Your browser is no longer supported

For the best possible experience using our website we recommend you upgrade to a newer version or another browser.

Your browser appears to have cookies disabled. For the best experience of this website, please enable cookies in your browser

We'll assume we have your consent to use cookies, for example so you won't need to log in each time you visit our site.
Learn more

Soil washing process

1 Excavated material is weighed to monitor amounts processed and recovered and passed through two fixed screens of 150mm and 100mm, along conveyors fitted with overband magnets. These remove ferrous metals, some of which are reclaimed. Oversize material is removed, crushed and then reintroduced to the system.

2 Screened material is wetted and placed on a 2mm horizontal vibrating screen and is disaggregated with high pressure water. The gravel (+2mm) fraction falls into a brick washer where it is agitated and washed by a counter current of water. A fine screen allows water to drain from the cleaned gravel before it is discharged into a collection bay. Material held in the counter current, such as coke, fragments of wood and plastic, flows over a 2mm vibrating screen and is removed. Water from the barrelwasher and the 2mm material slurry from the initial screen is pumped to the next stage. 3 The slurry is split and injected into two hydrocyclones that separateit into 63 micron to 2mm 'sand' and a 63 micron slurry. The sand fraction from both passes into an attrition scrubber that removes surface contam- ination. This clean material collects at the base of the module and more water is added. This second slurry is pumped to two further hydrocyclones where the fine fraction is separated from the sand. Slurry from the top of the cyclones is then pumped for dewatering (see 5).

4 The sand falls into a dense medium separator (DMS) where low density material (along with some fine sand) is separated from the sand fraction. Clean sand is emptied from the base of the cylinder where it is dewatered by a vibrating 63 micron screen and discharged to a collection bay. Water and low density material overflowing from the DMS is passed over an inclined fixed screen that removes the low density material (which is disposed of) and recovers the majority of the fine sand particles. Water and fine sand from the inclined screen collect in a tank and are passed through another hydrocyclone before being reintroduced to the process.

5 Slurry from the previous two stages containing the 63 micron fraction is collected and pumped into a Superflo high rate thickener, where flocculants can be added. This produces a sludge that collects at the base of the tank while the water flows over a weir at the top.

6 Most of the process water is collected in two storage tanks where clean water from the on-site water treatment plant is added to adjust for losses within the clean and waste products. The remainder is sent for additional solid removal using a cross flow clarifier. Water flows into a tank and passes through a series of inclined plates which accelerate the settlement of the solids. Water is recovered and reused in the plant and the resulting sludge reintroduced into the Supaflo thickener.

7 Sludge from the Supaflo thickener is pumped into a conditioning tank, and on to a continuous filter press, where additives may be introduced to enable solid particles to adhere to each other and repel water. The resulting filter cake forms the main waste product from the process.

Have your say

You must sign in to make a comment

Please remember that the submission of any material is governed by our Terms and Conditions and by submitting material you confirm your agreement to these Terms and Conditions. Please note comments made online may also be published in the print edition of New Civil Engineer. Links may be included in your comments but HTML is not permitted.