New Milton success for Haith
3rd September 2003
One company who fully acknowledge that environmental protection must play a permanent and fundamental role in quarrying are Haith Industrial of Worksop, Nottinghamshire; who for several years have been involved in the design, manufacture and installation of cost-effective waste-management solutions.Recently Haith Industrial have been involved in a complete feasibility and waste-minimisation study for the New Milton Sand and Ballast a private company that operate a land based quarry in Hampshire. The quarry is a sand and gravel facility supplying bulk products across the weighbridge.
In 1997 Haith Industrial using existing conventional lagoons installed a 6m deep cone thickener which received 135m3 per hr of effluent/dirty wash water which contained a 3% solids/silt contamination, this system evacuated thickened silt slurry to an existing lagoon which provided a solution to their current problem by increasing the capacity to re-use clean water.
In 2002 NMSB embarked on an expansion of the existing plant by adding a silt/dewatering system. After much investigation and overseas site visits they approached Haith Industrial to explore the method of dewatering of silt using a multi-roll filter belt. This was established by site visit and subsequent design of an interface for the existing thickener and re-use of an existing concrete support and penthouse fabricated structure.
Unusually the latter proved to be a challenge as the press would have to be located 3.9m above ground level. As the conventional method is to normally locate the press at ground level and convey the cake by inclined conveyor to a stockpile; an alternative design was adapted to accommodate the existing structure. A standard 2m wide machine was positioned toward one end of the penthouse which had to be relocated to a more suitable position which was modified to accept an under tray to catch dirty water for recirculation in to the system.
Positioning of the machine allows the material exiting the press to be dropped vertically into a bunker below. Haith also had to redesign the penthouse structure to increase head room which was done on site by the quarry engineers working to new drawings.
Utilising an existing thickener underflow pump; the thickened silt slurry is delivered to a buffer tank, which is a 24m3 capacity heavy duty picket fence type mixer vessel. The purpose of the tank is to eliminate any surge or fluctuation in the underflow from the thickener which provides a balanced homogenous feed to the press.
From the tank the silt/slurry is delivered by a variable speed pump to the press, essentially at 8-12m3 per hr flow. The silt/slurry is then chemically conditioned as it is pumped to the press by ejecting a combination of anionic and cationic flocculent which is added separately to the silt.
Anionic flocculent is added initially in a progressive manner at strategic intervals along the pipe creating a flocculent structure, cationic is then added just before the slurry is delivered to the press. The press was sized by original calculations from sample analysis which determined the silt production rate which led to the selection of the 2m wide machine; the Haith specification uniquely dictates a standard length of 6m which provides for extended free drainage to the press.
Over the last four months the 2m machine has provided the flexibility and process robustness of differing material qualities. In the past typically 30% dry solids, 70% water have been handled through the lagoon storage, this machine has provided the ability to generate dewatered silt slurry to produce a cake circa 65/70% solids.
This has produced a dryer product which means re-using 40% more of the silt volume than previously with the old system.
Working alongside engineers from the quarry Haith installed the system in 3-4 weeks, actual commissioning took 1 week and the plant was fully operational late January 2003.