Cleaner Production in Wool Scouring

ConAgra Wool Pty. Ltd.

ConAgra Wool Pty. Ltd. has investigated a number of cleaner production initiatives for their wool scouring operation. The increased recovery of wool grease and optimisation of sulphuric acid usage in the carbonising process has provided significant economic returns to the company.

Contact

Dr Richard White

ConAgra Wool Pty. Ltd.

89-103 Doherty's Road

Laverton North VIC 3026

Ph: 61 3 9360 6100

Fax: 61 3 9360 6170

Background

ConAgra Wool Pty. Ltd. is a wool scouring operation that supplies scoured and carbonised wool to the global market. The company participated in the Victorian Government's Cleaner Production Demonstration Project in 1992.

The Process

The scouring process involves washing the wool in a number of different "bowls" in open tanks which contain various cleaning solutions - that together comprise a scouring line. ConAgra has four scouring lines at its Laverton plant. After passing through the scouring line, the clean wool still contains some organic matter such as seed burrs. These are removed by carbonising, a process involving washing the wool with sulphuric acid, which dehydrates the organic matter, then rinsing and drying in an oven. The application of heat to the dehydrated organic matter turns it into carbon, which can then be easily removed as a dust material.

The scouring process generates significant quantities of waste, including:

• 400 tonnes per annum (pa.) of sludge (disposal costs in excess of $200,000 pa);
• 750 tonnes pa. of solid waste (disposal cost in excess of $60,000 pa); and
• 750,000 cubic metres pa. of trade waste effluent, with an annual discharge cost of $1.5 million. In addition the cost of feed water is around $350,000 pa.

There are two main effluent streams at the plant:

• rinse streams, which have a relatively low contaminant loading and can be discharged to the sewer after minor treatment; and
• heavily contaminated effluent from the washing stages, which does require treatment prior to discharge to sewer.

In the existing process the overflows from the washing bowls are sent to 'heavy solids' tanks, where the solids are settled out and the wash solution is returned to the wash bowls. The heavy solids tanks are located at some distance from the scouring lines, and as the wash solutions operate at an elevated temperature of 65 degrees Celsius, heat losses are experienced. In addition, the heavy solids tanks are nearing capacity as production volumes increase.

Current water usage is in the range of 20-25 litres per kilogram of wool processed. The long term objective for ConAgra is to halve this value.

Cleaner Production Initiatives

ConAgra participated in the Victorian Cleaner Production Demonstration Program in 1992. The outcome from this program was the identification of a number of potential cleaner production processes that required further research and development (R&D) work to establish their viability. Participation in the program has led to an ongoing interest in cleaner production. Initiatives have included:

• Installation of hydrocyclones on the wash baths of one of the scouring lines to enhance solids removal, and allow recycling of wash waters at each bowl. As two of the scour lines process wool that is almost identical, the installation of the hydrocyclones on one of these lines allowed direct comparisons between performance to be made. Energy costs were reduced because of the need for only one pump, and heat loss was reduced by not pumping to the remote heavy solids tank. This project is still at the R&D stage.

• Combination of solid wastes from the scouring process with solids from the grease plant to produce a material that could be suitable as a compost or compost additive. ConAgra is currently reviewing the possibility of developing this as a commercial operation.

• Improved grease recovery to achieve 35-40 per cent reclaim of all grease in the wool. Two types of wool grease are present in the wool - oxidised and unoxidised. The split between the two is roughly 50/50, with the oxidised grease being of poorer quality and harder to recover (because its specific gravity is closer to that of water). Current recoveries are 75-80 per cent for the unoxidised grease and negligible for the oxidised grease. The recovery efficiency has been enhanced by improved monitoring practices and tighter maintenance on the centrifuges. Formerly, grease recovery was running at 4 per cent of cleaned wool weight (incoming dirty wool contains 10-15 per cent grease). This recovery has increased to about 6 per cent of cleaned wool weight.

• Optimisation of sulphuric acid in the carbonising process. Tighter process monitoring identified opportunities to reduce acid consumption by 25 per cent.

• Optimising water usage and waste water generation. This project has only recently commenced, with the first stage aimed at quantifying effluent sources and contaminant loadings at the unit process level.

The wool scour line

Advantages of the Process

The increased recovery of wool grease has had both environmental and economic benefits. The increased grease recovery has reduced the amount of Biochemical Oxygen Demand and Suspended Solids in the final effluent discharge, with a corresponding decrease in trade waste charges. Another benefit from improved grease recovery has been an improvement in washed wool quality. This has enabled ConAgra to maintain its market share in the face of considerable competition.

ECONOMIC BENEFITS
Costs	$16,000 per line
Annual Savings	$27,500 per line
Payback period	0.6 years

Cleaner Production Incentive

The single most important incentive for ConAgra to implement cleaner production initiatives has been the potential cost reductions that can be achieved. ConAgra already pays a significant amount of money for waste disposal and discharge of effluent to the sewerage system, and there is always attention being given to opportunities to reduce these costs.

Barriers

Technical problems have been experienced with the hydrocyclones. Excessive rates of internal wear from the high solids loading has led to a search for more resistant construction materials. The initial hydrocyclones were made of a ceramic material, which has now been changed to a polyurethane material. Problems have also been experienced with solids buildup in the pumps, resulting in a decrease in pumping efficiency.

The initial R&D work indicated that the hydrocyclones worked effectively on the wash stages, and this was where the initial work was targeted. When ConAgra expanded the fitting of hydrocyclones to the rinse streams they found that they were not as effective as had been expected. This resulted in a level of disenchantment with the technology.

Accessing capital is also a barrier for ConAgra. This means that capital expenditures, even where the pay back is very attractive, need to placed into forward budget planning.

Case Study Prepared: January 1997 by ACCP

The Cleaner Production Case Studies Directory is part of EnviroNET Australia.

For more information contact:

Environment Australia
Environment Protection Group
PO Box E305
KINGSTON ACT 2604
AUSTRALIA
Email: cproduction@ea.gov.au
URL: http://www.environment.gov.au/net/environet.html