By LINCOLN T. WORK

Assistant Professor of Chemical Engineering Columbia University, Nciv York, N. Y.

T

opm ents in th e reduction of particle size of m ate­

rials. In this field, as in m any others, w h at has been considered im possible has on occasion become the sim plest m ethod o f p rocedure. T h a t m ay be illu strated by the recent developm ent o f 'd ry in g an d g rin d in g in one operation. F o r a long tim e it w as considered necessary to feed to a p u lverizing m achine a th o ro u g h ly d ried solid. M an y plants co n tain in g g rin d in g equipm ent have had included larg e and expensive d ry e rs in o rd e r th a t th e solid to be fed to th e m ill shall be so d ry th a t th e re is no tendency fo r caking d u rin g g rin d in g .

W e t g rin d in g w as know n only as the g rin d in g o f a pulp, a fluid m ix tu re o f liquid an d solid. I t w as a com ­ m on statem en t am ong m en w ho op erated g rin d in g equip­

m en t th a t m aterial to be pulverized m u st be e ith e r very w et or very dry. T h is did n o t p rev en t the developm ent o f a com bination of d ry in g an d g rin d in g in th e sam e m achine. A s th is new m ethod is now operated, m oist or plastic m aterial m ay be fed to th e m ill; b u t due to the d ry in g conditions w hich prevail w ith in th e m ill th e m ate­

rial being g ro u n d is really quite dry. S tag es in th e d e ­ velopm ent o f this m ethod a re briefly review ed.

M an y y ears ago the m ining in d u stry ad o p ted closed circu it w et g rin d in g in o rd e r to avoid the fo rm a tio n of too m uch v ery fine m aterial and in o rd e r to effect econ­

om y in pow er. In this m ethod the w et m ix tu re flows th ro u g h the m ill, is partially reduced to th e req u ired size, a n d th en flows to a se p a ra tin g device. H e re m aterial finer th a n a certain size is w ashed aw ay as finished, and the oversize is re tu rn e d to the mill w ith th e feed. M ore recently the sam e principle o f closed circu it g rin d in g has been applied to th e pulverization o f d ry m aterials.

T h e m ills a re sw ept w ith a ir, and th e fines as form ed a re lifted out by the air stream . T h e coarse g rits rem ain in th e mill until reduced to a suitable size. It w as b u t a sh o rt step beyond th is to em ploy w arm air. T h is dried th e m aterial w ithin th e mill and elim inated th e p re ­ lim inary d ry in g o f m oist m aterials.

T h is paper outlines a few of th e achievem ents in this com bined u n it operation of g rin d in g and d ry in g , and p resen ts an analysis o f the fa cto rs involved.

T h is com bination o f tw o o p eratio n s in one m achine h as been applied to a larg e n u m b er o f problem s of

pul-Fig. 1—Flow sheet of a R aym ond kiln-m ill installation in a chem ical p lan t fo r dehydrating and reducing a filter-press p ro d u ct to a fine dry pow der in one

operation

v erization. T h ese include th e g rin d in g of m in erals re ­ ceived in a w et condition, th e d isin teg ratio n o f filter cakes d irectly fro m th e press, an d th e d eh y d ratio n an d d is­

in teg ratio n o f cry stallin e h y d rates.

T h e p u lv erizatio n of coal offers exam ples o f th is com ­ b in atio n in the m ineral field. B e fo re d ry in g and g rin d ­ in g w ere com bined into one op eratio n th e coal h ad to p ass th ro u g h a p relim in ary d ry e r in o rd e r to elim inate th e m oisture, and the capacity o f th is d ry e r h ad to accom m odate an y w a te r co n ten t w hich th e coal m ig h t have. Since th e coal as delivered m ay ra n g e in m o istu re co n ten t up to 18-20 p e r cent, w hich is d rip p in g w et, th is pro ced u re w as cum bersom e and costly. W ith th e d ry in g an d g rin d in g as one o p eratio n it has becom e possible to feed coal of any m o istu re co n ten t d irectly to th e mill w ith only lim ited red u ctio n in pow er consum ption and mill capacity. T h e w a rm air w hich sw eeps th ro u g h the m ill dries th e coal and carries it d irectly to th e b u rn e r o r to a se p a ra to r fro m w hich it is sto red . In the direct firing th e w arm a ir also re p resen ts p re -h e a tin g ; and offers th e fu rth e r ad v an tag e o f m inim um d a n g er from sp ontaneous com bustion and fro m explosion. T h e com ­ p actness an d flexibility w hich re su lt fro m d ry in g and g rin d in g in th e sam e m achine w ere especially im p o rta n t

306 C h em ica l & M eta llu rg ica l E n g in eerin g — V ol.40,N o.6

in the a d ap tatio n of pulverized coal to m arine boilers.

O th e r m in eral m aterials besides coal m ay be dried and ground in one op eratio n w ith highly sa tisfacto ry results.

T hese include cem ent raw m ix tu re, lim estone, bauxite, phosphate rock, b ary tes and talc. A special exam ple of gro u n d m in erals is th a t of gypsum . W h e n pulverized and floated in a ir m ain tain ed a t th e p ro p e r tem p eratu re, the pow der is d eh y d ra te d into p la ste r of paris. O th e r crystalline m aterials besides gypsum have been g ro u n d and a t the sam e tim e red u ced fro m one state o f h y dration to an other. B lue copper sulp h ate m ay be converted to a m o n o -h y d rate in a single operation, and the u n ifo rm blue-w hite p ro d u ct w hich re su lts is useful in the insecti­

cide field.

T h e application o f the com bination of d ry in g and g rin d in g has been ex ten d ed to the field of disintegration.

Cake fro m th e filter press is d ro p p ed directly in to a h o p p er fro m w hich it is fed to th e g rin d in g mill (F ig . 1 ).

I t is d ried an d d isin teg rated , an d is rem oved as a finished pow der fro m the se p arato r. O rg an ic m aterials w hich are susceptible to h eat have been handled in th e sam e m an ­ ner. F r u it pulp, co ntaining fro m 80-85 p e r cent m ois­

tu re , is co n v erted into a d ry pow der. M ilk album en, a f te r p rocessing to about 65 p er cent w ater, is reduced to less th a n 8 p e r cent w a te r by a g rin d in g o f th is type.

T h e usual types of g rin d in g m achines m ay be used in co n ju n ctio n w ith such a dual operation. T h e cylindrical ball mill, the conical ball mill, and m any types of ring ro ller o r ball ro ller m ills, im pact m ills and b eater mills, w hich are em ployed in d ry g rin d in g m ay be used. F ig. 2 illu strates a w ell-know n type o f roller mill used w ith an a ir sep aratin g an d d ry in g system . T h e choice o f mill is

Fig. 2—R o lle r lype m ill w ith a ir drying and separation

affected by the n a tu re of the m aterial to be reduced and not prim arily by its m oisture content. T h e m oisture is taken care of by p ro p er control of the tem p e ra tu re of the air w hich is circulated. A certain am o u n t of air circulates th ro u g h the mill and se p arato rs an d back again to the mill. A p ortion is led aw ay th ro u g h a dust collector, and the m ake-up o f new w arm a ir is b ro u g h t into the stream and tho ro u g h ly m ixed w ith it b efo re th e a ir enters the mill. In th is w ay th e accum ulated m ois­

tu re in the atm osphere is m aintained a t a sufficiently low value to insure drying. T h e th o ro u g h m ix in g of th e new w arm air w ith the circulating a ir assures ex act an d u n i­

form tem p eratu re. T h e hot gases m ay be h o t air from a h eater o r heat recovery system , o r they m ay be flue gases or o th er in erts w hen th ere is d an g er fro m o x id a­

tion.

F eed in g M oist Solids

T h e feeding o f m oist solids to g rin d in g m ills which are not sw ept by air causes caking. T h e m aterial is pressed together until it form s h a rd lum ps an d p u lv eriz­

ing ceases. I t is a sim ple m a tte r to apply a cru sh in g load to a coarse piece of m aterial w h eth er w et or dry. I f th a t load can be applied directly, th e piece will be ru p ­ tured. H ence the m oisture problem is o f little con­

cern in coarse g rin d in g m ills, and it is only w hen a p p re ­ ciable quantities o f m aterial less th an sieve size a re p ro ­ duced th a t th ere is m arked im pedance to mill action.

A s the new su rface is produced it becom es w et, probably by a blotting action, as it comes in contact w ith w et m aterial. T h e su rface m oisture causes the p articles to cluster into small balls o r agglom erates.

A t first these agglom erates are p lastic; th a t is, th e y can flow into o th er shapes u n d er load, b u t they offer definite resistance to th e forces of g rinding. T h e re fo re w hen the cru sh in g load is applied, th ey slow ly change shape to conform , and th e load seldom reaches th e co arse particles on w hich it should act. T h e mill faces a lso become w et by contact w ith th e m aterial being crushed and the clusters adhere to the m etal. W ith repeated ap ­ plication of p ressu re these agglom erates becom e com ­ pactly grouped in rigid cakes into w hich the mill action no longer penetrates. T h e w alls of a ball m ill become coated quite deeply; and the balls, having little charge- m ixed w ith them , m ove on the su rface of th is cake..

T h e re is little tendency fo r dry in g to take place.

I f the mill frictio n is low, th ere is b u t a sm all am ount o f heat g enerated and the atm osphere is soon sa tu ra te d . In the case o f extrem e frictio n if th e mill actually reached atm ospheric steam tem p eratu re, free w a te r w ould boil aw ay, b u t th e d ry in g of th e m aterial being- g ro u n d would be very slow. H o w ev er, it tak es still higher tem p eratu res th a n 212 deg. F . to elim inate water- com pletely from the m aterial, fo r sorbed m o istu re is held well beyond the equilibrium te m p eratu res fo r evaporating- free w ater. T h e actual am ounts of m oisture req u ired to im pede g rin d in g v ary w ith th e m aterial, its fineness, and the m anner of mill loading. F ig. 3 show s th a t mill capacity is reduced m ore th a n one-half fo r th e three- m aterials show n w hen th e ir m o istu re 'v a lu e s a re in the- ran g e of 10-15 p e r cent. H en ce fo r all practical con­

siderations it w as necessary to op erate th e o rd in ary g rin d in g mill w ith either v ery w et o r v ery d ry feed.

In m ills w here th e m aterial is d ried d u rin g th e process;

Ju n e, 1 9 3 3 — C h em ica l & M e ta llu rg ica l E n gin eerin g 30T

o f g rin d in g die effect o f m o istu re is g re a tly re d u c e d ; fo r alth o u g h th e feed is w et, th e m aterial w ith in the mill is essentially dry. M o ist solid e n terin g fro m th e feeder m ay behave fo r a sh o rt tim e as it w ould in a closed mill.

T h e d ry solid alread y in th e mill ad h eres to th e su rfa c e of the m oist m aterial and rem oves the w a te r by a b lotting action. A s the sticky feed becom es d ried by diffusion of w a te r to ad h e re n t m aterial, th e m ass becom es m ore crum bly and d isin teg rates into sm aller lum ps. T h ese are m ix ed w ith m ore d ry m aterial un til finally the individual pieces o f feed m aterial a re essentially fre e d o f w ater.

T h e action o f the m achine h asten s th e m ix in g o f th e feed w ith th e d rie r m aterial alread y p re se n t and quickly p ro ­ duces a u n ifo rm ly d ry condition of the m aterial held w ithin th e mill. A t th e sam e tim e the feed m aterial is w arm ed, and the v apor p re ssu re of th e w a te r in it is increased to such a point th a t appreciable am ounts of w a te r a re evolved into the overlying air._{/ o}

T h is processing then b rin g s th e m aterial to a condi­

tion w here the mill action 011 individual particles can take place w ith a m inim um o f agglom eration. W h ile not en tirely dry, the m aterial is in such a condition th a t the fine particles do n o t ad h ere too m uch to the coarse o n e s;

also th ere is sufficient fluidity to the m ass so th a t the fines flow fro m beneath the mill loading, exposing th e coarse ones to d irect action. U n d e r these conditions th e fine particles m ove freely and are lifte d out o f the mill by the a ir stream . In the air se p a ra to r th e coarsest o f these are re tu rn e d by g ra v ity o r ce n trifu g a l force, w hile th e finer m aterial passes on to th e p ro d u ct collectors. D u r ­ ing the process of rem oval fu rth e r d ry in g tak es place, th u s reducing th e p ro d u ct to a v ery low m o istu re con­

tent.

B eh avior of M oisture D uring D rying

I t is necessary a t th is tim e to consider in som e detail th e behavior of m o istu re d u rin g th e d ry in g o f a m aterial.

R ecent w ork on the d ry in g of solids show s th a t th ere a re th re e stages involved in the d ry in g of a piece o f m oist m aterial. A s the a ir sw eeps over the su rfa c e o f a solid, v apor is evolved into it from th e fre e liquid film.

T h e relatio n is n o t unlike th a t w hich w ould e x ist if a ir sw ept ov er a n open co n tain er of th e liquid. T h e m ass being d ried is autom atically cooled to w a rd its dew point and th e v a p o r p ressu re o f the liquid is reduced. T h e ev ap o ratio n proceeds in the sam e m anner, only m ore slowly. A f te r a tim e certain faces o f th e m aterial be­

com e freed o f liquid, w hile o th ers a re still m oist and th e ev ap o ratio n ra te drops. A s soon as all of the free m o istu re is elim inated from th e su rface, the liquid in th e in te rio r o f th e m ass m u st diffuse to th e su rfa c e b e­

fo re it can be evaporated. T h is takes place v ery slow ly and d ry in g is m uch less effective u n d er th o se conditions.

T h e lim it of d ry in g is reached w hen the w ater is reduced to th e equilibrium m o istu re co n ten t o f the m aterial.

I t is well kn o w n in the field of d ry in g th a t th e re is a certain p ercentage o f w ater retained in th e solid. I t is sm all w hen th e h u m id ity is 0 p e r c e n t; quickly increases to a significant value w hen the h u m id ity is aro u n d 25 p er c e n t; slow ly increases beyond th a t up to a h u m idity o f 80 p er c e n t; and then rises rap id ly as the atm o sp h ere approaches satu ratio n . T h e v a ria tio n in th e absolute values has n o t been so fully stu d ied u n d e r a w ide v ariety o f te m p e ra tu re conditions.

308

T h is equilibrium m o istu re so rp tio n gains special s ig n if­

icance in th e field o f g rin d in g . M ost o f the m aterials being su b jected to p u lv erizatio n a re in h eren tly low in th e ir tendency to sorb w ater into the particle, b u t they have large su rfa c e areas exposed a f te r g rin d in g upon w hich such so rp tio n m ay tak e place. Som e equilibrium m o istu re so rp tio n values fo r g ro u n d sand an d fo r zinc oxide a re p resen ted to show q u an tities of m o istu re re ­ tain ed u n d e r d ifferen t h u m id ity conditions and the effect o f fineness upon th is value (F ig . 4 ) . C e rtain m aterials o f a m ore colloidal n a tu re m ay hold still la rg e r q uantities of m oisture. I t is reasonable to assum e th a t this type of curve holds a t th e h ig h er te m p e ra tu res used in d ry in g m ills, an d it becom es im p o rtan t to co n tro l th e h u m id ity o f the atm o sp h ere w ith in the mill.

A dvantages of W arm A ir

A ccording to th is concept it can be seen w h erein g rin d ­ ing m ills w hich a re sw ept by w arm air m ay offer d istin ct ad v an tag es in ra te o f d ry in g , po w er consum ption, and ch a ra c ter o f the p ro d u ct. T h e m o istu re in th e feed is quickly sp read th ro u g h a larg e m ass o f m aterial w hich is w a rm and essentially dry. T h e te m p e ra tu re an d the g re a te r su rfa c e accentuate th e d ry in g , leaving a d ry m a ­ terial to be su b jected to g rin d in g action. T h e m ark ed lo w erin g o f m ill capacity revealed in F ig . 3 is affected only in a m in o r w ay on account of the low m o istu re con­

te n t of the solid in th e mill.

T h e m echanism by w hich ra p id ity o f d ry in g is achieved

Fig. 3—Test-chart show ing relatio n betw een dryness of m aterial an d capacity of p u lv erizer

Fig. 4— E q u ilib riu m m oisture sorption

C h em ica l & M e ta llu rg ic a l E n g in eerin g — V ol.40,N o.6

can be su p p lem en ted to show how a u n ifo rm p ro d u ct is obtained. I t has been show n th a t fine particles d ry quickly. In c o n tra st, consider the d ry in g o f a filter cake which has alread y been m entioned as a strik in g exam ple of th e p e rfo rm a n c e o f th e d ry in g m ills. T h e a re a is sm all p e r u n it of m ass to be d rie d an d th e re a re long distances fo r the fluid to diffuse th ro u g h th e solid in order to reach the su rface. I f th e re a re dissolved solids in the fluid, these will tra v e l to th e su rfa c e an d produce in cru statio n s or efflorescence, and th e cake becom es non- u n ifo rm in com position. S uch in cru statio n s a re m ore difficult to g rin d and have a tendency to leave g rits in the finished p ro d u ct. T h e process o f g rin d in g an alread y d ry filter cake ten d s to m ake the p ro d u ct u n ifo rm , b u t the com bination of d ry in g an d g rin d in g is m ore likely to in su re u n ifo rm ity . T h e decom position o f h y d rates p re ­

sents a m ore delicate situ atio n w here th e d ry in g m ill is effective because of close te m p eratu re control and rapid p en etratio n of heat. T h e sm all particles m ay be b ro u g h t to the correct tem p e ra tu re w ith o u t d an g er of over o r u nder heating w hich com m only occurs in th e processing of large masses.

Since th e com bination of d ry in g and g rin d in g have been effected a num ber o f applications have been m ade ran g in g from m ineral pro d u cts to sensitive organic m ate­

rials, an d fro m coarse w et m asses to plastic m aterials like filter cake. T h e com bination takes advan tag e of the principles of dry in g to produce resu lts rapidly and w ith little if an y im pedance to grinding. D u e to th e in tim ate contact o f the m aterial w ith air, an d th e close te m p e ra ­ tu re control w hich m ay be m ain tain ed a high q uality o f

rials, an d fro m coarse w et m asses to plastic m aterials like filter cake. T h e com bination takes advan tag e of the principles of dry in g to produce resu lts rapidly and w ith little if an y im pedance to grinding. D u e to th e in tim ate contact o f the m aterial w ith air, an d th e close te m p e ra ­ tu re control w hich m ay be m ain tain ed a high q uality o f