Recent Developments in the Tennessee Phosphate Industry

Transcription

1 Preprint for Nsw York Mesting, February, 1939 Permiadion is hereby ginen to publish, with appropriate acknowledoment, ezc~pts or eummaries of this Prsprrnt not to meed one-third of the entks tezt of the paper. Permission to print in more eztended form subrepwnt to publication by the Institute must be obtained from the Secretary of the Institute. AMERICAN INSTITUTE OF MINING AND METALLURGICAL ENGINEERS Technical Publication No (CLABB H. INDUBTRIAL MINERALB DNIBION. NO. 78) DISCUSSION OF THIS PAPER IS INVITED. It should preferably be presented by the,contributor m person at the New York Meetrng February 1939 when an abstract of the paper wll be read. If thia is impomible discussion in wri'ting (2 cor;iea) Aay be sent to the Secretar. American Inatltute of Mining and.~&allurgical Engineers. 29 West 39th Street, New York. N. Y. &less special arran ement 18 made, discussion of this paper will close June 1, Any dsscumion offered thereafter shouli preferably be in the form of a new paper. Recent Developments in the Tennessee Phosphate Industry BY PAUL M. TYLER* AND HERBERT R. MOSLEY,~ MEMBERS A.I.M.E. (New York Meeting, February, 1939) STRATEGICALLY situated in almost the heart of the leading fertilizerconsuming area of the United States, Tennessee long has ranked second only to Florida as a phosphate-producing state. Since 1932 its share of the national output of phosphate rock has increased steadily, and in 1937 its production mounted to an all-time record. More significant even than the increase in tonnage are the forward strides in technology. The Tennessee Valley Authority's research program has led to quasicommercia1 operations using hydroelectric power to manufacture concentrated phosphatic fertilizer that is used to further the farmer education program designed to streamline the fertilizer business and halt the appalling waste of farm land by soil erosion. Even more stimulating to chemically minded imaginations is the production of elemental phosphorus by a private company that not only has succeeded in extracting it from the phosphate rock in electric furnaces but actually has contrived to capture it and to transport it safely in tank cars. New values have been created for low-grade phosphate deposits, and the economic life of reserves has been lengthened while the waste of former years goes back into the mills for recovery of its values by modernized methods. Tennessee's phosphate-mining industry dates from the discovery of blue rock in Lewis County in December Its known reserves are of the order of 101,500,000 tons, which compares with the production of 825,099 tons in 1937; that is, 21 per cent of the phosphate-rock output of the whole nation. The total output reported from 1894 until July 1, 1938, was approximately 19,000,000 tons. Official estimates of reserves are overly conservative and evidence submitted before a joint Congressional committee in November 1938 places "probable" reserves at more than 6,000,000,000 tons. Manuscript received at the office of the Institute Dec. 1, Published by permission of the Director, U. S. Bureau of Mines and of the Board of Directors, Tennessee Valley Authority. * Chief Engineer, Nonmet8al Economics Division, U. S. Bureau of Mines, Washington, D. C. f Chief of Field Office, Tennessee Valley Authority, Columbia, Tenn. Copyright, 1939, by the American Institute of Mining and Metallurgical Engineers, Inc. MINING TECENOLOQY, March Printed in U. 8. A.

2 . 2 RECENT DEVELOPMENT8 IN TENNEBBEE PHOBPRATE INDUSTRY ' ' Most of ~ennessee's production to date, but less than one-fifth of its reserves as previously estimated, has, been "brown rock," the residual l~roduct of ~veathering a.ntl nat,ural concentration of certain phosphatic.@rdovici$n limest,ones. Over four-fift'hs of the so-calletl known 'reserves but only 10 per cent of the past prodnct,ion is "blue rock," which occurs as bedded deposits or lenses in the latest Devonian or ea,rliest i\lississippian shales ant1 sandsto~les. A third variety of Tennessee phosphate is "white rock," coinprising chen~ical replacements and clel~osit~ions in Silurian and Devonian formations. The phosphatic limestones from which the brown rock cleposit,~ are ' derived are of great areal extent a.nd are arched gently in the Cincinnati geanticline. This structure has been traced from the Great Lakes to the Gulf, but the brown phosphate-rock tleposits are developed only in t,wo domes, one. rising in south central Tennessee and the other in t,lle blue grass region of Kentucky. Elsewhere theil~ecls lie too deep t'o I)e reached by t,hk agencies of weathering. The phosphate muck or matrix represents the reinnins of the Hennitjagel Bigl~y, and Cannon hecls (Middle Ordovician) and of tlie Leipers fornlation (Upper Ordovician). The favorable conjunct,ion of good \venthering ancl erosion condit.ions restricts t,he minable occurrences to relatively narrow belts -m~hicl~ represent the outer margins of an irregular oval cent~-lring nbolit. C'olumhia, Tenn.. This oval is not, more tl~an 50 miles wide, but its nort,h-so~~t~ll axis overlaps 11ot,ll the A1nb:tma and Kentucky borders. I11 t'lie river nren the phosphatic beds have been renloved by erosion, and t,lie int,ersection of the gently clomecl becls with the surface tra.ces n complicnt,etl pat.tern by reason of t,he rolling llills ancl mrincling strea.m. Bl)out'SO per cent of the brown rock mined in the stn.te has come from an nren about 15 or 20 miles long in Mn.ury County, near Mt. Pleasant', 011 the soutliwest.erly borclelof the clome ~vl~ere geological conditions I~ave been unusually fn.vorable for the development of rich, ea.sily worked cleposits. However,.similar deposits are founcl extending clocknrise nromnd tile clome in Lewis, Hicliman, l\~illiamson, 34nrshal1, Giles, ancl otller 11~:1r-l)y rr),~llt,ies. The pllnspllatic mat,ris or "muck" ranges from 2 to 20 ft,. or nlore in depth, ancl t,he overburden of soil, cla,y, and slight,ly plnos1,hntic muck 1.aage.s'fronl :illnost 0 to 20 ft. or Inore in t~llickness. For t,he ~.irher beds as mucl1a.s 6'ft,. of overburtlen nlnp be ikmovetl for every foot of phosphate, but n. 3 to 1 ratio is applied to the lower-grade deposits. Ss in.most rc.siclual, deposits, the contact bet,ween t'he phosphate antl bedrock,i$.often l~ndulating and broken by li~nestone pinnncles, l~orses, or "stools " of u~~clecomposecl rock. On the ot,her hand, enlarge(-l joint.!: or "cutters" ~fille:;l 7iiith resiclual phosphate reach down into the originn.1 l,hosphntic limestone..'.'hn,t band," ".ri~n," or lic~llar" del~osit~s may encircle an ~

3 PAUL M. TYLER AND HERBERT R. MOSLET 3 entire hill, but unless they come close to the sumillit may run into a core of unaltered limestone only a short clistance from the outcrop. Accorcliiig to tlie miners, they "lime out" or (the bottom) "jump up." Close esainination of the liiilestoiie horses or pinnacles reveals that they are banded with alternate layers of phosphate rock ancl recrystallized calcium carbonate. Slablike pieces of "plate rock" in tlie matrix imcloubteclly are derived from this source. A brief review of the available, literature might leacl one to assume that the phosphatic linlestolles before being weathered were relatively uniform in composition over most of their areal estent, but closer observation would suggest great variations in the phosphate; for example, at the Akin property, about 6 rililes northeast of Columbia, there are beds of solid phosphate that apparently were formecl in place. Conimercial deposits of bluk rock are confined to the phospllatic portion of the Hardin sandstone horizon that crops out at the base of the Chattanooga black shale; they are stratigraphically higher in tlie dome and geanticline tllail the Orclovician phosphatic limestones, which in R'Iississippian (or Devonian) times may have contributed phosphatic material to the blue rock formatioii. In central Tennessee, tlie blue rock ranges from a compact, oolitic or conglomerate variety analyzing as high as Sl per cent B.P.L. (bone pliospliate of liille, CaaPZOs) to lo~vgrade phosphatic shale or sandstone. Beds 1s in. thick are coilsidered minable, but thickness varies greatly even over short distances, so that individunl deposits or lenses are never large. Thickness ordinarily ranges froill a few inches to 3 or 4 ft., representing the top and enriched portion of a thick (core-drilled to about 100 ft.) phosphatic limestone, which is progressively leaner in phosphate at depth. Most of the blue rock minetl has come froin tlie Gordonsburg district in the valley of Swan Creek in Hickman and Lewis Counties and in tlie Leatherwood district along Leatherwoocl Creek in Maury ancl Hickman Counties. An area \vest of Tucker's Bencl on Duck River ancl a sinall area south of C:enterville in Hickman County also are workable, and nlinor cleposits have been reportecl in Wayne, Perry, and JVilliamson Counties. Low-grade phosphate reported from eastern Tennessee, however, is not blue rock. White rock occurs illainly in Perry and Decatur Counties near the Tennessee River and in Johnson County in eastern Tennessee. The deposits are of Tertiary or post-tertiary age. RiIost important is the lamellar variety found filling pre-esisting solution cha~lnels and as a matrix of angular chert fragments, less often as replacement of limestone. Individual deposits are characteristically irregular and ususlly rather small. Widths of 18 and possibly even 30 ft. have beell reported, but in tlie Beech River tract in Decatur County the average is about 5 ft. Owing to the mode of their origin, they are never likely to be very deep. A number of small mines have been opened, the first production being '

4 4 RECENT DEVELOPMENTS IN TENNESSEE PHOSPHATE INDUSTRY reported in 1898, but the total out8put of ~vhite rock has 11een under 25,000 tons. Aluminum pliospl~ate, reseml~ling v:triscit,e " in coml,osition, has been found in two Ten~iessee localit,ies :inel rvns nlinecl in Alarsllill County 395 miles i~ort~l~east of LewisGurg, but not. on a significant scale. Earth augers or post-hole diggers are used in prospect'illg brown rock. a For prelinlinary investigations holes at the corners of 500-ft. sc1u:lres may suffice, but before mining is. begun tlle spacing ma.y 1)e 3s close RS 50 ft. Changes in the color and texture of the cuttings tlsunlly mn.rli clept,h of. ' overburden anel thickiiess of matrix; esperiencecl opern.t,ors c:tn also distinguish readily between low-grade 3,11c! 11igl1-grade 111:ztris 11y t,he (lolor and by the feel. Samples, bowever, should be t,alie~l at regular intervals and certainly whenever a change of color is not'ecl. Commercial recoveries may be estimated by washing the reject. from t>he borehole samples in a tub and weighing and analyzing t,llr c,o:lrse cut,tings rt~iil fine s:intl from which the slime has been remo\;t.tl. The drill logs may be clleckecl further Ily sinki~ig pit,s st a fe~v points. Some operators cut a vertical channel 1 ft,. sclynrk on one wall of the pit and wash the pllo+l~ate layer; the weight of' wnshings cliviclecl by the thickness of the sample gives tlle probnhle recovery per cubic foot,. Mult,iplying this factor by the minable n~unl~er of scluare ft.et ailel t,lie a.vernge.. thickness of the deposit and dividing Ly 2240 gives the pro.bable: yielcl in long tons froin a given blocli. From 1936 tlirougli 1938 the Tennessee Valley But.horit,y spent an average of $5 an acre in prospecting, nlost,ly ljy auger holes on t,he corners of 200-ft. squares. These holes were, plot,t,ecl on a map ancl t.1~ inillable area was nlensured by means of n glanimet'er. Jiolr~mes of overl~urtle~ and of phosphate were then calculatecl by mult,iplying the number of square feet of minable area by the number of feet of average t,hicknes. of the overburden aiicl bf'tl~e phospllnt,e bed, respectively. Inasniucli a* numerous determinations showed an average weight. of per cu. ft.. volumes were reclllcecl to long dry tons by clivicling by 25, ~vl~ich is roughly the number of cul~ic feet occupiecl Ly a ton of materi3,l in the,ground. The average B.P.L. content was c~alculatecl by multiplying the number of 'feet of thickness of tl!e phosph,zt~e bed at ench prospect hole by the weighted average per c,ent of B.P.L. shown i11 tae sninples froir t,hat hole. By adding the results from all holes in tile minnl~le area ant then dividing by the sum of phosphate-bed t~hickness in the min:zl~le area the average B.P.L. conte~it of the area was ca~lculateil. For prospecting a large tract near Franklin, Tenn., tlie T.V.A. con. trac,ted for auger holes at a minimum price of $1.15 each for holes up tc 10 ft.,. plus 20# per foot for additional depth up to 20 ft., plus 356 pe.

5 PAUL &.I. TYLER AND HERBERT R. MOSLEY..,5 foot from 20 to 30 ft.. This \vorks out to :t total of $6.55 for :t 30-ft.. hole. An additional payment of 506 per foot was nlatle for. all holes over 30 ft,. deep. For digging, sampling and refilling pit,s (to return 1,~ilcls ill contlition for fn.r~ning) the 'contr%ct.price was 606 for t,he first. foot plus 50f per foot up to 10 ft. deep; for deeper pits, tliere wa,s :i sliding scale, so that a 10-ft. pit costs $6.35; a 20-ft. pit, $16.35; ant1 still deeper pitas, $16.35 plus $1.60. For this work T.V.A. furnisliecl augers and 1:)n.g~ for. 171~. 1.-MINING DEEP MUCK (GOOD BEDROCK CONDITIONS). Matrix londecl into cars on hott.o~n hy snl:tller dr:tglinc. New track in foreground laid for nest cut. Large esc:trnt,or 011 upper be~lch is being moved for\vnrd fir111 t,o right to stsrt stripping overhurdell (not sho\\,n), \vhich will be cast into rninecl-()lit, dren where cars now stnncl. Spoil pile from previol~ st,ripping in left l),zokground. ta,king samples, \jut the contractor supplied all other equipment, iilcluding a truck :t~icl slnnll tools as well as labor. He also undertook to transport, t,he samples 40 or 50 miles to s 1:iboratory for analysis. Hancl mining persists, even in the brunrn-rock field, on small isolatecl tracts in the large pits where the ~nuck goes clown into cutt.ers c~r hangs up in cavities in the limestolie liorses aacl cannot be reached with a bucket. Formerly hand mining was the main sourcc of lump rock, but improvenient,~ in washing metllocls nncl increasing utilization of fine sand aucl phosl,hst.ic! c1a.y c~ombinecl wit11 high wage- scales have tended to elinliilate hnncl work ~vlienevcl. possil)le. I11 tlie brown-rock field phosphnte occurs in clay pnrt,ly in tlie for111 of plate rock, bron-n to gray slabs locally as nlucll as 24 in. t.hick! 1 ~1t also

6 grading down t,hrc!ugh s:tncl sizes t,o virt,ually colloiclal mnteri:~l. In the earlier d:tys overl:)ttrtlen was reniovecl by llallcl or by scrapers anit the phosphat,ic nlrlck w:ts loosenecl \\.it11 a pick, anil only the lunips (say 1-in. or larger) wore londecl into c::trt,s wit,li a spall fork. Most,of the cleposits lyi~~g not t,orr clecp I:)c!nent,ll the sill-face wcre originn.lly "highgraclrcl" in t.liis fashion :u-~tl hive sul~srcl~~ent~ly Ixcn mint!d over again. Plate rock from em:i,ller Gper:~t,ions urns dried on corclwcrocl ric!ke! requiring about 1 ccjrcl of wootl for each 10 tolls of' rock. At presc~~t. virtually 3.11 ininiug is by tlmgline cscavators using I,!:;-ycl. buckets or la.rger :tncl 45 to 11.5-ft. booms. Most of the draglines are n~ouilted oli csterljillnr tre:tds, I>ut some have nralkbr mountings and n few r:tilrontl-type machines are still in service. The s:l.me m:tchines rnny Ije usecl for renloving oi~cl:burden, altjhough longer 1:)ooms are rleecled where the o~rerburilen must bc cn,st a. consicleiable clistance. Most of the overburtlc!n is c1:tye.y soil :mcl selclon~ contains any boulclers :is nlucll as 2 ft. in cliaunet,er. In n t,ypical operat,ion ~vlic$re Ijedrock conditions are goocl, a relntiv'ely a&rronr ci~t is ipade across the minnblc: wen. Stripping is carriecl in. aclv:~,nce of mining; at one propert,y t\vo esc~vators are employed (Fig. I), hoth operating on t,he s:~.me bench, one reaching dowii to 'bedrock ttlld initling the ~nuck ant1 the other strippitlg overburclen ail0 casting it :>,cross the I~ench into,n par:tllel cut that has previously been niineil out. After tl~e systenl is well st:trted, very little of the overburden is hanclled twice ancl only tlw: 1ntlc.k mt~st 11c~5, haulecl a\irn.y. In sonle pits tracks are la.ic1 on the bottom of the minecl-oiit area? cars being loaded witllout re\:ol\~ing the shovel'i~lore than about 90". Or (Fig. 2) the same c1r:tgline used for mining rnay be llsecl for stripping,' during tlle wait for cars or at other t,imes. However, :is iiotecl later, 1:trge buckebi suitable for economical stripping may not be used so effectively in inining llilless Ijedrock contlitions tire goocl. Where 1)ecIroclr conclitions are tiot goocl, aiid solnetil~les for other reasons, the track is 1:ticl on the stripp6cl muck. Where the ore in cutters goes down a fen. feet belot~r the noimal surface of bedrock, the clean-up g:tng follows t,he esctvator ancl tlirows the inuck to :t spot nt ~\.hich it cn.n Ile picked up by t.he bucket. Occ~sionally, the hancl-shovelecl muck is loacled into a steel box similar t.0 n cluarry pan, which can Ije attnchecl to t11e hucket or pickecl up by :t cnnt,ilever crane. When the :tmount of muck below the lime table is fn.irly large (Figs. 3 :tncl 4) tlle clean-up gang may workml~el~incl the clr:tgline, throwing the muck into great piles that call be loaded nrllen. the clr,igline comes back on the return cut. Haul;~.ge is t,ypicnlly in 4-yd. Western side-dump cars, having a cspacity ecluivn.lent. t,o about 3.75 long tons of tlry muck, cir:twtl over 3, 36-in. gauge track I~)y clonl-firecl locomotives, usually in tarnins of 12 to 20 cars each. ~nrl-clknz~~ tramcars of 3-ycl. cn,pac.ity or smallel- are used at some properties, but t,hey are clifficult to discharge, pn.rticularl;y when left fillecl

7 PAUL M. TYLER AND HERBERT K. MOSLEY 7 with wet muck for many Irours. Usually it is necessary to have el;ough cars to afford stor:tge sufficient. to keep tlre mill opera.ting steadily through tlle periocls of nonii:~l clelnys in mining :tncl h:tuling. Pneumatic-tirecl trucks a.re not favorecl by most operators cincl there seems to be a general feeling that tlie irle:tl ~netliocl of t,aking tl~e muck fro111 mine to washer would l~e in fairly large cars. The larger the car, of course, the niore easily it coultl l~e fillecl at tlie shovel, the more storage cap:~city it ~vould have, ant.1 the easier it \voulcl be to i1isch:~rge. The limiting factor, where installa.tion cost ia not the main consiclerat,ion or nrliere existing equipment FIG. 2.-MINING AND STRIPPING \VITH SAIIE EXCAVATOR (GOOD BEDROCK CONDITIONS). Dr:tglirle nr111 track both on top of stripl~ecl ~natris. Mnrks of h~~ckct t.eeth foreground \\.here 1:ist load mas taken. After train is loaded ant1 hefore it returns from w:tsher? overburden l):tnk (right) may be d ~ anil ~ g cast, into t~~ineil-out :).re:& :~.t left. lleecl not l~e scmppcd, is thc clifficultv and espeiihe of mai11t:tining nlicl shifting 1le:tvy rails ancl ht:~liclnrcl-gauge track. The ol~erntion of the hlonsnnto Clirmical Co., rclntil rly n newcomer, clepnrts some\~.hnt fro111 customary practice in thc clistrict. Tl~is cornpany en1ploy5 a 4-ytl B~~cyruq-R'Ionighnn \valking dr:lgline for digging :tnd especially tlesignecl bottom-dtuiiip E11clic1 Trac-Truks for hauling. These tractor trucks are of 12-cu. ytl. water-level cnpncity and take an average loacl of 12 tons of phohph:~te rock. Tlic tires are 18 by 24 in. Unlike other operatioils in this district, this nii~ie ~)roduces two kinds of muck, which are readily clistinguishecl in tlie bank by tlieir color. Throughout the clistrict overburden is generally lighter in color than the minable muck, but tlie high-gmde nluck is generally lighter in color than

8 8 RECENT 1)I:VI:LC)l'RIENTS IN TENNESSEE PHOSPH:irrF, INDUSTRY the lo\vrr-grnile I~III~:~, I:~eo:tr~sc. of it,s content of plat,e rook a11d co:lrse sand (ortlinn.rily g!:iy r:~.tlier 1;Iian " I)ro\\rn "). In t'lle Estc:s Bend area, where the Monsanto nine is sit,ustecl, nl~out 55 per cent of the 1-r-~innble muck is relnt,ively I~igl-I grnclb (nver:~gi~lg 2:3 per cent P&5) and can be used directly for rnsking sinter. 'J'lle re~nnining 45 pcr cent is somewhnt lower in phosphate colltei~t (averaging 17 per cent P20h) and goes to the washer. The tractor trucks deliver either grade of material, as required, to the primary 1.1opper at the sinter plant or to the respective FIG. ~.-R'~ININC. AND STRIPPING FROhl SAME BEN(:H (DIFFICULT BEDROCK CONDITIONS). Limestone 11orsc:s :111tl stools. :ilsl-).piles of ~n:lt,ris,cll~g hy hand from cavities (Fig. 4) are ready to 1.e t,:rlicl~ I I I'J' ~ clr:~gli~~c IIIIIIC~~, c~t,. stockpiles for first,-grade and scccjncl-grnclc ~ mk. RccI:L~I~~II~ tlie st,ock-' piles is :tccomplishecl by bullcl~jzers n.hicl~ scrape off tlie sr1rfnc.e :tnd propel t,he material to n Ilopper. Bcclrock conclitions ia this mine are clirtracterizecl by closely apacecl ridges or limcst'onc lio~ses with the muck este~ltlitlg down illto the cut,t,ers to clcptl~s of 50 ft. or more (Fig. 3). The clmglinc 11t1cket reaches clown into tl~ cutt,ers nnil t,akes 011 t as mt~cl~ ns po~sible, then cololnil Ial)or6rs! n;orki~~g \zrith pick :tntl sllovel close to the bank, tlig the muel< out of t'lle trnvitic.sin tl~c rock :1.11d throw it i~nto piles, \rhere the buclict cnn 1:1ick it up (Fig. 4). 011 tl~is property,very little r~mov:tl of overburclen is necessary, ancl bl~le clrngline and tractor trucks operate on the same level. Sonle portion of tl16 operating t,ime is consumecl in c:tsting overburclen and waste material..is the excavator itself is at the top of the bank, the operator call decicle ~vl~ether 9 bucketful is t,o be cast away from tlie bank into the minecl-out nrea or

9 PAUL M. TYLER AND HERBERT R. MOSLE,Y' 9.. hoiitc'tl over the eclge a11cl clumped in'td'the waiting tractor t,ruck or onto a temporary storage pile, for first-grade or seconcl-gmde muck, respecbively. ' Dipper-stick sho\~els have been tried. in the tlistrict but because of their" li~nitecl reach have beell abancloned: ' Hyclritl~lic ~i>ining was atteinptecl in Tennessee 2nd wns abandoned years ago. The nlatris is fairlytougli, but high-pressure nlonitors would probably cut it, ancl there arlirregular deposits where hydraulicking' would eliminate a great deal of l~antl work a.rouncl the linlestone horses ancl possibly in the.cutters; bi~t for various reasons, including the topography ancl the general cliffi- PIG. 4.-CLEAN-UP GANG DIGGING PHOSPHATE MATRIX FROXI CAVITIES BY BAND. cdty -of clisposing of large \:olumes of water-even cluring seasons when an ample supply coulcl be obtninycl-it \\,us uneeononiicnl. Formerly hydraulicking was used to a. consiclera.ble estent for stripping but, following Floricln land-pebble experience, lnrge clraglines \Irere fount1 t,o be more economic:l,l.... Owiiig to the value of lane1 for farnming in much of the brown-rock titiitory, mining met,imods that permit reconclitioning of the Iancl are much, to be clesirecl. Where beclrock conditions aie. gogel (Fig. 5), ri&onditioning may be accomplished merely by leveling off the over-' burden East 1,ehincl the mining operations, using a bulldozer in some instances. In inany of tlie mined-out areas, hokever, the pinnacles'are high, and where there was not enough overburden to cover.tliem, ghostlj. ribs of barren i-ock remain after decades of erosion, and... even small ti%s :, akd'mideigroivtlm find only a nlcager foothold. ' ' '

10 10 RECENT DEVELOPAZ~NTS ' IN TENNESSEE PHOSPH.ATE INDUSTRY, Blue rock outcropping on a sicle trill is nlinad ljy stirface stripping and open cut mltil overburden re~nov:~l costs too muc11 ; then untlerground mining, usl~nlly room-a.nd-pillar, is employed. For an amplifiecl clescription, see references 2 and 17 it the end of the paper. Workable thicknesses rarely esceed 5 ft. ancl average probably 2 St,. Mining costs accordingly tend to be consic1er:tbly lliglier than in the brown-rock field, where mining can be mechanized nlore readily, owing to the. larger scale FIG. 5.-GENERAL VIEW ACHOSS MINED-OUT AREA FROM TOP OF STRIPPING. Notice Lighter color of bank of overburdcn'itl place (right. fc)rcground). In this, nren lilllestorie horses or pi~illizclcs are virt,ually al~sent, nr~d lnritl 1nn.y I,e reconditioned by levelingo\rerburdet~ that has been cast into p:~rallel cuts, :iftcrnl:ttris 112s 11eerl ~llilled. of ~pernt~ions. 111 tlle Gorclonsbl~rg district at least, t,lie blue rock is capped by tlic Hardin sandstone ("skull rock") whicli m:tkes a strong roof requiring little support. Phosphate rock ancl sand occur in Tennessee muck or matrix, plus variable amounts of flint, sandy clay, nnrl a rather tough gumbo. Limestone fragments are found occasionnlly, usually in the form of stray pieces of bedrock that can be picked out by hand. Mechanical renloval of calcium carbonate is riot difficult and the separation, afterdispersiorl of clay, is nlai~lly between phosphate rock and flint or slnd j~articles. The phosphatic muck in the brown-rock field varies greatly in B.P.L. content, and the distribution of phosphate between coarse aiicl fine sizes varies even more. Some deposits yield practically no plate rock while others contain solid masses several feet thick. In some parts of the field

11 PAUL M. TYLER.4ND HERBERT R. IMOSLEY 11 t,he clay nncl sli~nes carry off very little phosphate, w11ere;ts in other deposits a goocl clcal of phosphn,t,e npl)en.rs t,q be :~.ln~ost if not quite colloidal ancl the slinles carry as much as 40 per cent B.P.L. Until a.. few years ago only cleposits; tl!at \vould yield a relatively large quantity of s wasbecl rock ancl sancl carrying 70 or preferably 73 per cent or more B.P.L. were considerecl niorkable; in still earlier years, 80 per cent rock was proclucecl. h/iuck. that carried too large n percentage of its pliosphatic value in slimes was almost \vort,i~less, but now such muck can be. concentratecl and srnelted by the new furnace processes to be discussecl later, provided it does not contain too mucli silica. (the opt,imum liinesilica ratio is 1.2'at present). A little 50 to 60 per cent material is ground FIG. ~.-L)RAC; CLASSIFIERS AND CONES ARE USED EXTENSIVELY AT YOME '~ENNESSEE WASHERS. for fertilizer filler, stock feed, anel other purposes, but such sales are snlnll nncl the main clemnnd is still for high-grade sands suitable for making s~lperphosphnte by the :wid pro.cess. Most of the nluck that conies to the conlmercinl plant,s at present yields plate rock sufficiently free froin silica to be shipped without further concentration, after :tclhering clay 'is remover.l, 1,ut genera.11~ it contni~ls n litt,le flint ancl sometimes nus st be crushed nucl concentrnted. Screen ana.lyses a.nd washer cuperience, lio\vever, bring out the fact that lnost of the l~articles bet\veen &out 2s and 100 mesh are almost pure phospl~nte. h/iateri:tl finer than nbout, 100 mesh is progressively coata.minated with more nad more silica grains; in some deposits anything.finer than about SO mesh cannot he classed as commercial-grncle matrix ~vit,liout concentration.

12 12 RECENT DEVELOPMENTS IN TENNESSEE PHOSPHATE INDUSTRY The steps in more econonlic~al utilization of Tennessee brown phosphate collsequently are marked by steady improvement in the utilization of finer and finer sizes. First came hand mining, which rejectedvirtually. everything under about,lg in.. This was followed by log washers, which, rejected only part of the sand along with tlie clay. Later, by screening all of the log-washer protluct, a separation could be nlade at any definite point over 'about in. On bhis basis, at all.but the best deposits; rougllly 50 per cent of the phosphate was still lost in the rejected sands ancl slime. As drag classifiers, cones, nncl various settling devices were gradually nclcletl to the.flowsheet, recoveries improved, but as the propor- MlllECAPS.. I 1 JET ' - _ T ~ ~ ~ DISCHAPGE.., I. A t 1 ':. HOPPER ' 6WiT WCR 7HICKEIIEP FOUP Ilr 17-FT JET 1~11~5' 1. 1 OVERFLOW 7 4 F h E 7, TiVO 8.30-FT WWRZ-DECK, CLASSIFIERS.,, ' J JET.. OVERFLOVI DISCHARGE i 1 &'FT DOUBLE LOGLVASHER, MUD POND r' DISCHRRGE OVEiFLm 16-FT. ALLEtl '. -. HYDRAULIC COtJE OVERTLW OILCHARGE f 25-FT. DOLIBLE LOG WASHER"! $ '. o v E R v H & % E WET Rocti i STORAGE 1. OVERFLOW Y' 36-IEJSPPAY TROMMEL~ i DISCLARGE t32-it1.. +J/,!.IN 'L+ -3'i3 mu m. STOR-E~ WET COCK STORAGE WAZTEc PHOSZATE ROCK 1 WET fiock STORAGE FIG. 7.-GENERALIZED PLOWSHEET OF MODERN BROWN-ROCK WASHER. a Rotary wasllers may be nsed at this stage to break up clap balls. b Linliting screen varies fro t.o?i inch. L c Fli~lty plate rock ]nay be cruslled or ground and returned to circuit (either at tronlmel or jet tank). d Single-spigot pyrnll~iditl hydraulic classifier. Four hydraulic c1:xssifiers in series. J May he sent to flotntion plant.. tion of sand inherent,ly increases inversely with the size of t,he m:tterial, cone s:tncls usually carried uncler 6s per cent B.P.L. This was the state of stff:tirs until the intr~cluct~ion of frot,h flotat,ion, ivherel>y :t separ a t' ion. between silica and phosplinte grains could be effected. Sh:iking tables liad been given a thorough trial.' ~ltliou~h some of t1he 1:trger sand grains can I>e sloughed off by habliug, the clifference in gravity between pl~ospl~nte mid santl is not 'sufficient to allow 3 clean sep,zr,ztion. Flotation nfforcls an effective means of concentrating nllnlatlerinl in the critical range of silica contaminat,ion clown to about 300 mesh, but even yet no meci~nnic:tl proc'ess has appeared for recovering slimes. Fortunately, at many deposits the minus 300-mesh fractions carry less than 20 per cent B.P.L., sncl with heads running 50 to 60 per cent B.P.L. this permits a recovery of.70 to 55 per cent, which is probably the comnlercial range in t,he district at present. At one property, the overflow averages 11 per 1 TYiO 8-FT.ALLEN DEVI?TEPItIG CONES

13 PAUL RI. TYLER AND HERBERT R. MOSLEY 13 ~ent.1'20~ as against a theoretical limit of 8 per cent. At this property evep the flint carries.4 per cent P205, so the actual recovery based upon mineral conlposition is some~.hat better than theb.p.l. recoveries alone might indicate.,..the essential steps in \vashing Tenliessee nlatris are: (1) dispersing the clay.; (2) screening (if necessary re-treating oversize); (3) recovering coarse sand; (4) recovering fine sand; and (5) co~lcentrating fine sand by flqtation. The log washer is the. best known device for separating and washing clayey material ancl is the one.most commonly used.in the Tennessee field.....:one company for many years has run all material through a "mulcher" -ayerticn.l cylinder fecl from the top with moist muck, which is passed. through rolls to break up any fragnlelits over about 2 in. Attached to a ce;n.tral revolving.shaft are 10 series of arms or spokes, which stir the material anil mix it thoroughly. The discharge from this unusual machine has a consistency of heavy cream, which passes.to a rotary scrubber with a 1-in. scalping screen, where more water is added. Oversize goes to a picking belt for rellloval of clay balls and thence to a log washer, where it is. joined by the undersize of the scalper screen, which meanwhile hn.s been partly clewatered. Even in the log washer the consistency of the pulp is much thicker than usual, the definite policy at this plant being to use as litt'le water as possible until all of the matrix is thoroughly wetted. Although contrary to the usual practice, where great quantities of water are added immediately, the muck being sluiced into the washer with monitors, this idea for reducing the formation of clay I~alls may be a counterpart of paintmakers' experience-in aclding vehicle to. pigment better results are obtained by incorporating the licluicl a litt,le at a time. Another well nlallagecl washer in the district eliminates log washers altogether anil depends upon a 60-ft. scrubbing cylinder for dispersing the,'clay. This cylinder, originally a drier shell, is equipped with lifting arms to raise the material. A good deal of the mashing of the clay balls is accomplished by the larger pieces of plate rock ancl when the proportion of plntc rock is small, more rock or other grinding medium is ~tldecl. Soclium silicate in the proportipn of 4 lb. per ton of solicls is sometinles used to aicl clispersion of the clay. At several plants all nlnterinl over about :?.<ti in. is separatecl into two sizes; the coarser of which is hand-picked to remove clay balls, limestone fragments or conrar. flint. Everything ualcler $is-in. is usually sent to a drag or Dorr rn,ke classifier, sometimes after a preliminary screening at about!.$ in. At lliost of the plants the arrangenient,~ of drag or rake classifiers are rather complicated, some being in parallel ancl others in se~ies. A rllocler~l trencl is to have classifiers in series as well as in parallel tq.ma.ke cleaner protltict,~. As lllany of the plants havechanged their..

14 flowsheets over aacl over again, settling cones may be retained in the finesand circuits ex7en here a nlechanical classifier might clo a betterjob. The net result, however, is to produce: (I) coarse sctncls that colitain enough phosphate to beshipped as high-grade ant1 (2) low-grade sands, generally finer, that contain n. goocl deal of silica. At present only three plants are equipped with flotnt,ion machines, ant1 where flotation 'is,not employed the cone sa~lcls or other fine products llave to be sold at a substantial discount from the prices of high-grade rock. The typical feed to flotation machines carries 69 to 70 per cent B.P.L. while concentrates carry 74 per cent or better, and are much reduced in iron and alumina. The tailings from flotation nlay co~lt,aiil over 60 per cent B.P.L. and are grolmil and used for direct applic.ation to tlie soil, for fertilizer filler, or locally for phosplloric acid manufacture. Most plants have l~yclroseparators or thickeners for removing fine sands, the 6nn.l separation being typically at about 250 mesh: Overflow from a liyclroseparator go,es to slinle ponds. Slime-pond tailings from fornler operations are being reworked and the recoverable sa~icls concentrated by flotation. Water collsun~ptioil is high, ahout 10 to 15 tons per toil of solids at, most, washers. During part of the year at least most ~nills reclni~n water fro111 their sli~ne ponds and cluring very dry seasons nlay be obliged to reduce operating time. The sand discharge from the drag, rake, or bowl classifiers ordinarily contains arotuld 30 per cent moisture but drains to an average of 10 to 15 per cent I~efore going to the driers. R.otary driers are gei~e~!ally employed, n~lcl the tendency seenls to be to feed the wet lnaterial at the hot end, to reduce stick'ing. The cliscl~nr~e ca,rries 1 or 2 per cent moisture, and the rockre,zches n. n~n.simun~ temiern.ture of about 300" F. At a typical plant, 35 tons per hour is dried in a 7 by 30-ft. shell. Most of the driers are cod-fired, using atitomn.tic stokers. Powderecl coal has also been employed where contamian.tion with n,sli coulcl I)e permitted. The clriers, although unlinecl, give long years of service, :is the rock is not al~rasix~kand the shell cn.n be pntchecl Ijy xirelding. Sintering of phosphate rock is incrensing in amount,, n.s this process not ouly raises the B.P.L. cont~ent-for esample, fro to 76 or even 77 per cent-but destroys orgauic nlatter and otliernrise makes a product that is preferred for the making of phosphoric acid and ot,her chemicals by wet met11ocis. Calcining temperatures run from 2200" to 2400" F. A1thoug;h coal-dust firiug has been triecl, most of the si~ltering kilns are oil-fired. Matrix and.rock for furnace processes may be no8ulizecl in rotary kilns, like Portla~ld cement, but Dwight-Lloyd ecluipment is employed to

15 PAUL M. TYLER AND HERBERT R..MOSLEY 15 make sinter at the Monsanto plant, a blend of raw, nearly dry matrix and pliospl~ate sand being mixed wit11 about 10 per cent of fuel,. One nodulizing plant rnaterinlly reduces coal consumption by burning carbon monoxide from its furnacing operations. Inasmuch as tl~e matrix for these processes nust be driecl anyway, further consideration may be given to air separation to remove clay in the form of dust, instead of washing or to supplement the washing process. The average value of Tennessee phosphate rock as reported to the Bureau of Mines by producers was $4.05 per long ton in 1937, but this figure includes a certain amount of sinterecl ~natris and a good deal of 'low-grade material. A little rock or muck carrying less than 60 per cent B.P.L. is used for various purposes ancl sells for only $2.50 to $3 a long ton f.0.b. washer. Generally speaking, st,andarcl grades of Tennessee rock are sold on a competitive basis with Florida land pebble plus freight,. Published quotations for phosphate rock are inaccurate, because virtually all sales are under contrnct at prices negotiated privately between buyer and seller. Actually 72 per cent rock is at present worth about $4.50 a ton, and 75 per cent rock is around $5 a ton f.o.b.,tennessee shipping point, compared with $2 and $2.50 a ton respectively for l&nd pebble f.0.b. Florida drying plant (subject to fuel-oil and lal~or differentials). Highgracle Tennessee ground rock, used fairly extensively in Illinois and Indiana for direct applicatioa, sells for $7.25 per short ton (carload lob) in bulk, plus around $2 estra for.shipment in 11ag.s. This materisl is ground 85 per cent through 300 mesh and carries approximately 75 per cent B.P.L. Freight rates to Wilson Dam, Ala. (Riluscle Shoals) from Columbia or MOLII~~ Pleasant are $1.62 for on high-grade rock, $1.32 on furnace sinter, and $0.99 ancl on unwashed muck. Further refinements in mechanical methocls of treating phosphatic muck doubtless will be made, but a limit seems to be placecl upon recoveries by ordinary settling nacl flotation processes by the slinles which in some portions of the Tennessee bron.11-rock field run high in phosphate (30+ per cent B.P.L.). Iron ancl alumina, because they use up acid and increase the percentage of "unavailable" phosphate, and silica, because it is a diluent, cannot be tolerated except in relatively small percentages in material for making superphosphate by usual methods of acidu1,ztion with sulphuric acid. Flotation or agglomerate tabling successfully separates impurities down to 200 or possibly even 300 mesh, but not until smelting n~ethods were comn~ercializecl was there any feasible method for utilizing colloiclal or, semicolloidal phosphate contaminated with clay or fine flint particles.

16 Ferropliosphorus has been niacle commercidly for several decades in blast furnaces but in relatively s~llall cluantities; moreover, lump rock is generally preferred for this purpose. Electrothernial snlelting of a mixture of phosphate rock, sancl, and coke to make phospl~orus was begun on a coin~nercial scale at Niagara Falls in 189.5, but not until 1920 was the elect-c furnace usecl in rnaking phosphoric acid. Tlie Swarm interests, at Anliston, Ala., instead of collecti~~g the elelilental phosphorus as it volatilized in the furnace, allowed it to burn in gas mains to pentoside, which was caught in water.sprays to form phosphoric acicl. Acid and phosphate chemicals producecl in this way are of high purity because impurities :%re not volatilizecl and can be rcniovecl in the slag. The Anniston plant was later acquired by the h/ionsanto Chemical Co., which, after due consideration, purcliased phosphate lands ancl built a large new plant near Columbia, Tean., for producing and recovering elelnental phosphorus. The first of its three furnaces in Tennessee began operations on June 15, 1937, mld the first tank car of elemental phosphorus was presently on its \\ray to Anniston. Prior to 1936 it was customary, even in electric-furnace smelting, to use ~vasliecl rock, principally Tennessee brown rock and Florida larid pebble, containing 66 per cent or rnore B.P.L. and then to adcl crushecl quartz or sancl to furnis!) the necessary silica for slag formation. The Tennessee Valley Authority and the IvIonsanto Chemical Co. have pioneered in utilizing unmashecl matrix, tlierel~y avoiding the loss of phosphate v:tlues in the slimes which are wastecl i11 washing. Nodulizecl or sinterecl muck is also Iwing l~tilizecl in the new plant of the Victor Chemical Co., whicl~ came into operation on June 1, 193S, near Mount Pleasant, Tenn. Other plants arc. uncler consideration. Tlle various processes investigatecl I)y the Tennessee Valley Authority haye been described It is pertinent to point out here that the ratio of lime to qilica in electric-funlace charges must be regulated carefully. Satisf:tctory functioning of esperimental furnaces 112s I~een ol~tained wit11 lime-silica weight ratios ranging frotii 0.83 to 1.28, but the cost of cnergy coilst~niecl per unit of P?O, increase? ar tli~ lime-silica ratio clccrease.s, so tlirtt ratios lower than 1.10 cannot be smelted econo~nically and a somctt~hat higher ratio is desirable. Within tlie percrllt:xges commoaly fotuncl in Tennessee muck, 31unlina can replace silica as a fluuing agent lnole for mole, 1 per cent A1203 being equivalent to 0.59 per cent SiO?. Iron, on tlie other hand, is reduced in the furnace and conlbines \vith pliosl,horus to form ferrophosphorus. The production of ferrophosphorus, therefore, is unavoidable and is directly proportional to tlie iron content of t(1ie charge. Hitherto ferrophosphorus has been considered a \~nluable by-product, recently having been quoted at $7.5 a ton for the 24 per cent grade. Nearly all tlle ferrophosphorus consumed in this coulitry is procluced as a by-product of phosphate-smelting operations, and further expnnsion in

17 PAUL XI. TYLER AND HERBERT R. MOSLEY 17 by-procluct out,put nlny llot Ile re:itlily :tljsorl)ecl in normal oons.un1ption channels. Unless econonlical rnet,hocls for cclnvertil~g ferro~~l~osphorus into fertilizers and other c11emica.l proclur.t,s can be tlevelol~ecl, any large increase in the output of the alloy might clepresa it.$ v:tlue to the point where it.s production tvould be a liability ra.ther than an asset'. 111 some part,s of the Tennessee field the matrix, even though it c:trries nlucl~ of its phosphatic v:tlues in the slimes, can.be sintered alicl used directly for electric-furnace or blast-furnace smelt.ing. I11 the Estes, Bend region, where both T.V.A. ancl Monsnnto linve been mining, rouglily 55 per cent of the muck can be used directly for t,he production of furnacegrade sinter and 45 per cent must be washed. The sl~t~icipated recovery of B.P.L. in the washing opera,tion is around 60 per cent but tlie combined recovery, including the B.P.L. contained in tlie muck that does not 1i:ive to be washecl, runs well over 80 per cent, a figure that could not even be approached wit,h this particular muck by the most efficient combination of known mechanical methods. Better teclinology has been the nleans not only of expanding but'of continuing phosphate production in Tennessee. Deposits rich enough in plate rock to be worked by old-time nletliods liave long been exhnusted, and for ninny years the proportion of fine concentrate llas been n~ucll greater in Tennessee than in other l)llos~~hate fields. Year after year, as the Illore easily and cheaply minec1,deposits are worked out, the average ratio of overburden to nluck thickness tencls to increase ancl tlie physical conditions of nlining, groy Inore difficult. Rlechanization has not progressecl a.s fast in Tellliessee a,s in Florida, I~ut,he output per man ' per year rose in 1937 to 575 tons compared with 543 tons in 1929 and 312 tons in In 1909 it was 207 "tons, a substaatia.1 drop from 271 tons in 1902, thus nlnrking the diminishing out,put by hand mining following 6lie exhaustion of easily worked blue rock ancl rich brown rock deposits and before the expansion in nlechanical methods got well under way. A local observerlg has stated that t,he present goal of a nlillion tons a year c,nn be producecl in existing large pla,nts with fewer than 1600 people, nrhile the same output produced by hand n~inilig and in snlall plants would keep 5000 enlployecl except in dull seasons, ~vlien they could all be farming. Even discounting the fact t,hat at present almost one-third of tlie output is of phosphatic nluck requiring 'libt'le or no washing, t,he foregoing figures,, largely from Census data, reveal a great inlprovement in efficiency. It is unfair t,o corn11:tre operat,ions in Tennessee tvith t,l~ose in Florida. In Florida costas are lower nntl output per ma11 is about three tinlev a.s high as in Tennessee, but in Teiine~see mining c~nclit~ions are Inore difficult nncl so mucli larger n proportion of rock is recoverecl in fine sizes

18 1 that more treatlnellt is required. I Tennessee, though furnishing only 22.1 per cent of the total national'output,, employed 41.6 per cent. of all the workers in the phospliate industry in the Unite'd States.,. Su~nn~nrizing t,he progress out,linetl briefly in t,his paper, nre fincl that better recoveries have been quitme as important as improved nlechailical equipment,. Of first importance is the progressive recovery of finer and finer sailtls by better sett,ling, later sul>plementecl by flotat,ion, and, finally the increasiug utilizat,ion of unwaklied or only pa.rt,ly wa.shed matris by furnace processes. Details in t,he concentr:tt,ing c-lepartment, include finer screens, better nleclianica~l classifiers, - and better sett,ling equipment, together with the placing of inlportant machines in t,andem so as to make their work more effective. hlechanically, too, the washers 'are bett,er designed, and hand labor is being n~ininlizecl by more conveying, elevating, ancl punlping ecjuipment. Ilinsrnuch as lnost, of the mills have been rei~lodelecl instead of rebuilt, they clo not have the immaculate appearance of modern design, but they are nlucll improved from'the -stanclpoints of both efficiency and safety of the workers. Economies in the drying step have been lnainly in nierlianicd stokilig and, where possible, larger units, coml:)inecl with- more at,tent.ion to handling the rock into and out of the sljell. More attention to mechanical control and ulliforln operation coiubined with grntlillg and blending f:icilitiesaided at some plants by the possibility of lnaking high-gracle flotation concentrates ('for sweete~lilig~~-has resulted in the production of more unifornl products and better ability to cater to the needs of discriminating buyers.. The increasing sale for sands aid muck containing under 60 per cent B.P.L. is a further fnct,or, not only br6ndenil?g the range of proclucts that can be sold but, actually helping to increase the proportion. of high-gtacle concentrates t,liat bring t,he highest prices. Out, of this lnny colne a well-defined tbacieiicy to elirnii~at~e micldle grades and to procluce for the general chemical market and acidulating plants highgrade rock carrying well oyer 73 per cent B.P.L., using the remainder of't,he mashed sands to blelltl mitli unnrnshecl m:tt,ris for use in making.. furnace siuter. If the dell~aild for sinter grows sufficiently,, such a trencl would inean bigger profits for operating companies and better conserva- tion of a ~~alnnble national resource in t,his region, whose locatio~l rnnkes the so~newllat limited reserves of estraorclinary economic value. Apart from t,he displacement of hand by machine escavntion, technologic c.hn11ge has lincl less effect on mining operatiolls t,lian on washing. klost tleposits either are too snlallor have not sufficiently favorable bedrock coaclit,ions to permit the use of much larger equipment with the,.accornl~a~ying economies that have been gninecl elsewhere. Eveu a 4-yd. bncket may he too large ancl :it some pits a 11?-yd. bucket may be nlore econoinica.1 because it is so much more flesible, gets don711 into the. cut.t,ers and into crevices in the horses, and so recluces llnnd shoveling.

19 PAUL M. TYLER AND HERBERT R. MOSLEY 19 Progress in mechai~izat~ion can be measurecl by the yardstick of power equipment. In 1902 the total of all prime Illovers iristalled.was 1410 hp. By 1919 it had jii~n~ecl to 7070 hp. In 1929 it clecliiied slightly, to ' 6639 hp. but meanwhile the horsepower of electric motors operated on purchased power hacl jumped from none in 1902 to The total power installation increased from 7168 hp. in 1919 to 11,811 lip. in Subsequently, further large increases have occurrecl ancl while steam or Diesel power is still usecl on shovels and for.hauling from the ~nines, inclivicliial power plants have been abanclonecl beca,use cheap electric power no\\- is svailable everynrhere for purchase. Per wage earner employecl, the installecl horsepower has grown from 4.61 ill 1919 to 9.57 in 1929, more tl~nn doubling in 10 years. Although none of the Tennessee operations can be colisidered large compared with the big Florida enterprises, the industry has become concentrated in the hancls of the folloning competing companies: Armour Fertilizer Works; Charleston R'Xining Co., Inc.; Fecleral Chemical Co.; Hoover & Mason Phosphate Co. ; Iilternational Agricultural Corporation; a,nd Monsanto Chernical Co. These six companies have coi~tributecl over 90 per cent of the t>otal output, of the state since 1928 ancl t,ogether with T.V.A. have ~~roclucecl o17er 98 per cent since 1936,~although t1leburenu of Mines has canvassecl regula.rly a list of 25 to 30 more or less active operators. The Census Bureau, ignoring operations contributing less than $2500 worth of products, reported 12 enterprises and 12 mines in 1929 comparecl with 19 enterprises and 23 mines in In 1919 the total capital invested was only $14,657,494; it has increased greatly since then but no figures are available. Alnlost the last of the sn1a11 mines discontinued operations in 1938 but even at present, although few enterprises eniploy fewer than 100 wage eariiers, none employ as nlany 3.s 250, at least in corijunctioli with their ~nining and washing operations only. All these companies except Hoover & Rqason are largely integrated, producing superpl~osphate and/or phosphate chemicals, and elren Hoover k h:[:tson 11a.s gone into t,he marketing of ground phosphn.te rock to farnlers. Whereas 37.5 per cent of the value of the Teiiilessee phosphate rock output was spent for wsges in 1919, the proportion in 1929 was only 32.5 per cent, notwithstancling $100 increase in average annual earnings per worker-from $750 in 1919 to $850 in During the latter year the highest average earnings ($938) were received by workers in plants of nledium size (51 to 100 employees), where employment was more regular than even at the largest plants. After 1929 wages clid not decline, ancl until about 1935 the general level for common labor remained at 1215 to 17.!& an hour. Later, 2lomever, it rose to 25b, and after the C.I.O. strike the minimum was raised tmo'30$ an hour niitll sorne cdnlpanies paying as high as 40$. 'I'he wages of shove1,operators range from 70 to S5$ an hour,