648
INDUSTRIAL AND ENGINEERING CHEMISTRY
public or industrial water supplies and for sewage, not for waters of the types and concentrations of acid mine waters. T h e investigations referred to have already developed some d y t i c a l procedures which, ditrering fiocthose of the past, give promise of yielding h f o m t i o n much needed if the problem of acid mine d r w is to be d v e d . SILT CONTROL
i
1
I n Pennsylvmia, where practically all of the anthracite is located and most of the silt is pbduced, the Water Board called upon all the operators to remove settleable solids from colliery waste waters before discharging them to stream The board has received hearty cooperation from the collieries. Silt removal is construed as meaning not less than the equivalent of the removal effected by one hour of e5cient quiescent sedimentation under laboratory conditions. Because of the high specific gravities of the so&, this sedimentation wiU remove all but tbe colloidal particles. With properly designed devices, a nominal %hour retention, or, in the case of meohanidly cleaned clarifiers, an upward velocity not excee&ug two feet per hour, will generally meet requirements and give an effluent with relatively low or almost eero turbidity. Special requirements apply to colloidal mlids. Many collieries have already stopped the discharge of silt by devicea ranging from simple silt basins (as contrasted with their former silt banks) to mechanically cleaned 180-foot diameter clarifiers and froth flotation p h t s removing to minus 200 Fesh. One company's installations serving three collieries arb reported to have cost over $1,500,000. Another company's two plants are to cost over $300,000. Stoppage of silt discharge is well in band, and work is begiwhg in Pennsylvania on removing the silt now in the Schuylkill, one of the most heavily silted streams. As of
Vol. 39, No. 5
1936, the United'States Army Engineers estimated (I)that there were 24,000,000 cubic yards of silt and culm in the river above Faimount Dam a t Philadelphia, with a 650,000ton annual influx. In 1926 experienced geologists (6)estimated that there were, all told, 900,000,000 tons of the material in the Schuylkill and Susquehanos. Rivers. For many years coal washeries and river d r e d w have been reclaiming the h e sizes of coal washed down from the coal fields. At Harrisburg in the 19twelve-month, k central generating and B central s t e m heating p h t together burned 230,330 tons of this river coal, according to the Pennsylvania Power and Light Company. Today the washeries are gradually going out of busineas, however, and the days of blocked sewers and silt-Nled stream c h e l a are paasing. Let us hope that, through d a r coopsration between the coal operators, public 05cid5, and research workers, a way will be found to stop the piwent noas pollution of acid mine waters. IITERATURE CITED
(1) Bmrd of E h g h ~ r for s Rivers and Harbors on Schuylldll River,
Pa., House of Representttiivw Domment 183,76thCongr.. 1st Qnm&*"
.-..,
2) Carpenter, L. V.. -d
Herodon, L. K.. W. 5's. Univ., College o l Eng , Bun. Series 19, No. 2 (Sepr. 19331. (3, Rerndou. 1.. K..and Hod-, \V. W.,Ibrd.. 37, No. 2-1 (Aug > 01*\
W"",.
(4) Hodge. W. W., Ibid., 38,No. 7 (Jm. 1933).
(5) Pennsylvania Dept. of Health, Rapt. on Sealiug Absndoped Coal Mines, July 1, 1938. (6) Bider. J. D., Frsser, Thomas, and Ashrnesd, D. C.,Topogrsphic and Gedwc Survey of Pa.. Bull. MI2 (1928). (7)U.8. Dept. of Interior. Minerals Yesrbook, 1944. (8) U. 8. Public Health Service, Ohio River Pollution Control Re&. House Dooument 266,78th Congr., 1st Seasion. P R E B - ~ before the Industrial Waste Symposium st the 111th Meting of the Arssucm Casrraar. S o a ~ mAtlantio , City, N J.
Industrial Wastes
. COAL WASHERY PLANTS W. JULIAN PARTON Lebigh Navigation Coal Company Incoqmrahd, Lanaford, Pa.
A
NTHRACITE has been mined in Pennsylvania for over one hundred years. The industry bas increased in siee until the annual production was approximately 56,000,000 tons in 1946. Incident with the mining and preparation of anthracite for market, much undermzed h e coal for which there has been no market is produced and has to be discarded. In addition to tlus unmarketable h e coal, considerable tonnages of reject material become mixed with the raw coal in the mining operation and must be brought to the surface. After separation from the coal this reject mateyial must be stored on rock ban@. The disposal of these waste materials has become a serious problem in tbe anthracite region, since much of the coal occurs in narrow valleys which have become congested by the mining operations and the towns. Large amount8 of both the unmsrketable h e r a k e d coal and refuse rock have accumulated near the mining operatiom. A8 a result of thegbortage of disposal areaa and because of the extreme di5cdties of retaining the h e s t - s h d partides, some fine coal finds its way into the streams. Erwion from old b& of accumulated fine coal wastes is another source of material which enters the streams.
After the coal is mined and brought to the surface of the ground with ssmciated impurities, it is sent through "breakers" or preparation plants where the coal is separated from the impurities and sized for market. In the early days of the industry all the coal was prepared dry, and only the large pieces were marketed. The fine~izedmaterial was discarded to waste piles. AB the output of the industry increased during the nineteenth century, uses were developed for somewhat d e r sizes down to and including pea coal. Early in the twentieth century, sises smaUer than pea were developed and used, but it was not until after World War I that preparstion of the so-called steam sizes began commeroially. This was largely No. 3 buckwheat which is screened over a '/ainch round-hole screen. At preAent various sizes dorn to and including No. 5 buckwheat are prepared from current mining and bank reclaiming operations. No. 4 and No. 5 buckwheat and smaller sizes are generally used for steam production in large boilers. Since 1907 processing of anthracite has undergone extemive changes as a result of market demands for cleaner coal and the gradual, but progressive, utilization of tbe h e r sizes. Wet washing methods were developed which used water to effect separation
May
I W
INDUSTRIAL A N D ENQINEERING G i i E M l S l i i Y
h4
of the refuse from the coal. During the cleaning operation the bne-sized coal becomes suspended in the large quantity of water required for the cleaning proceea. A survey (6)indicated that the weighted average of silt or fine mlids in the breaker discharges waa approximately 13% of the marketable coal. The proportion of silt or h e solids is much higher in the southern and middle anthracite 6el& than the average figure for the whole industry because of different geologic and mining conditions. Without doubt the problem of treating b r a & waste water m d the diapoaal of the solids is of considerable magnitude. The objectives of the industry in the treatment of the waste discharges have generally been aa follows: to recover and prepare for market aa much of the bne-siaed coal as possible to satidy the increasing demand for this materd; to cowrve the remaining material by storing in such a manner that it can be salvaged to meet future demands; and, by removing these solids from the breaker waate discharges, to clarify the water to a point where it can be discharged into the streams or re-used in the cleaning processes. PBEPARATION O F COAL
The run-of-mine coal is prepared for market by separating from the coal the particles having a high a& content and screening tha clean coal into fractions of various sizes. These operations are accomplished in breakers or cleaning planta. The various market s i w of anthracite are given in Table I. Many Merent cleaning processes or combinations of proceases are employed in the oleaning planta to separate the cod from reject material. AU the processes in general use depend on the difference in speci6c gravity between the cod and the refuse to accomplish the separation. The system employed a t one operation is as follows: RBw &em cod is dumped into a pocket a t the top of the cleaning plant, from which point the coal flows principdy by gravity throngh the plant. Raw material from the pocket is passed over bull shakers equipped with screen plates with 6-inch round perforations. Makin1 over the shakers (lump size) is
Table I. Market Sizes of Anthracite Frsation Name
Round-Hole Sueen Openiw, Inchaa Thmwh O"e*
.
Figure 1. Silt Settling and Impounding Dam
discharged onto platform picking tables where rock and other refuse are separated by hand. The rejected material from these tables is by-passed to refuse bins, whereas the lump coal is ground or crushed to egg and smaller by toothed rolls. .Material smaller than 6 inches that passes through the bull shaker is then combined with the crushed hand-picked material and is fed by conveyors to main reciprocating screens. Egg, nut, and pea siaes from these screens are sent to a sand-flotation cone, 18 feet in diameter. In this sand flotation prows, coal is separated from refuse in an artificial heavy medium of sand suspended in water. The specific gravity of this medium is such that the merchantable coal flmta while the rufuse Sinks to the bottom. * Both the clean coal, which flows over a suitable lip with an excees of sand constantly fed to the cone, and the refuse, which sinks to the bottom of the cone and is witbdrswn through a double &e, are passed over separate screens for removal of the remaining sand from these products. In addition to desanding the clean c o d product, shakers screen the coal into broken, egg, stove, nut, and pea sizes. These individual sizes are then delivered to their respective loading pockets. In a similar manner, buck and rice siaes from screens are cleaned in c o w and screened into separate sizes Barley size, also made by the main screens, is sent to a Hydrotator coal cleaner, 7 feet in diameter. The Hydrotator is -uti-
.
From the standpoint OEs8muu d-
the cod and d' rook minerals &re stable.
*
...
- ~ ~ - ~ ~ , f r o i this inRecommen'dations ~~
vestigation were of considerable assistance in increasing the effectiveness of settling facilities and preventing erosion of hanks into streams, s o that the two sources of mine-waste pollution were cheoked. 6. Plans far improving the settling and dispassl facilities for hesized mine wastes have been prepared by the various operators. Same of these improvements have been completed, some are under construction, and the remainder will he made as soon as the n e c e s s a r y e q u i p m e n t becomes svailahle. TREATMENT FACILITIES
WASTESILT SEITLINQAND IMWhen the waste solids oennot be prepared rtnd marketed, they are usually discharged into surface basins where the suspended solids are settled' and impounded. Storwe of the solids in this manner makes it possible ta recover the cod at some time in the future when the market will absorb this fine material, Figure 1 shows a silt settling dam for storing these waste solids. The location, size, and construction of settling dams for disposal of silt are governed principally by the quant.ities of fine sizes and volume of rrater to he handled, the topography of the areas available for the location of the dam, and the material zvailahle for the construction of the berm or breast to confine thc slush. All plans should allow a progressive expansion in height or area to maintain continuous and efficient settling areas. POUNDINQ BASIN.
INQUSTRIAL AND ENGINEERING CHEMISTRY
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May 1947
INDUSTRIAL AND ENGINEERING CHEMISTRY
hIaterial for the construction of the breast should contain a certain amount of fine sizes to fill the voids and prevent undue leakage. Cleaning plant refuse is generally acceptable for this purpose and is usually used. Sometimes the formation of a layer of silt over the breast of a dam is necessary to serve as a filter bed to prevent, undue loss of solids through the breast, when too much coarse material is used in its construction. The successful operat,ion of a settling dam depends on the formation of a quiescent settling pool of sufficient area t o allow the finest solids to settle. When sufficient pool area is provided, clear water can be removed through suitable overflow drain boses. ..iceording t,o W ,C. Muehlhof and L. D. Lamont, a 2hour retention time in the pool and a depth of 20 t'o 36 inches at the uverflo\v point are generally required. vertical drain bos or well is usually used to remove t'hc clarified water from the surface of the dam. B pipe line running from the base of the well under the breast of the dam discharges the water a t a point beyond the limits of the foot of the dam. Concrete, cast, iron, or creosoted hardwood pipes can be used for the horizontal drain under the breast. The vertical drain pipe or wooden overflow drain box must' be a t a distance inside thc. breast to permit extensions vertically to the ultimate height of t h e settling dam. A subst.antia1 base must b e provided t o support the vertical section of the drain. One drain for evcry 1000 gallon. per minute of overflon- is advisable. ~IECHAZICAL SETTLING TAXKS.In many instances me%chanical settling tanks or thickeners arc used t o produce a t'hickened sludge from the waste solids and a clarified overflon of excess water. Thickener tanks are circular and have relatively flat bottoms. Slowly revolving rake mechanism are used to move the settled sludge to a central point of discharge. h thickener 180 feet in diameter is shown in the foreground of Figure 2. The settling tank can be constructed near the operat'ions to minimize the amount of pumping required. The clarificd Lvater is usually recirculat'ed for coal cleaning but can be discharged direct to the streams. The settled solids, no\v in a concentrated condition, arc pumped t.o a storage dam with a minimum of pumping equipment and power because of the decrease in the volume of material to be handled. Also, more efficient operation of the wast'e impounding dam is possible because of the reduced volume of water to be handled. Smaller pool areas and fewer overflorv drain boxes are required. Figure 3 shows a layout for the treatment of Coaldale breaker waste n-ater by using a 180-foot thickener tank in conjunction with a silt-impounding basin. The clarified water from the thickenw tank is returned to t.he breaker for re-use. Operat'ing results'of this settling tank arc as follows: Kaste-water thickener feed, gal./min Solids jn feed, % Solids in feed, tons/hour Clarified oi,erflow, gal./min. Solids i n overflow, % Sludge underflow, gal./min. Solids in sludge, Yo
solution is used to treat 8100 gallons prr minute of water before it, enters a thickener 100 feet in diamet,er. The overflow from this tank carries 0.02 to 0 , 0 6 5 solids, and the underflow sludge carrier 17.0Y0 solids by a-eight. RECOVERY FACILITIES.Orie of the most recent' developments in the recovery of fine anthracite which is discharged from t h e cleaning plant with the waste water is a froth flotation recovery plant (5')at the Tamaqua Colliery of this company. I n addition to the recovery plant, facilities are provided for the complete rlarification of rvashery water, recircuIat,ion of this clarificd water for breaker use, and t,he disposal of the refuse solids in a waste impounding dam. Figure 2 s h o m a vimv of the settling tanks and t,he flotation plant. The froth flotation process used to clean the coal sludge to pniduce a marketable product is a wet concentration method; t t e desirable mineral (coal in this case) is caused t o flmt to the surincc. of a body of pulp rrhile the refuse remains subniergcd. Bubbli~* are used to elevate the mineral to the surface in the form of a froth or foam. The valuable mineral adheres t 3 the bubblos 11+ muse the surface of the mineral remains unwel ted, whereas the, refuse particles become wetted and remain in the pulp. Thib preferential action is due to the different pol:irit,y of the SUI>ytances. The wettability of a mineral surface can be controllid hy using reagents which selectively coat the niincral with a filna of the desired pnlarity. I n froth flotation of coal, oil is used lit:cause it has an affinitp for carbon and a n aversion to the u-ual inipuritics of coal. Figure 4 is a flow diagram for the complete recovery pro)ci:t. All the waste water from the cleaning plant is collected in sump :t outside the breaker structure; from there pun-~pB delivers it at 7000 gallons per minute t,hrough a 16-inch line to hydroseparaior C' (75 feet in diameter). The classifier is set to give a separation at approsinlately 200 mesh. The minus 200-mesh overflow is discharged by gravity int,o 180-foot thickener 0 for final settling of the solids. The classifier underflow sludge is pumped to the plant by the 6-inch quadruplex diaphragm pump E. Hydrobeparator F (10 feet in diameter) is used to nialie a second classification on the sludge solids a t approsimate!y 28 mesh. The underflow from this classifier containing the coarse particles is olt~vatedby 6-inch diaphragm pump, G, and is discharged on two 4 X 6 it. vibrating screens, H . These screciis are dressed n i t h appm\;imatc~ly 28-mesh cloth. Screcxn ovt>wizc product flou 3
7500 6.1 115.0 5700 0 05
1800 21.5
Reagents are used in some instances in the viash R-at'er to increase the rate at which the solids settle in thickening tanks. Lime and starch solutions are most commonly used for this purpose. Coalescence of the finely divided particles in water to form floccules or aggregates of solid particles results in a much more rapid settlement of the solids in water. K h e n a suitable reagent is used, the size of the settling tank required for a given flow is considerably smaller. At one colliery a causticizcd starch
653
Figure 5.
F r o t h F l o t a t i o n Cell i n Primar) Circuit
652
INDUSTRIAL AND ENGINEERING CHEMISTRY
Table 111. Size Analysis of Flotation Plant Product Wai&. % 0.3 1.0 2.7 4.7 9.7 12.3 14.0 18.6 16.0 9.0 12.8
,
,I
\
C"mvlati"e Weight. % 0.3 1.3 4.0 8.7 18.4 30.7 44.7 83.2
78.2
87.2 100.0
directly to dewatering equipment. The screen underflow and the overllow from the IO-fmt c h e d e r flow to the flotation circuit. The flotation feed is conditioned in 12 X 12 foot agitator tank, J , at B pulp density of 1to 2.33 (30% solids). Reagents are fed to the circuit by reagent feeder 1. The conditioned pulp is then diluted to a pulp density of approximately 1 to 5 (18% solids) and is fed to two bank@of four froth flotation cells K (capacity, 100 cubic feet each) for priinary coal recovery and cleaning. These cells are shown m Figure 5. Additional reagents from reagent distributor L are added to the feed weir box of cells 2,3, and 4 of m h bank of flotation machines. Clean coal removed from these cells flows ,directly to the clean coal sump. Dilute tailings from the priniary cells are rethickened in thickener M (pfeet in diameter) and are then retreated in a scavenger circuit for final recovery of coal. The scavenger circuit consists of a 6 X 6 foot agitator tank N,and two bank8 of four froth flotation cells 0 (cqmity, 24 cubic feet each). Coal froth fiom these cells flows to the dewatering equipment, and the tailings flow to a vertical refuse pump which delivers it to waste thickener D. Dewatering of the coal products is ~ocamplisbedin a 54 X 72 inch Bird centrifugal filter P. The cake product is dropped onto a conveyor and is loaded directly into cars, until the installation of an indirectdirect rotary hest dryer Q'(11.5 X 60 feet) oan he made to dewater this product further. The effluent from the
Vol. 39, No:S
centrifuge and the overflow from the plant %foot thickener are pumped to the conditioner t a n k for dilution water. The c h i fied water from waste thickener D is delivered to the breaker for re-usc hy the 500C-gallon pump, R. The thickened waste underflow solids are delivered to impounding dam S by sludge pump T delivering 800 gallons per minute. The recovery plant produces aRprogimately 500 tons of h e mthraciteper day (two shifts) with an ash content of 11 t o 13%. The siee analysis of this product is given in Table 111. In addition to recovering marketable cod, the waste water is clari6ed by the thickener tank. The overflow of this tank carries 0.01% solids. Mast of this overflow. 5MM gallons per minute, ia returned to the breaker for re-use. CONCLUSION
Coal operators are minimiaing Stream pollution by retaining mine waste solids a t the operations. Considerable information
and data have heen obtained by research and development work on means of preventing pollution rn well as means of recovering a marketable product from the waste solids. Several new recovery plants and impounding facilities have recently been completed. When construction materials become available, additional facilities will be installed to reduce further the pollution.cauaed by find-shed mine wastes. LITERATURE CITED
(1) Muehlhof, W. C.. and Lamant. L. D., Am. Inat. Mining Met. Engrs., Tech. Pub. 1959 (1945).
T ~ o n sAm. . Inst. Mining M a .
UlYRlEI"
1
Stock Piles of Coal Recovered from Streams below an Anthracite Field
PEI*."L"AN,"
DEPA*ITNINI OF " € U T *
