It is known that mud treatment plants are in a closed loop, in order to reduce the amount of water used to make the mud and consequently reduce costs. Such a mud treatment plant must adhere to predetermined environmental pollution restrictions.
Normally, mud treatment plants should treat the mud in such a way that it can carry out the following useful functions for drilling:                stabilizing the walls of the excavation thanks to hydrostatic thrust, linked to the density of the mud;        limiting the losses of mud in the excavation by permeation        acting as a transportation medium for the detritus during drilling through the use of known pumping systems;        lubricating and cooling the drilling tool.        
It is known, moreover, that the optimal density value of the mud is around 1.10 t/m3, at the maximum 1.15 t/m3.
The use of muds with higher densities is problematic since:                greater powers are required for the centrifugal pumps generally used for the evacuation of the detritus, increasing the costs for making the excavation;        such muds tend to more easily sediment on the surfaces of the excavation creating a deposit generally known as “cake” that must be removed at the end of the excavation in order not to jeopardise the hydraulic seal and/or increase the permeability of the foundation;        it does not allow the casting of concrete directly in the foundation because the small difference in density with respect to concrete could cause mud to be incorporated inside the product jeopardising its strength and permeability.        
The muds used in excavations are generally mixtures of water and bentonite and possible polymers, with dosage of about 30-50 kg of bentonite for every m3 of water.
The main function of said bentonite and polymers is to increase the density of the mud at the same time increasing their stabilizing effect for the walls of the excavation. Moreover, such clean mud, when it deposits on the walls, fills possible pores and small fractures in the ground, limiting the loss of the mud itself during drilling.
Once the drilling step is finished, normally, there is then the step of making the foundation. Generally, the concrete is cast inside the excavation to make the foundation. The casting of the concrete is carried out leaving the mud in the excavation. The concrete is carried inside the excavation through special pipes that allow it to rise, gradually replacing the bentonite mud, thanks to the difference in density, said mud being recovered from the surface, regenerated and stored for subsequent drilling.
The muds contained in the excavation during the step of making the foundation must have a density of less than 1.15 t/m3 and a sand content of less than 4% since, in the case of higher sand concentrations, the quality of the end product would not meet the required standards, with possible inclusions and consequent problems, like for example structural yielding.
It is also important that, for the purposes of the foundation, the mud has a low slime and clay content. Such slimes and clays, indeed, tend to deposit on the walls of the excavation generating an excessive thickness that can jeopardise some physical characteristics of the foundation. Such a phenomenon is very harmful when making adjacent rectangular panels, jeopardising the hydraulic seal of the structure.
Normally, during drilling, the mud that is recovered from the excavation, containing drilling detritus, is sent to the treatment plant. In such a treatment plant, such muds are purified of the coarsest detritus and as far as possible of finer detritus.
It is known that sands have a particle size of more than 74 μm; slimes have a particle size of between 74 and 2 μm and clays, finer still, have a particle size of less than 2 μm.
In order to perform these functions the plants currently in use consist of two plants in parallel, as illustrated in the attached FIG. 1.
FIG. 1 illustrates in detail a main plant (1) comprising a first portion of plant, which in turn comprises:                a main mud storage tank (11), containing the mud ready to be sent to the excavation;        a mud treatment plant comprising a first grizzly screen (12), a mud processing hopper (13), a first cyclone stage (14) and a second cyclone stage (15) and, finally, a dewatering screen (16).        
The mud indicated by an arrow (17) arrives full of detritus, is treated in order to be able to be reused and goes back to the excavation as indicated by the arrow (20). Up to now this plant only manages to correctly separate larger sized detritus and sand from the mud, but it is not very effective for finer substances dissolved in the mud such as slime and clay.
Such a first plant also comprises a second tank with clean mud, not represented in the figures, which is inserted into the excavation before carrying out the casting of concrete in the step of making the foundation, to be certain of carrying out the casting for the foundation with a mud that is as clean as possible and with the correct concentration of sand.
A second portion of plant (2) in parallel with the first portion of plant quoted above, comprises a pump (18) that takes mud from the main tank (11) and sends it to a first machine (19), normally known as centrifuge, capable of separating the finer parts like slime and clay from the mud. This particular plant is called centrifugal plant since the mud is centrifuged at very high speed in order to be able to gravimetrically separate the solid particles suspended in the water.
The mud, arriving from the drilling, on average has a density of about 1.3 t/m3 The grizzly screen (12), generally, exploiting vibrating screens, separates the coarser detritus, like for example pieces of rock or gravel, from the mud arriving from the drilling. The flow rate of mud (121) coming out from the grizzly screen (12), with a density of about 1.25 t/m3, is conveyed into the mud processing hopper (13). The mud processing hopper (13) is generally divided into many volumes. From a compartment of this hopper the mud is pumped to the first cyclone stage.
In such a first cyclone stage there is a separation of the sand from the mud. Normally, in output from the cyclones (14) there is a mud having intermediate density (141), about 1.18 t/m3. Such mud comes out from the cyclone (14) in the upper part and is conveyed into the mud processing hopper (13); on the other hand, exiting in the lower part from the cyclone (14), there is a mud (142) that is very dense and very full with sand that is conveyed to the dewatering screen (16) in order to reduce the amount of water from the muds produced making them shovelable.
Normally, the cyclone stage can also comprise more than one cyclone, connected in parallel to one another. Every cyclone can also be in vertical position or in inclined configuration, still keeping the outlet of the light mud in their upper part and the outlet of the heavy mud, filled with separated solid, in the lower part.
The mud having intermediate density, taken from the mud processing tank (13) is conveyed to the second cyclone stage (15). In such a stage (15) a light mud (151) with a density of about 1.15 t/m3 is separated, coming out from its upper part and being conveyed to the mud processing hopper (13) and then to the main storage tank (11). The second cyclone stage manages to separate mainly slime and partially clay from the mud. When exiting, in the lower part of said second cyclone stage, there is a flow rate of high density mud (152), oscillating between 1.2 t/m3 and 1.5 t/m3, which is conveyed to the dewatering screen (16).
The centrifugal plant (2), installed in parallel, has the task of taking mud from the main tank (11) already treated by the plant (1) described previously. Inside the centrifuge the mud is lightened, reducing its density gravimetrically, thanks to the separation of part of the solid fraction suspended in it. The treated mud is conveyed into the main tank (11) with a density of about 1.08 t/m3.
Normally, the flow rates that a single centrifuge (19) can manage, depending on its size, internal geometry and the density of the mud going in, are variable between 15 and 50 m3/h; whereas the flow rate arriving from the excavation towards the main plant (1) can vary between 300 and 450 m3/h. Generally, a single centrifuge is not sufficient to keep the content of fine solids, such as slime and clay, contained in the muds under control and therefore, in some cases, it is necessary to use more than one centrifuge, all in parallel with respect to the main plant (1).
Using a single centrifuge (19), in the case of ground very full of fine solids, a mud would be obtained whose density increases as the working days goes on. Such an increase in density is caused by the fact that the plant with centrifuge (2) cannot extract a sufficient quantity of fine solids from the mud since the centrifuges currently used have the possibility of processing lower flow rates of mud than those involved in the field of foundations as quoted above. Known plants, therefore, use many centrifuges for every drilling machine, with a negative impact on the plant costs, maintenance and bulk.
If a plant were used without centrifuge in the presence of ground with slime and clay, during normal operation there would be a continuous increase in density of the mud since it is not possible to separate slime and clay with a simple mud treatment plant.
In known plants, in the absence of a centrifugal plant, after a few days of work, the density of the mud would reach values of about 1.25 t/m3 making drilling difficult. In this last plant configuration it is therefore necessary to periodically dispose of a portion of mud and reintegrate it with bentonite fresh mud and/or clean industrial water.
In the attached FIG. 2, the curve referred to as “a”, illustrates the quality progression of the density of the mud in plants without centrifuge as the days pass.
Normally, the disposal of the muds is a big problem in building sites since it has a high cost.
Moreover, it is known that it is necessary to refill with mud, of a more modest amount, due to the fact that a part thereof permeates through the walls of the excavation due to an imperfect seal of the walls themselves. Such a loss, if not refilled, would cause a further concentration of fine material in the process mud.
Again in FIG. 2, in comparison to the curve “a” there is the progression of the density of a mud of a plant in which a centrifuge has been added in parallel. Such a curve “b” shows that the problem is only delayed, reaching a critical density after a longer time. Finally, the curve “c”, of FIG. 2, shows the progression of the density of the mud in plants comprising many centrifuges in parallel. It can be seen that the problem of reaching a critical density is further delayed but without ever solving it definitively.
The plants present in the prior art foresee to carry out the periodic disposal of amounts of muds full with fine material and reintegrating it with fresh bentonite mud and/or clean industrial water.
Moreover, it is known that the use of many centrifuges has a very high cost and requires large spaces to fit them on the building site, which can be problematic in building sites located in heavily urbanised areas.
From U.S. Pat. No. 7,540,837 B2 a mud treatment plant is known for the field of oil drilling, comprising a control system that in turn comprises viscosity and density sensors positioned inside the main storage tank. Depending on the data obtained from said sensors, the operation of the centrifuge that draws and conveys fluid from the tank itself is adjusted.
Such a patent describes an embodiment of the system in which part of the solids separated through centrifuge are reinserted into the main tank to keep the viscosity and the density of the fluid at a desired level, in the case in which it becomes too low.
The solutions described by the aforementioned patent do not solve the problems quoted earlier and, moreover, it tries to solve the problem of increasing the density of a mud that is not very dense. The problem of using a mud that is not very dense to make the foundations, in general, never occurs, since, as stated above, the density of the mud tends to increase as the excavation proceeds.
Moreover, the centrifuge takes mud from the main tank, which does not have controlled characteristics of density and presence of residues.
Finally, in the plants described up to now, the operation of the centrifuge is only optimised if the muds inside the main storage tanks (11) are properly homogenised and the variations in density entering the centrifuge are gradual.