MIXING TANK, PRIMARILY FOR DISSOLVING FERROUS SULPHATE GRANULATE IN WATER

Abstract: A mixing tank (1) for the continuous mixing of ferrous sulphate granulate with water. The mixing tank (1) comprises two parallel plastic pipes (2', 2'') positioned helically on the bottom (3) of the tank (1), the upper sides of said pipes having the same water discharge outlet area per area unit of the bottom (3) of the tank (1). As a result, water is always distributed evenly for percolation through the ferrous sulphate solution, thus always producing a ferrous sulphate solution of a "saturated" concentration. The manufacture of the mixing tank (1) is therefore considerably quicker and cheaper than the manufacture of known types of mixing tanks, said mixing tank also being more economical in operation and smaller in dimension than known types of mixing tanks having the same output.

Description: Title: Mixing tank, primarily for dissolving ferrous sulphate granulate in water Technical Field The present invention relates to a mixing tank, primarily for the continuous mixing of a predetermined quantity of ferrous sulphate granulate in the tank with water to be supplied to the tank via at least one water supply device provided at the bottom of the tank, thus producing a ferrous sulphate solution to be taken from a defined collection area of the tank and to be dosed for discharge into a sewage purification plant. Background Art Typically, mixing tanks of this type have a capacity of 10-15 tons of ferrous sulphate granulate and thus an output allowing them to supply a sewage purification plant serving a town of 4,000 - 50,000 inhabitants with a constant and sufficient quantity of ferrous sulphate solution. A control system connected to the tank ensures that the dose of ferrous sulphate solution to the sewage purification plant is proportional to the flow quantity of sewage. In the sewage purification plant, the ferrous sulphate binds phosphates present in the sewage, said phosphates consequently precipitating as phosphate sludge. It is necessary to equip the known types of mixing tanks with expensive mechanical agitators as well as a control means to achieve a fairly uniform, predetermined concentration of the ferrous sulphate solution during operation, said solution preferably being a so-called "saturated" solution, where the concentration is always known because of the chemical equilibrium. In known types of mixing tanks not equipped with mechanical agitators, it is impossible during operation to produce a ferrous sulphate solution having a uniform, predetermined concentration. Furthermore, these mixing tanks have to be of such a size as to be supplied with disproportionately large quantities of ferrous sulphate granulate in relation to the ferrous sulphate solution produced to ensure that the solution is of a fairly and approximately uniform concentration. Another problem in the known types of mixing tanks without agitators is the ever-appearing undesirable cone-shaped cavity formations in the ferrous sulphate granulate as said granulate dissolves, thus allowing water to percolate too quickly through the granulate in the centre of such a cone-shaped cavity formation. Consequently, it is expensive to manufacture all the types of mixing tanks mentioned above, said tanks either having to be equipped with expensive , mechanical agitators or having to be of such a size as to ensure a fairly uniform concentration of the ferrous sulphate solution produced (said concentration however not being guaranteed) and said tanks also being uneconomical to operate because of excessive concentration variations in the ferrous sulphate solution. Disclosure of the Invention The object of the present invention is therefore to provide a mixing tank of the type mentioned above, said tank being inexpensive to manufacture and allowing the production of a ferrous sulphate solution of as uniform a concentration as is possible, thus ensuring economical operation. According to the present invention, the mixing tank is characterised in that one or several water supply devices are provided in such a way than said device(s) is/are principally positioned evenly to cover the entire bottom of the tank and that the entire extent of said water supply device(s) principally has the same water discharge outlet area per area unit of the bottom of the tank. This allows a mixing tank to be manufactured, wherein there is a constant production of a ferrous sulphate solution having a known and precisely predetermined concentration of ferrous sulphate in water owing to an evenly distributed discharge of water over the entire bottom of the tank, said water thus percolating at a constant rate through the entire quantity of ferrous sulphate granulate. During operation the mixing tank is therefore considerably more economical than known mixing tanks of the same output. The mixing tank according to the invention is also considerably smaller than known mixing tanks of the same output, and the manufacture of said mixing tank is therefore both considerably quicker and cheaper. If the level of the non-dissolved ferrous sulphate granulate in the mixing tank constantly exceeds a specific, sufficient level and if at the same time the quantity of water supplied per unit time and the supply pressure of water are below specific levels, the result is a sufficient percolation passage and a sufficient water percolation time through the ferrous sulphate granulate, thus always ensuring a production of a "saturated" solution of ferrous sulphate. This ensures a constant production of a ferrous sulphate solution of a uniform concentration (the concentration variations of a "saturated" solution being 2-3% under normal conditions), at the same time allowing the most economical operation of the mixing tank. According to the invention, at least one water supply device may principally in its entire extent comprise two or more separate water supply devices provided in such a way as to cover separate, smaller areas of any defined area of the bottom of the tank, said defined area being covered by said water supply device, and the water supply device may be connected to a water inlet means provided in such a way as to supply water in predetermined quantities per unit time and period of time optionally and alternately to the separate water supply devices. Consequently, the so-called cone-shaped cavity formation always appearing approximately in the centre of the ferrous sulphate granulate during the dissolution of said quantity and therefore producing an uneven concentration of the solution and resulting in uneconomical operation, is constantly shifted in the quantity of ferrous sulphate granulate and is therefore of an infinitesimal size. This allows the use of an even smaller quantity of ferrous sulphate granulate and thus a smaller mixing tank to produce the same output, whereby the size and thus also the construction price of the tank may be reduced even more. Furthermore, all separate water supply devices may, according to the invention, principally and preferably have the same water discharge outlet area per area unit of the bottom of the tank. Moreover, the mixing tank may, according to the invention, principally be cylindrical with a circular bottom, and the separate devices of the water supply device may comprise two parallel pipes positioned helically to cover the bottom of the tank, the helical centres preferably being close to the centre of the bottom. As a result, the mixing tank is particularly economical to manufacture in view of its output. Furthermore, the mutual radial distance between two adjacent pipes in each of two successive convolutions of the helices may be 10 cm principally anywhere in the tank, and each pipe may on its upper side and in its entire extent be provided with holes at a mutual distance of approximately 20 cm, said holes preferably having a diameter of 1-2 mm. The manufacture of this discharge system is particularly simple and quick. Finally, in a particularly advantageous embodiment of the invention, the upper side of the water supply device may be provided as a grate or a sieve. Brief Description of Drawings The invention is explained in greater detail below and with reference to the accompanying drawings, where Fig. 1 is a side and partially sectional view of the mixing tank according to the invention, said tank being dug into the ground close to a sewage purification plant not shown in the drawing. Fig. 2 is a top view of the mixing tank, and Fig. 3 is a sectional view along the line III - III of Fig. 1. Best Mode for Carrying Out the Invention Fig. 1 shows a mixing tank 1 comprising a water supply device 2 provided at the circular bottom 3 of the tank and a filler hole 4 with one end above the level of the ground, the embodiment shown being a mixing tank dug into the ground. The filler hole 4 may be opened-and closed by means of a cover 5 operated by means of a winch 6 so that the tank 1 may be filled with ferrous sulphate granulate through the filler hole 4 from a lorry platform to be tipped when required at regular intervals . At the bottom 3, the mixing tank 1 is provided with outside attachment devices 7 secured to holding means 8, said holding means principally being large concrete blocks with tie bars in the ground. Moreover, the mixing tank 1 comprises a defined area principally in the shape of a vertical pipe 9 for accumulating the ferrous sulphate solution produced, before said solution is dosed for the sewage purification plant through a pipeline system not shown in the drawings. On top of the mixing tank 1 and above ground level there is a pipe connection means 10 connected to a water supply device not shown, the sewage purification plant and a system, not shown, to control the water supply to the tank 1 and the solution of ferrous sulphate dosed from the tank to the purification plant. The mixing tank dug into the ground principally ensures a constant, uniform temperature in the tank, resulting in economical operation during conditions of hard frost. The tank may, however, equally well be positioned on the ground. The mixing tank 1 is principally made of cast fibreglass- reinforced polyester with an internal smooth surface. Having a capacity of 10-15 tons of ferrous sulphate granulate, the tank may have a diameter of 3.35 m and a height of approx. 4 m. The pipe 9 has a diameter of 400 mm. Thus the tank 1 has a capacity sufficiently large to supply a purification plant serving a town of up to 50,000 inhabitants, said quantity being extremely large for such a small tank. The water supply device 2 principally comprises two parallel plastic pipes 2' and 2", said plastic pipes preferably being polyethylene pipes. The pipes 2' and 2" are principally positioned helically to cover the bottom 3 of the mixing tank 1 at a radial distance of approx. 10 cm with the helical centre preferably in the centre of the bottom 3 of the mixing tank 1, cf. Fig. 3. The pipes 2' and 2" are connected to the pipe connection means 10. The pipes 2' and 2" are primarily secured inside the bottom 3 of the mixing tank 1 by means of radial stringers or the like not shown. On its upper side and over its entire extent, each pipe 2' and 2" is provided with holes at a mutual distance of approximately 20 cm. Preferably, the holes have a diameter of 1-2 mm. This ensures that anywhere in the tank the water supply device 2 has approximately the same water discharge outlet area per area unit of the bottom of the tank 3. The water supplied is therefore discharged evenly over the entire bottom 3 of the tank and is allowed to percolate evenly through the entire quantity of ferrous sulphate granulate. If the level of the non-dissolved ferrous sulphate granulate in the tank constantly exceeds a specific, sufficient level A (in the present embodiment, at some 6-700 mm above the pipes 2' and 2"), and if at the same time the quantity of water supplied per unit time is 300-1,000 1/hour and the supply pressure of water corresponds to that of tap water, the result is a sufficient percolation passage and a sufficient water percolation time through the ferrous sulphate granulate, thus allowing a constant production of a "saturated" solution of ferrous sulphate in water. Half of the ferrous sulphate granulate filled into the tank thus being dissolved, the ferrous sulphate granulate level is typically at B. The upper liquid level is typically kept constant at C. The solution is accumulated in the pipe 9 and is dosed for discharge to the sewage purification plant. The production of a "saturated" solution of ferrous sulphate granulate always being ensured, a very high output is achieved in the mixing tank 1 along with really economical operation. Water is supplied alternately and at predetermined intervals to be set by means of the control system from the water inlet means not shown through the pipe connection means 10 to one pipe 2' or to the other pipe 2", but never simultaneously to both of the pipes 2' and 2". The result is that the undesirable cone-shaped cavity formation (said formation always appearing in a quantity of ferrous sulphate granulate because of the internal friction and the friction against the tank wall from the granulate) always disappears when the water supply is switched from one pipe 2' to the other pipe 2" or vice versa. This ensures that water passes in the same flow direction when percolating through the quantity of ferrous sulphate granulate to allow a constant production of a "saturated" solution. The required minimum quantity of ferrous sulphate granulate in the tank before refilling may also be reduced considerably. The device may be varied in many ways without thereby deviating from the scope of the invention. The upper side of the water supply device may be provided as a sieve or the water supply device may comprise a large number of nozzles positioned evenly over the bottom of the tank.

Claims: Claims 1. A mixing tank primarily for the continuous mixing of a predetermined quantity of ferrous sulphate granulate in the tank with water to be supplied to the tank via at least one water supply device provided at the bottom of the tank, thus producing a ferrous sulphate solution to be taken from a defined area of the tank and to be dosed for discharge into a sewage purification plant, c h a r- a c t e r i s e d in that one or several water supply devices (2) are provided in such a way that said device(s) is/are principally positioned evenly to cover the entire bottom (3) of the tank (1) , and that the entire extent of said water supply device(s) (2) principally has the same water discharge outlet area per area unit of the bottom of the tank (3). 2. A mixing tank as claimed in 1, c h a r a c t e r¬ i s e d in that at least one water supply device (2), principally in its entire extent, comprises two or more separate water supply devices (2', 2") provided in such a way as to cover separate, smaller areas of any defined area of the bottom of the tank (3) , said defined area being covered by said water supply device (2), and that the water supply device (2) is connected to a water inlet means provided in such a way as to supply water in predetermined quantities per unit time and period of time optionally and alternately to the separate water supply devices (2', 2"). 3. A mixing tank as claimed in 2, c h a r a c t e r¬ i s e d in that both of the separate water supply devices (2', 2") principally have the same water discharge outlet area per area unit of the bottom of the tank (3) . 4. A mixing tank as claimed in 3, where the mixing tank is principally cylindrical with a circular bottom, c h a r a c t e r i s e d in that the separate devices (2', 2") of the water supply device (2) comprise two parallel pipes (2', 2") positioned helically to cover the bottom (3) of the tank (1) , the helical centres preferably being close to the centre of the bottom (3). 5. A mixing tank as claimed in 4, c h a r a c t e r- i s e d in that the mutual radial distance between two adjacent pipes (2', 2") in each of two successive convolutions of the helices is 10 cm principally anywhere in the tank and that each pipe (2' , 2" ) on its upper side and in the entire extent is provided with holes at a mutual distance of approximately 20 cm, said holes preferably having a diameter of 1-2 mm. β . A mixing tank as claimed in 1, c h a r a c t e r¬ i s e d in that the upper side of the water supply device (2) is provided as a grate or a sieve.

Inventors: HOVAD KARL ERIK, HOVAD, KARL, ERIK
Applicants: TUNETANKEN AS, TUNETANKEN A/S
Application year: 1990
Main classification: B01F3
IPC codes: B01F1, B01F15