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Case Study - Pivot Geelong

De-watering of Process Pond Sludge, Australia

Client: Incitec Pivot Ltd, Geelong

Consultants: Permathene Pty Ltd, Sydney

Contractor: Green Waste Environmental, Melbourne

Date: March 2004
 

Incitec Pivot manufactures super phosphate at a plant in Geelong, Australia. A byproduct of super phosphate manufacture is formation of silica in fluorosilicic acid (H2SiF6). The silica and acid, along with any entrained super phosphate dust is captured in the process ponds for storage. The level of solids content in the process pond tends to increase over time, as the pond is not agitated. Every few years, the solids have to be removed to restore storage capacity. Due to the highly corrosive nature of the acid, the means of solids removal are expensive, messy and potentially hazardous.
 

Objective

The objective of the project was to separate the solids in the process pond (primarily silica and super phosphate) from the liquid (approx 20% fluorosilicic acid). The solids removed can be recycled back into the product, while the liquid can either be bled back into the process or treated. In order to return the solids back into the process in a timely fashion, the level of moisture content in the solids has to minimized.
 

Method

Syntex High Strength Woven bags were used to remove the solids. These bags met the primary requirement of being able to separate the very fine solids from the process liquor, whilst also providing a high solids content cake after only 5-10 days of drying. The heavy individual yarns are woven into a unique twill pattern to form a strong geotextile with superior hydraulic characteristics.  A piping manifold was manufactured to supply sludge to up to six bags (10m long x 4.5m circumference). Sludge was supplied from the pond via a Sandpiper diaphragm sludge pump. This system had a major SH&E benefit by enabling the bag filling to be done remotely by only one or two operators. After the bags were full (approximately 20 tonnes of 50% solids sludge), the supply piping manifold was disconnected and relocated to enable a further 6 bags to be set up. The first set of bags was then left to dry for as long as possible. Drying periods varied between 5 days to 3 weeks. A longer drying period is better, however due to the very high solids content of the supplied sludge, and the ‘pressure filtration’ effect provided by the bag, the cake was spade able almost from day one. After the drying period, the bags were split open and the sludge picked up using a large front-end loader. The sludge was further dried and conditioned before retuning to the process.


Results

The results were excellent with an estimated 250 tonnes of sludge removed using 14 sludge bags. The sludge removal was possible during normal operation and did not disrupt plant operations. Additionally, the high solids content after drying allowed the sludge to go to secondary conditioning within 2 weeks of removal when previously this used to take up to 6 months. The bags also had a low visual impact. The high strength of the bags allowed a large volume of sludge to be removed at a high rate. Each bag was acting as a pressure filter, with incoming sludge forcing liquor to be filtered out through the bag mesh depositing solids behind. The cost savings using this method are difficult to quantify, however we believe that Incitec Pivot have saved up to A$100,000 by using this method over other sludge removal techniques.