Background of the Invention

This invention refers to a centrifugal dryer for washed and screened silica sand that works without heating the sand itself. The silica sand is the basis of a multitude of products, from paints to plasters, cements etc. For the production of most of these products the sand, once washed and screened, must be delivered with a very low humidity content, that is, the sand must be dry. The traditional method of performing this drying operation is to pass the sand through a cylinder placed horizontally and rotating around its axis. At one end of the cylinder is placed a diesel or gas burner, whose combustion fumes evaporate the moisture present in the sand, such fumes are then extracted at the other end of the cylinder and filtered before being released into the atmosphere.

The above methodology has the advantage of being practical and easy to use but has the fundamental disadvantage of being highly energy intensive. In fact, only a part of the thermal energy of the fuel is transferred to the moisture to make it evaporate; one part goes to heat the sand itself and the structure of the cylinder and is then dispersed to the outside environment. In addition, energy is dispersed with the heat still present in the outgoing fumes.

Summary of the Invention

The machine covered by this patent allows to dehumidify the silica sand or any type of granular material without the heat input generated by fuel, taking advantage of the centrifugal effect and the lowering of pressure through the Venturi effect. Thanks to the centrifugal force of the vertical cylinder the water present in the sand is separated mechanically so without the need to provide the latent heat of evaporation. The moisture that is not mechanically removed by the centrifugal force is removed, up to a certain amount, by lowering the air pressure, which is more energy efficient than providing the latent heat of evaporation at ambient pressure that would otherwise be necessary. In fact, as mentioned in the paragraph above, only a part of the heat provided by the fuel goes to the water, being a considerable part transferred to the grains of sand and the outdoor environment.

For the above, the machine covered by this patent can find the following two applications within a sand drying plant. 1. Dehumidify the sand to a level acceptable for delivery to customers. Depending on the acceptable maximum humidity content, the required granulometry and the type of sand, dehumidification obtained through the machine covered by this patent may be sufficient and therefore does not require a subsequent passage through a combustion drying plant.

2. Reduce the humidity in the sand so that the next step in a conventional burner drying cylinder with exposure to combustion fumes can occur with a significant fuel saving. In addition, the reduction in the humidity content has the effect of increasing the productivity of the burner drying plant, which, if not linear with the lowering of the humidity content, is anyway considerable, with obvious sizing benefit for a new plant. It should also be noted that the first application of this invention is likely to be that of integration to existing plants, precisely to reduce their consumption and at the same time, if necessary, increase their productivity.

Detail Description of the Invention

The machine consists of a vertical axis cylinder and a series of 4 bulkheads (1), (2), (3), (4) of concentric triangular section that force the sand to follow an alternately up and down path under the centrifugal force caused by the rotation of the cylinder until it exits the cylinder itself. Each bulkhead is uniformly provided with holes through which the water that separates from the sand as a result of the centrifugal force is drained.

The wet sand, coming from a wash and/or screen, is dropped at the entrance placed on the axis of the cylinder (6). The fins (7) at the base of the cylinder's axis and the four plates (9) that delimit the four sectors of the cylinder apply a sufficient force for the sand to go against the wall of the first bulkhead (1); being this tilted counterclockwise from the vertical, the component of the centrifugal force along the bulkhead makes the sand slide upwards.

Once the sand arrives at the extremity of the first bulkhead (1) and passes its edge, it is then projected against the upper end of the second bulkhead (2) which, being tilted clockwise from the vertical will make the sand advance downwards. As with the first (1) bulkhead, the surface of the second bulkhead (2) is provided with uniform holes that allow the drainage of water separated from the sand thanks to the centrifugal force. Water separated from the first and second bulkhead (1) (2) is extracted through the piping system (8) placed circumferentially at regular intervals one for each of the four sectors of the cylinder. These four pipes discharge the water to a drainage system (10) fixed to the base of the machine, from here it is then sent to the decanting tank of the washing plant, here not shown because it is not the subject of this patent application.

Once it has reached the lower end of the second bulkhead (2) the sand is projected at the base of the third bulkhead (3) and the cycle just described is repeated.

Unlike the passage along the first (1) and second (2) bulkhead now in the third (3) and fourth (4) bulkhead becomes dominant in the dehumidification process the effect obtained by the lowering of pressure due to the Venturi effect generated by the air acceleration due to the increased centrifugal force and the decrease in the section through which passes the air itself. In fact, although the area of the cylinder surface at a given radius increases when increasing the distance from the center of the cylinder, the area available for the air to flow as the radius increases between the third (3) and fourth (4) bulkhead decreases because of the geometry of the bulkheads and the fact that part of their volume is closed, not in communication with the air in transit through the cylinder. Practically between the third (3) and fourth (4) bulkhead as the radius increases, the volume available to the air decreases, as a consequence the air has to increase the speed by losing pressure for the Venturi effect. The drop of pressure facilitates the evaporation of water still present on the sand grains and the speed of the airflow along the path outlined by the bulkheads prevents the increase of the partial pressure of water vapor which would otherwise hinder the evaporation of other quantity of water.

In addition to the above the sand is spread on the surface of the third (3) and even more so of the fourth (4) bulkhead on an ever-increasing surface. The air passing through the orifices of the bulkheads removes moisture from the sand by again taking advantage of the Venturi effect applied this time at the level of each individual hole of the bulkheads.

It should be noted that while in the first (1) and second (2) bulkhead the water was removed mainly in the liquid state through the holes of the bulkheads, in the third (3) and fourth (4) bulkhead the water is removed mainly in the state of vapor through the holes.

The surface of the third (3) and fourth (4) bulkhead in contact with the sand is provided with wedge shaped steps (19), whose function is to mix the grains of sand during their motion along the bulkhead increasing their overall exposure to the air flow.

In addition to the centrifugal force and the reduction in volume for the air to flow as the radius increases, a draft fan (11) is connected to the carcass of the machine (12), sucking air from inside the third (3) and fourth (4) bulkhead, which are in communication between each other through tubes (13) placed at regular intervals circumferentially. The addition of the draft fan has the dual function of sending the vapor and air extracted from the machine to the filtration system before being released outside and of increasing the depression and air flow inside the cylinder, both factors that increase the dehumidification of the sand.

The inlet of the draft fan is connected on the top of the machine's frame on a female circular sector (16) within which the corresponding male part (17) of the rotating cylinder coaxially spins. The sealing between the male and female part is guaranteed by sliding seals (18). The gap between the male part (17) of the cylinder and the female part of the frame (16) is such that contact between the two parts is avoided once the cylinder is in motion and subject to vibrations.

At the lower end of the fourth bulkhead (4) the sand detaches tangentially from the edge and is collected by a conical shaped chute (14) that conveys the dehumidified sand to the base of the machine itself where the exit is placed (15) over a roller tape, not shown in the figure, which will send the sand to the storage area.

It is to be noted that the conical chute is crossed by the drainage system pipe (10) so that no mixing occurs between the drainage water and the dehumidified sand at the exit.

The degree of humidity of the sand at the exit of the machine, keeping fix the moisture of the sand at the entrance, the speed of rotation of the cylinder and the suction power of the downstream draft fan, depends on the grain size and the sand flow rate which in turn influences the average time the sand stays in the cylinder. The degree of purity of the sand affects the dehumidification capacity of the machine. For example, if clay is present, this will delay the flow of the sand through the bulkheads of the cylinder, thus decreasing the overall flow rate.

In general, the higher the average time the sand stays in the cylinder and the lower the degree of filling of the cylinder itself, then the higher is the percentage of humidity extracted from the sand.

The cylinder is mounted on a series of springs that damp the vibrations generated by the rotation of the cylinder, by the inevitable asymmetry of the distribution of sand at the entrance and then during its path through the 4 bulkheads until the exit where continuous dynamic stresses occur in the radial direction due to the separation of the sand from the edge of the cylinder.

The above mentioned vibrations facilitate the descent of the sand along the exit chute (14) and generally help the sand flow inside the cylinder by preventing the formation of accumulations that could delay the passage through the bulkheads. In the lower part between the first (1) and the third (3) bulkhead, in the center of each of the four sectors of the machine, there is a radial sliding opening (5). The four openings are activated simultaneously by a rod coaxial with the cylinder pin.

The function of the openings is to evacuate the sand in case for any reason there should be a blockage of the sand between the bulkheads or in any case in order to be able to inspect the inside of the cylinder.