SW is an off-site treating technology used for the reclamation of contaminated soils. Its aim is to recover around the 60% of materials and so reduce quantity and danger of waste elements which have to be brought to the dump.
The main operations constituting the SW are based on selecting of the granulometry and washing the materials composing the solid part of the soil allowing the partial or total transfer of contamination to the liquid used as washing agent. The selecting process of the solid phase is followed by a chemical-physical treatment of the ore-pulp obtained, in order to concentrate polluting substances in dried mud and allowing the recirculation of waste water.
The portion of soil with greater granulometry (sand and gravel) is recycled, whereas the smaller ones, upon drying, are brought to definitive disposal.
The range of contaminants which can be removed through this treatment is very wide and the efficiency in removal is related to the granulometry of the soil. For gravel-sandy groundmass there’s a high efficiency, while for slit-clay ones it’s lower.
Treatment phases can be summarized as follows:
|Hopper with conveyor and coarse riddling system|
Material which has to be treated is fed, by mechanical arms, in the loading hopper, which is the starting stockpile of waste material that have to be treated. The solid, stocked in the loading hopper, is extracted from the lower part of the machine though the conveyor with metal blades, which provides to feed it on the next conveyor installed under the hopper.
Furthermore, the hopper is equipped with a superior grid used to eliminate coarse material. Separation of material happens by overturning of the grid activated by hydraulic pistons.
To complete the system, under the conveyor with metal blades, is installed an additional conveyor which has the function of gathering and cleaning out.
|Conveyor belt to the cylindrical scrubber|
The belt following the hopper is essentially composed on a loading belt with a rubber sheet, kind “V” (central horizontal roller with two inclined lateral rollers).
On the loading belt of the cylindrical scrubber is installed a magnetic separator to divide, by the waste material which has to be treated, possible metal ferromagnetic components which can be dangerous in the next treatment phases.
The magnetic separator is composed of a supporting framework where a permanent magnet (which attracts metal units) and several components, allowing the progress of the rubber belt (which causes the removal of metal units picked up) are installed. With this system foreign metal units are separated and gathered in an appropriate container.
In this section is realized the first cleansing, where coarse material is separated from finest one and eventually to them adhered. Washing is done using purified water coming from the following chemical-physical treatment.
The cylindrical scrubber is equipped with internal mechanical rotating blades, adequately shaped, and spry nozzles. The system creates an energetic and turbulent agitation of materials which have to be washed by a counter-current movement between cleaning water and material treated.
In the cylindrical scrubber the gravel, with sizes from 5 to 100 mm, is risen by blades rotating action, brought on the top of it and then loaded in a specific storage bin.
The screened waste, coming from the cylindrical scrubber, is composed on a watery sandy current (<5mm), slit and clay (<0,063) and light material. It passes through a specific vibrating screen, where thin elements, with dimension larger than 5 mm, are eliminated by the system, whereas the remaining material reaches the group for sand refinement.
The ore-pulp outgoing from the cylindrical scrubber is transferred to an hydrocyclone system, where, thanks to gravity, the sandy part is separated by the watery one which consequently will include all the finest remaining parts.
The pump installed in the gathering ore-pulp bin feeds the hydrocyclone, which has to separate sandy portions by the water, containing even parts<0,063 mm and concentrate the solid part in order to make the sandy ore-pulp suitable to the treatment in the attrition cell. The water-sandy mixture outgoing from lower nozzle of the hydrocyclone, containing around the 50% of solid elements, falls down in the loading box of the attrition cell. Quantity and quality of the blend is controlled by an automatic method by the automation system which manages the opening of a pneumatic valve installed near the unloading nozzle. The automation uses a pressure indicator installed directly on the recirculation pipe of the hydrocyclone pump to manage the system.
The blend outgoing from the superior nozzle of the hydrocyclone, water and portion<0,063 mm, is directly destined to the chemical-physical plant.
In this phase is considered feeding with purified water. Sand passes through the attrition cells which, for mechanical action generated by two turboemulsioners, causes an energetic rubbing of sand grains. The surface of grains is exposed to a mechanical treatment to remove polluted adhering elements.
The attrition unit is composed on two cells of carbon steel covered inside with abrasion-proof material, which supports agitators with of shaft with several blades.
In the attrition cell, for impact among particles caused by the intense turbulence of agitation system, there’s the physical separation of polluted elements that are strongly linked to the surface of sandy particles. The mixture of water, sand and polluted elements outgoing from the attrition cell, falls in a gathering box and from here is brought to the secondary hydrocyclone group.
The material passes from attrition cells to secondary hydrocyclone section, which separates sand, which wasn’t divided after the first section of hydrocylone procedure, from the thinnest portion remaining. The blend of water and sand outgoing from the lower part of the hydrocyclone is gathered by gravity to the next section.
The hydrocyclone has same features of the first one, but a drainer sieve to dry the sand on it is applied.
|Following the cylindrical scrubber there’s a screw, which separates the material unloaded. The solid portion, in fact, is pushed out by the screw whereas water and thinnest elements flow by sunken channels positioned inside itself.|