Design Features of the WRONZ Comprehensive System Design
A number of factors influenced the basic design, some related to existing (and desired) emulsion scouring practice and others arising from the need to improve certain aspects of the process.
The initial design sought to accommodate the following provisions:
firstly, scourers should be able to operate their plant economically under a variety of conditions, including: with or without grease recovery; replacing all or part of the first bowl liquor without losing recoverable grease or heat; running the last bowl of the train either as a hot standing rinse or as a cold running rinse and drawing liquor and solids from the bottom of both the main scouring bowl(s) and the side tank(s).
Secondly, solids should not accumulate in any part of the liquor circulation system except where they can be specifically removed. This necessitated redesigning the side tank.
Thirdly, suitable pumps, valves and pipelines should allow suspended solids to flow easily. Purge lines and isolating valves should facilitate rapid clearing or replacement should a blockage or breakdown occur.
Fourthly, the amount of discharge liquor (flow down) should be metered and should be easily adjustable by management. All other alterations in liquor flow rates resulting from an adjustment in flow down except those to and from the heavy solids tanks should be automatically compensated.
Fifthly, a wool grease recovery plant, by virtue of its dual role as a grease and dirt removal unit, should be an integral part of the liquor circulation system. This necessitated completely redesigning the two-stage recovery plant with emphasis on insuring trouble free operation for long periods while scouring any type of wool.
Finally, wherever possible, automatic control should be used to remove decision-making from the bowl minder, transferring this function instead to the plant manager15 Other design considerations included: removal of all fibre from liquor flows, recovery of heat from all the hot liquor discharge and eliminating the carry-over of contaminated liquor with the wool.16.
These design requirements formed the basis for the design of the prototype plant commissioned in August 1972. The plant was installed in conjunction with a new 1.8 metre-wide five-bowl scouring train fabricated throughout in stainless steel.
The WRONZ comprehensive scouring system was at first designed with conventional scouring trains. It was a liquor-handling system that rationalised the scouring process and made the continuous operation both technically feasible and economically desirable.7
The system also includes a primary effluent treatment by reducing the total volume discharged and flow-balancing the remainder at the same time as it removes settable solids, wool grease and heat
In this system, first-bowl scouring liquor is withdrawn continuously from the bottom of the bowl (instead of being intermittently discharged as was the convention) and the heavy dirt settled out in a special tank. 9 The settled liquor is heated, passed to a grease centrifuge and then a portion recycled to the first bowl and a portion (able to be measured and thus controlled) discharged to waste via a heat exchanger. As part of the system, all leaks of scouring liquor are avoided and no discharges are made to waste other than heavy dirt from the setting tank or measured flow down via the heat exchanger.10
Operation of a scour plant in this way brought about dramatic savings in energy and water used, and benefits in extra wool grease obtained, also a reduction in pollution as more grease is recovered rather than discharged. 11
About the time the WRONZ system was introduced, the New Zealand industry started to install ultra-small final rinse bowls to carry out mothproofing and also hot rinsing when economic.12
In 1977, CSIRO released their Lo-Flo process at a trade demonstration at Geelong. This encouraged the industry in New Zealand also to consider using smaller bowls for scouring. This approach obtained more support when Jamieson was able to show that wool immersed for a short time in a conventional bowl and squeezed was scoured to the same extent (as judged by the removal of grease and dirt) as wool that had remained in the bowl and traveled the full length before being squeezed. 13 In other words scouring with a short immersion time was a practical proposition. 14
The important technical features of the WRONZ with mini-bowls are:
- Draw-off is from the bottom of each bowl only: side-tank flows are completely pumped back to sprays or weir boxes via heat exchangers which enable precise temperature control to be maintained;
- Bowl volumes are small (about 2250 litres/bowl) and the bowls now have close-fitting, easily raised covers which give further savings in energy;
- Fibre removal precedes solids removal, a wedgewire screen usually being employed;
- Solids removal is by gravity settling in either a rectangular heavy solids tank (HST) with baffles and a slow speed stirrer for normal settling duty or a circular settling tank (CST) with a surface scraper and sludge thickening if flocculate is employed; sludge discharge from the HST is conveniently controlled by the PVS-developed Trim waste; the sludge level in the CST must be maintained at a certain depth in the tank to allow thickening to take place - an ultrasonic sludge detector, similar to that used in Trim waste but deployed differently inside the tank, is suitable. In a comparatively recent development (1991), WRONZ has introduced hydro cyclones to replace the solids settling tanks;
- Decanter dewatering has been employed on the sludge discharge so as to produce a spadeable solid; alternatively the 20-30% slurry can be carted away or included as one of the components in the feed for incineration;
- The clarified flow from either type of settling tank is heated in a remunerative exchanger and passed to a first stage (primary) centrifuge, either a nozzle or self-opening type, both of which allow for discharge of sludge and dirt during operation. The grease emulsion so produced is pumped to specially designed jacketed storage tanks where, under the influence of uniform heat (about 95oC), the emulsion cracks thermally and dirt and water can be run off;
- The flows of degreased liquor from the primary centrifuge are collected in a balance tank which automatically controls the amount of liquor recycled to the first scouring bowl via the regenerative heat exchanger (which cools the recycle flow to scouring temperature);
- The discharge of effluent is controllable and indicated on a flow meter. This flow down is taken preferentially from the dirtier (jet) discharge of the two flows from the centrifuge by the automatic action of the balance tank. Waste heat is recovered from this flow before final discharge; the important point is that at this stage management can note, select, and control the amount of effluent run to waste or to further effluent treatment; the flow down rate is selected according to wool type and throughput. 20
(1) R.G. Stewart, WOOLSCOURING AND ALLIED TECHNOLOGY, Third Edition. Caxton Press, (Christchurch 1985), pg1
(2) Ibid. pg1
(3) Ibid, pg1
(4) Ibid, pg49
(5) J.Bremner, WOOLSCOURS OF NEW ZEALAND: Tales of the early industry, Caxton Press, (Christchurch 1985), pg125
6) Ibid, pg125
(7) R.G. Stewart, G,V. Barker, P.E. Chisnal, and J.L. Hoare, THE WRONZ COMPREHENSIVE SCOURING SYSTEM, WRONZ REPORT NO.25,
WRONZ, (Christchurch 1974), pgl
(8) 1bid, pg1-2
(9) Bremner, WOOLSCOURS OF NEW ZEALAND, pg125
(10) Ibid, pg125
(11) 1bid, pg125
(12) Stewart, WOOLSCOURING AND ALLIED TECHNOLOGY, pg53
(13) Ibid, pg53
(14) Ibid, pg53
(15) Stewart, WRONZ REPORT NO.25, pg3
(16) Ibid, pg3
(17) Ibid, pg4
(18) Ibid, pg4
(19) Stewart, WOOLSCOURING AND ALLIED TECHNOLOGY, pg53
(20) Ibid, pg55
(21) Bremner, WOOLSCOURS OF NEW ZEALAND, pg125
(1) J Bremner, WOOLSCOURS OF NEW ZEALAND: tales of the early industry, Caxton Press, (Christchurch, 1983)
(2) G.Crawshaw, New Zealands Wool scouring Under Control, Australasian Textiles, (September/October 1992)
(3) E.L. Greensmith, THE NEW ZEALAND WOOL COMMISSION, New Zealand Wool Marketing Corporation, (Wellington 1976)
(4) R.G. Jamieson, Measurement And Analysis Of The Effluent From A Mini-Bowl Scour, WRONZ REPORT NO.90, WRONZ, (Christchurch 1982)
(5) R.G. Jamieson, Continuous Monitoring Of A Mini-Bowl WRONZ System Scouring Train, WRONZ REPORT NO.95, WRONZ, (Christchurch 1982)
(6) R.G. Stewart, WOOLSCOURING ALL TECHNOLOGY, Third Edition, Caxton Press, (Christchurch 1985)
(7) R.G. Stewart, G,V. Baker, P.E. Chisnall, and J.L. Hoare, The WRONZ Comprehensive Scouring System, WRONZ REPORT NO.25, WRONZ, (Christchurch 1974)
(8) R.G. Stewart, P.E. Chisnall, J.M. Flynn, and R.G. Jamieson, Studies In Wool scouring Grease Recovery And Dirt Removal In Optimised Liquor Handling, WRONZ REPORT NO.36, WRONZ, (Christchurch 1975)