Process water recovery at a deep navigation colliery

fhkds

By G J C Head of Research and Development. Richard Mozley Ltd, Cardrew, Redrulh, Cornwall, UK This paper was presented

lo the Filtration

Society (by Richard Mozley) al a meeting on Control of Air and Water Pollution Processing fndusfries, Coventry, Ccfober, 1989

in the Minera!

Thickeners have traditionally been used to dewater tailings streams containing fine coal from washeries. When British Coal built a new Baum Jig washery. at Deep Navigation Colliery in Wales, UK, instead of a thickener they installed high performance hydrocyclones and horizontal belt filters to do the same job. Advantages included lower capital cost, easier operation and maintenance and considerably reduced space requirement. The hydrocyclones dewater and thicken underlows from dewatering screens and centrates from centrifuges. The filtered hydrocyclone underflow forms a fine ccral product. Hydrocyclone overflow is returned to the jig as make up water. The paper describes in detail the design and operation of the washery circuit, highlighting the benefits gained from the use of hydrocyclones. IN ~t~t:N’r ycats much rcscarch h;ls been cwricd out into the USC of high cfficicncy classifying the newly dcvcloprd small diumctcr. Coal’s hyrtrocycloncs for the trc‘atmcnt of coat fines (I. “. British research nnd dcvclopmcnt arm - Hc~idquartcrs Tcchnic:tl Dcpnrtcmpirlcnl process models which mcnt (HQTD) - h:tvc dcvclopcd ClCitrly demonstrate the cconornic :tdV;~nIngcs to IX gained by the use of i~ydrocycl~~n~s in pl:tcc ol the more conventional thickoncrs or froth flotation (.l)

In IOX7 British Coal replaced the under-capacity Chance cant sand me&:\ wushury at Deep Nnvi@on Colliery in South Wolcs. UK, with a new pre~ir~iti~n plant built around a three c~~nlp~lrtrn~nt Baum jig. Foliowing the advice of HUTD the new plnnt was dcsigncd without thickcncrs or flotation but with an instillhltion ol 25Omm diamctcr high pcrform:mcc hydrocyclones to do the same job. The plant wus complctcd and commissioned during I%7 :md hns
AL PRODUCTS FIRE COIL TO AQERTHAW POWER

Fig 1. Deep

Navigation

Colliery

Coal Preparation

Plant Flowsheet

1. Skip Hoist 2. Bunker 3.125mm Screen 4. GEC Rotary Coal Breaker 6.25mm Screen 6. Ro-pro Sc:*?ens 7. Bunker 8. Suam Jig 9. Shale Elevators 30. Heavy Middlings Elevators 11. Light MiUdlings Elevators 12.lOmm & lmm Double Deck Middling Screen t3.8JD Chzher 14.1 mm Static Screen 15. lOmm 8 Imm Ooubte Deck Product Screen 16. Ciassilying Screens 17. Rail Wagons 18. Crusher 19. 1Omm Screen 20. Don Valley Centrifuges 21. Sump 22. 12 x 10 Hydrocyclone Feed Pump 23. Mozley Hydrocyclones 24. Baum Head Tank 25. Degritting Cyclone 26. Br;:fer Tank 27. Dorr Oliver Horizontal Belt Filters 28. Sealing Tank 29. Vacuum Pump 30. Clean Water Tank 31. Spray Tank 32. Bretby Mixer

March/April 1990

Filtration

& Separation

Allis-Chalmers bunker.

and

Ro-Pro

screens

goes

to a 150 tonne

ROM

Wut I’rcutmcnt. Ril\v con1 is fed from the bunker to-a three compartment Unifloc SilUlll jig with a capacity of 300 lonncs per hour. .Opcration is controilcd by u micro chip processor with a Hurst pneumatic system as back-up. Rchrsc from all corn artmcnts is removed b perloratcd bucket clcvators. From the Plrst compartment, shn lyc is rcmovcd and conveyed 10 ‘rc’ccts’. Refuse from the second compartment. known as ‘heavy mid b*Imgs’ may bc either conve cd to rcjccts or to an Allis-Chalmers lowhcad douhlc deck mid * Y-Imy screen fitted with IOmm and lmm apcrturc decks. Rcfusc from the third compartment. known as ‘light middlings’. is cithcr combined with the ctcan coal product or sent to the middlings screen. Screen oversize is crushed in a BJD swing hammer crush*:r and rcturncd to jig feed. Imm oversize is conveyed to rcjccrs. Irnrll undcrsizc goes to the hydrocyclonc feed

Fig 2. Thlckenlng Cyclones at Deep Navigation Colliery Circuit

The washcry circuit is divided into rhrcc basic sections: prctrcutmcnt; dry fines treatment: wcl trcatmcnr and wet fines trcatmcnt (see flowshcct.

Fig I).

I’retrwtment. Run or mint

coal from the skip winder or rcctaimcd from ground stock is [cd by conveyor 10 a 12Smrn (Sin) apcrturc screen. Screen ovcrsizc is trcatcd in :LGEC rotary coat hrcakcr. the targc material (shutc) from which goes to tip whilst the hrokcn material and screen undersize goes to dry I’incs trcatmcnt. Dry Fines Treatment. Coal from the prctrcatmcnt stugc is trcatcd on two Allis-Chalmers Rip!-Flo screens scalping at 2Smm (lin). Undcrsizc goes to two rotating probability screens (Ro-Pros) rhc cut point of which. hcncc ash content of the undersize, can bc adjusted by varyinp the speed or rotation I-(‘, Ro-Pro undcrsizc (known as ‘dry dul’l”) IS convcycd to a Brcthy miser whcrc it is hlcndcd with other lines bcrc,rc dispatch to Abcrthaw power station. Ovcrsizc from the

Wilt

Imm aperture static product screen littcd with 1Omm and Imm apcrturc decks with sprays. Screen ovcrsizc is convevcd to a classifvlne screen which eradcs the coal into four prod&s (cohblcs, I;&@ nuts, small n;ts and peas) which arc deposited directly into rail wagons by boom loaders. Thcrc is also a facility for crushing everything finer than IOmm if .icquircd. for blcndmg with the power station supply. Product screen 1Omm undcrsizc (Imm oycrsizc) is dcwatcrcd in two Don Valley horizontal vibrating ccntrifugcs and comhincd with the ‘dry duff’. lmm undcrsizc goes IO the hydrocyclonc feed sump. Wet Fines Treatment. lmm undcrsizc from the middlings, static and product screens. to vcthcr with centratcs from the centrifuges and filtrates from the belt blltcrs. , all go to the main hydrocyclonc feed sump. A 12 X 10 Warman centrifugal pump delivers the slurry to 12 Mozley Ten Inch high performance hydrocycloncs. Hydrocyclonc ovcrilow goes to a large header tank from which Baum jig backwater. screen spray water and taundcr flush water is drawn. Hydrocylonc undcrllow is stored in a stirred buffer tank which Cccds two Dorr Oliver 10m x 2m horizontal belt filters each with a design capacity of 17 tonnes per hour producing a cake with 25% moisrurc. Flowrate to the filters is controlled by automatic valves actuated by K-Ray solids concentration measuring devices. Magnalloc 156 llocculant. together with the cationic llocculant Magnalloc 1597. if necessary. is mixed in an Allied Colloids ‘Auto Jetwet’ polymer dissolving unit and dclivcrcd into the feed line by Mono pumps. Rotamctcrs and valves govern the flocculant dosmg rate allowing control by the operator. The speed of the belt filter is controlled by an inverter panel rather than a ‘Carter’ gcarhox. Tracking of the cloth is controlled by photo clcctric cells driving a linear motor acting on a roller. The cloth is cleaned by several banks of sprays on the front rollers, the water for which is pumped either from the clean water head tank or direct from mint water. Sealing water for the vacuum pumps is also drawn from the clean water head tank. Filter cake is blended with the dry duff and centrifuge product for delivery to the power station. Operation-Fines

Circuit

Ash content of the ‘duff’ from the ccntrifugcs runs steadily at bctwecn 6”/0 and 8%. The belt filter product runs at between 20% and 24% ash. Ro-Pro screen undersize (‘dry duff) has a variable ash content of anything up to 47% which is controlled by varying rhe speed of rotation. hence cut point, so that the ash content of the finished blended product (feed to Aberthaw power station) runs at 19.6% ash Thickening Cyclones Construction. The ihickening

cyclones consist of twelve Mozley C630 unitscs fitted to a polyurethane lined overflow collection tank with integral feed distributor. Ovcrnow tank and walkway are mounted on top of a polyurcthanc lined underflow collection tank (Fig 2). Stainless steel shells fitted with polyurethane liners are .used for hydrocyclone bodies and body cxrensions. The body liners are divided into upper and lower sections enabling replacement of the lower section (which is in the region subject to the most severe abrasion) without having to replace the whole liner. Vortex findcrlinlct caps arc manufactured in cast polyurethane. Dimensions. Dimensions of the individual cyclones are as foil~ws: Overall length lntcrnal diameter Length of cylindrical body cxtcnsion Included angle of cone

2.2m 250mm 0.4m 10 dcg

The complctc installation measures 4.6m high by 3.3m diameter (4.4m diamctcr to the outside of walkway). Operation. Operating at a pressure drop of 35psi (24OkPa) each cyclone treats I Ijm’lh of slurry. D50 cut point is between 50 and 70 microns. coal hasis (dcpcnding on feed slurry solids concentration).

Fig 3. Elliciency Curves for Mozley Ten Inch Hydrocyclone Filtration

& Separation

March/April

1990

equivalent to 32 and 45 microns ash basis. Typical efficiency curves arc shown in Fig 3. The hydrocylonc installation treats a total Row of 138Om~‘/h. Feeti solids concentration varies bctwccn 10% and 20% solids W/W. wi:n

119

.

typically.SO% of the soiids finer than 38 microns and 90% finer than 250 microns. The underflow is run at the rclotivcly low solids cdncetwtion of 30% to 35% solids w/w. in order to maintain a fine cut point and to ‘bleed ofT ultrafines which hnve no other csca c route from the circuit. By this means. the circulating load o P fines nround the washery, and in the hydrocyclonc overflow. stabilises at a solids concentration of 8% to 12% solids w/w. The solids in the overflow t picnlly huvc n size analysis of 90”/0 finer than 38 microns and 9 J .g% finer than 250 mtcrons. The underflow solids are IS”/0 finer thtm 38 microns and 80% finer than 250 microns. Over 90% of the coal coarser than 75 microns is recovered to hydrocyclonc underflow. Maintenance of the hydrocylonc installntion is Maintenance. minimiscd because there are. of course. no moving arts. it amounts to occnssional rcplaccmcnt of worn components Pset below), and removal of the occasional piece of tramp oversize blocking an inlet (usually found to bc picccs of ceramic tilt chute lining measuring over 50mm m diameter). Component wear life_ It WIS antici xttcd thnt as a result of the relatively coarse nature of the feed so I*Ids (up to lmm), wear of the hydrocyclonc components would be n problem, hence the selection of hydrocycloncs manufactured in polyurethane. a synthetic rubber renowned for its abrasion rcsistancc. Having operated for a period of 20 months, it has been found that although wear in the vortex finder cap, body extension and upper body liner !zzve been minimal, wenr in the lower body liner und spigot insert (which arc made in an 80” Short grade of polyurethane) has resulted in replacement of these components at four monthly intervals. As B result of this, comparative trials arc at present in progress using lower liners nnd :i igot inserts manufactured in n), a softer, 60” Shore polyurcthanc. bP , 95% alumina ceramic and. c), a newly developed metal reinforced ceramic material. Initial results indicate that the 60a Shcre polyurethane lasts 50% longer than the 80” Shore polyurcthnne and thnt wear of both types of ceramic material is minimal. It is anticipated that USC of ceramic lower body liners and spigot inserts will virtually eliminate problems with wear.

Conventional

Wet Fines Treatment Circuit

Table

i. Comparative

Capital Costs lnstelted hydrocyclone equipment

Capitat Cost. pounds sterling : Floor Area Reouirement. m2 :

50,QOO 15

Alternative conventional equipment 150.000-220,000 177

which the ‘top water’ or ovcrffow, contnining the finest size fractions, would have been used us Buum box back water. The thickened slurry from the bottom of the settling tower. containing scttlcd-out coarser solids, would have been pumped to the thickener. Thickener underflow would huvc been dcwntercd on a horizontnl belt filter (similar to the one installed) and the overflow. or ‘clnrificd water’ would have been used for sprays etc. Table I compares capital cost and spncc requircmcnt for the two alternatives. It demonstrates that the advantages to bc gained from the use of hydroc clones include considerably lower capital cost and less than

IOU/u oft Y,c floor

nrca rcquircmcnt.

Conclusions

High performance hydrocycloncs are being used successfully at Deep Navigation colliery in place of ;I conventional settling tower and thickcncr to trcnt wet fines. Advantngcs include significantly reduced capital cost. grcutly rcduccd floor arca rcquircmcnt and minimiscd maintcnoncc.

HEFERENCFS I. Brookcs. Ci F Jt Vickcrs. F. l’hcorv and Pructirc nf Fine Cod Circuh Design. In! Bnginccriag. Mmprcp. Symposium on Innovalivu I’klnt :d I’nrccsscs for Minurnls Doncancr. M:~rcti/April IYN7. 1. Rouec. B D. Claycon. J S and Drtrokus. G F. Conlirm:ltion 01 MoJclling Technic ucs lor 3rd Im Ccwlcrcncu on Ilydwcyclones. Oxfor d VU. Sm:llt Di:bmclcr Cycloncr. SeptcllthurlOcc~llrcr 19X7.

.

The conventional circuit, which was considered as an alternative. would have used a 9m diameter, 23m high settling tower. and a 12m diameter thickener in place of the hydrocyclones. The fines would hnvc been pumped to the settling tower from

HAVER POROPLATE is a laminate made from metallic woven wire cloth, bonded together by diffusion (sin’cering). Poreaize and oermeabilitv can be predetermined. POROPLATE made from Stainless Steel is generally fabricated into filterplates, centrifugal baskets or filter tubes. Particle-free filtration in the range from 150 to 1 micrometer is possible. HAVER 8 BOECKER Wire Weaving Division since 1887 P.O. Box 3320. Ennigerloher Str. 64 D-4740 OELDE Westfalen Federal Republic of Germany Phone: + 49-25 22-301 Telex: + 89476 haver d Fax: + 49-25 22-30 404

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MarchlApri

I 1990

Filtration & Separation