.

Silicc there :ire no filt,ers or other slurry-cle\t~a.t,cri~ig ilc?\'i(l(:s i l l the fltr\\:sl~ect, cont,rol of t,lic slurry moist,urc assumes more. than oldinnry iml~(jrtal~cc?. Tho 10 I>y 300-ft,. killis nt Lceils httvc n ~lominnl ~ n p n . ~ i t , y of 2000 11111. per il:l.y, :~.ncl t,Ilis c::~.pncit,y 1in.s bocn nt,t,n.ilr~:il.

,LAUNDER TRUSS I

E L E V A T I O N

Thc killis nt Loccls are e:~cli equipl)ecl wit,h unit coal l~ulvt.rizc-.rs, so t,Ilnt \\:it11 tllese nl~cl t,lw clrnft n\~nilnblo :L clefinit,e 111:~sinluln 1lum11cr of British thermal units cnti bc introcluced i l l t i t,he kilns. Tllercfnrc i t 1)ecomes i~nport , :~nt not only to lieel:, thc slurry llloist,urc unifonn h u t also t,c~ Irccp i t as lonf n.s possible. An a,lmost C C ) I I R ~ : X I I ~ l11111111er cjf B.~,.I.I. is recluired t o b11n1 one cubi?, foot of slurry, about the sn.nlc n.moullt of Ile:it, being ncccssnry t o cvnporn.t,e \\:ntcr :is t,o clrivc off cnrl~ou c.lioxiclc, 2nd tho l~iglier the solicls cor~tcnt of n. cubic foot of r:~.\\~ slurry, the highcr \\:ill I)r. the clil~kcr output c~f t,llc ki111: T h e kill1 shoulcl propc:rIy bc the bot t , lc~~eck of t , l~c entire opern.t,ion.

As st,nt,ecl above, tlle moisturc content ine\rit:ibly incre:ises \\:it11 extremely fine slurry. Therefore ill tlie raw-grillding opcr:ttion s. t,\\~ofolcl o1)jei;tive l~ecomes recluisite. First, everything must be grollnil tc~ :i

limit,il~g pnrt,ir:lc size, which pesmit,s chemical com~~inat ion in the 1;iln. In vt'her ~ \~o rds , tranlp oversize must be elirninn.tetl. This is t,l~oroughly noc~o~~~plisherl by closed-circuit grinding. Second, overgrincling and thv ~xotluction of ultra, fines must he nvoiclocl, since these t,end t,o liolcl mc~)isture. This o\rergrint:lii~g may be men.surec1 I)y L'specific surface," or square. centimet,ers of swf:tce per gmm of tlry solids, ns ileterlniiled by the \Vagner Turl~itlimet,er, ~ \~h ich registers the i ~ i t , e r f e r e ~ ~ ~ e to t,raizsnlit,t,ecl light of n clilut,e st~spension of fine pnrticles under st:tndnrtl condit,ions. This is t.llr me:tsurement, t,hnt is applied to finishrtl cement, but so far as linc~\vu it llas I ~ C V C ' ~ I~efore been :l.pplied tro t,he closecl-circuit grinclii~g of raw nlat.erin,ls.

Tlle sizing :tnnlysis of Leeds slurry is given in Table 1. The specific surfnce of the slurry from unit No. 2 in that t1nl)le is 4100, a.11~1 t,he 11~~isttu.c~ is 35.5 per cent. It is appnrent that specific surface is n very convei~ient, single figure for expressing the relative fineness of :t sample of ground mnt,c.- rinl. I t mn,y npply in m:tny inst,:tnces other tha.11 in tlie cement in(-lustry.

As nlent,ionecl, in order to reduce the specific surfn.ce, overgri~iclilig in tile 11lills 11lust be minimized. The specific surface of the slurry va.ricxl from n1:)out 3750 to 4400. This seemed high, co~npa~recl ~vi th the 1600 t.0 2!)00 surface t,o ~vhich t.he clinker is usunlly ground, ljut there wa.s nc~) compnr:~t,ive esperience n,vailnble on ot,her mlv sll~rries. It is \\.ell known that in ortler to minimize the procluctio~l of fines in n mill, 611e ~ i r ~ u l n t i ~ ~ g lun.cl must be high, n11t1 also that t,he pulp in the rllill inust be as thick ns possiHe.

Originally t,nble feeders were used tpo feed t,he ~llills, but t,hese gn.ve :I. widely varying mte, alicl ~111~0 tlle sctkil~g of the water valvcs was prn.c.:t,icnlly const,a~~t: t,l~ere nTns a. ri)nseqneat \?nrint,ioa in the? dilut,io~i in the prinlnry mill. To o\rerc.ome this v:~~ria.tioii, co~~s t~n~ l t -~ve ig l~ t feeders were i~~st,a.lleil.

The reason for tlesiril~g n high prima.ry circulnting lonil is tlint the 11ltm fines :ire procluct?d largely from the sha.le. This rllaterinl goes most ensily to :t eli~ke, t,he limest.one xilcl s:~,nrlstone both being harder. After pnssi~~g throl~gh the ~llill int,o t.he pri11l:iry classifier the slime overflows, goes t,o the 1,otvl ol:issifier, tvherc: it ngniil prompt.ly overflon-S, n131~cr gett,iilg int,o t . 1 ~ scconclnry grii~diilg cil.c~~it.. Bfost. of the grinrling (and i~vergrincling) of the sh:tle! t,berefore, is cnrriecl ollt in t,lir prim:\.ry mill. If t,he circulnt,ing load is lo~v, the. holding time in t'llc lnill is ilic!rense~l, :incl n higher specific surface results. If t,lle circ111n.ting 1os.d is i~lcrensc'cl, becat~sc of sl~osteiling t,he lloldi~lg time in t,he mill, oIrergrilltl- ing is minimizecl. I

Tile various steps tnkell so far h:i\;e recluced bile specific surf:lc,e to nl,out 37.50, while holding the perceat,nge of minus 200-~nesh constn~nt. With t,he aclclitio~lnl st,eps conteml~lntecl, i t is espectecl t,lln.t n still further recluction will lje accomplislled. As a &eck against specific surface

:LI.II~S, :tnd (:very 2 ft,. of tlepth directly a t t,lle rake arms, a t the ce~rt~cr :~.licl c\lery 25 ft,. or.~t\\l:l.rtl to tlie periphery. The est,remely int,erest,i~~g r c s r ~ l t , ~ of th is st1rvr.y are sllo\\.~i in Fig. 6. Sn.mples were t,estecl for mc~ist,urc?, :tnd for resicluc ~ I I I S 200.

The first pc)int tli:tt is inln~ediately apparent is t11n.t. 2 ft . belo\v t,Iic: surface of the \vat,er the slurry is a1re:tdy 50 per cent, solicls. Thickening is very m.l~i~:l nt first), then proceeds much more slo\vly to\vn.rcl f ind clensity.

Anot,llei striking thing is the segregn.tion of co:trse particles ratlier liigll up in the t,liirkener, a t the center ancl for 25 ft,. o r ~ t frc~m t,lle center. As is t o Ije espected, not much plus 200 mn.terinl gcts out n10re th3.n lialf\\ray to t,he periphery. Thc recirculati~ig pumps \vcre not in opera t , io~~ nt the t,ime this sllrvcy \vns made.

As nrigina,lly instnllecl, trhe recircul:tting pump suction \\Ins clo\vn in tlie clisclin.rge cone. T h e clischnrge lines cleliverecl tlie slurry, 33 and 67 ft. o11t from the center, just in front of the blndes, tlie lines running out on each of the long anns . IVllile the blcntling afforclecl 11y the recirculating pumps \\Ins escellent from a cllemicnl point of view, i t t,elided to increase ancl nlake cyclically va.rin,ble the moisture ill t,he slurry.

From the thicke~rer survey sho\\~n in Fig. 6, i t is oljvious thnt the pump s ~ ~ c t i o n shoulcl not be in the discharge cons, b u t in t.lic zone of Iiigh residue, higher u p a t the center. Conseclucntly the surt,ion lines hn.ve been hingecl n,nd nlncle n,cljustable, so tliat the intn.kes c:tn be pl;~.cecl a t any elcsired elevation. At tlle same time the discharge lines have been matle n.djustnBle. Before this \vns clone, a.n :ittempt wna made t,o dis- charge material from nhout 11nlf1va.y up in the hliickclier, 3.t. the int,o tlie thick zone ;lt the hotton), two-thircls of tlle way out. As urns t r ~ be esl)cctecl, this \\Ins not successful, resulting in a high-moisture slurry.

It has, of course, been fou~lcl clesirnl~le t o \\lit,hclm\\r slurry from tlie thickener n.t :I, fairly uniform rnte, 2nd the pumping sclicdule has 1)een so ana.nged. If no slurry is recluirecl a t the kiln-feed t m k s n.t tliat time, the slurry m:ty I)e r ~ t u r n e t l t o the feecl la~uicler.

With graclually improvecl oper:ttion, the slurry moisture has been reduced to abor.~t 35 per cent. Pencling some c11:inges in speed, nncl other chnnges to lo\ver t,llis moistme still fr~rt~her, t,he second t,hickener, \\,liicl~ normally is idle escept \ \ ~ l ~ e n speci:tl cements are recluirecl, has been 11setl both for storage :tnd for a slight aclclit,ional moisture reduct,ion. The tunclerflom from t,llis second tllickencr, \vliich constitutes the kiln feed, has been con~is tent~ly lo\ver th:tn 32 per cent, or about n. 3 per cent recluc- tion of moistawe in t,lie seco~id tliickcner. No recirculation is prncticecl in this unit. I t is hopec.1, liowe\rer, t,hn.t \vhen tlie severnl changes in oper:tting at:ljustme~lts 11011~ being carried out in the first t l~ickener have been completeel, this low moisture will proire readily obt.ainable in t,hc> underflon~ of the first unit.