VORTEX FLOCCULATION OF SOLIDS SUSPENDED IN LIQUID

Abstract: A method of promoting the formation of flocs comprising the steps of passing a liquid having suspended particles and an added flocculant as a vortex flow through a tapered conduit where the conduit is circular in cross section and increases in cross-sectional size in the direction of liquid flow through the conduit.

Description: VORTEX FLOCCULATION OF SOLIDS SUSPENDED IN LIQUID. This invention relates to the separation of solids from the liquid in which they are suspended. Suspended solid particles in a liquid such as water will float or sink at a velocity which is a function of the size of the particles, the specific gravity difference between the solids and the liquid and the viscosity of the liquid. The smaller the particles, the lower is the settling velocity, which effectively becomes zero for small "colloidal" particles. If the particles settle rapidly they can be separated from a liquid stream more readily. The settling velocity can be increased by collecting the particles into "floes' which are agglomerations of the particles, where the floes act as one composite particle and settle more rapidly than the particles making up the floes would settle. The creation of floes is usually promoted by adding chemicals to the liquid, either coagulants or flocculants, which act upon the electrical charges on the particles. The floes grow by collision between particles in the fluid as it is agitated or mixed and the particles are held together after colliding by forces of electrical attraction. For floes to grow and settle rapidly the fluid mixing is such that the particles collide in the velocity gradients created by the mixing. This invention is concerned with the manner of mixing liquid having suspended particles with flocculating agents to promote the accelerated growth of floes. It is known to promote flocculation using stirrer type apparatus. With the method and apparatus provided by this invention uniform sized floes can be formed in a controlled manner, this is not possible with conventional stirred apparatus. A practical use for the method of the invention would be for the treatment of water. For example, water from reservoirs where contaminants have mixed with the water, such water being for use as drinking water or for commercial purposes. Another example would be for the treatment of water which has been contaminated by use, such as industrial processes or in a sewerage system. Broadly stated, the invention in one form comprises a method of promoting the formation of floes comprising the steps of passing a liquid having suspended particles and an added flocculant as a vortex flow through a tapered conduit where the conduit is circular in cross-section and increases in cross-sectional size in the direction of liquid flow through the conduit. The invention also provides a method of promoting the formation of floes comprising the steps of mixing a liquid having suspended particles with gas under pressure and expanding the gas/liquid mixture to a lower pressure and passing the mixture as a vortex flow through a tapered conduit where the conduit is circular in cross-section and increases in cross-sectional size in the direction of liquid flow through the conduit. The invention further provides in addition to the above method the additional step in which a flocculent is mixed with said liquid- at or prior to entry into the inlet of the conduit. The first and third methods identified above involve a rapid mixing of the particle supporting liquid with the flocculant forming chemicals to promote an even dispersion of the chemicals in the liquid and also frequent collisions between the particles in the liquid. The large shear forces and high velocity gradients created by the rapid mixing creates an environment where the development of large floes is prevented and an equilibrium floe size develops for a particular velocity gradient. In view of this the floe size and the settling rate can be varied by increasing or decreasing the velocity gradients in the liquid. If rapidly settling large floes are required the liquid is mixed rapidly to disperse the chemicals as they are added and the velocity gradients are gradually reduced to allow the floes to grow to the desired size for the required settling rate. The apparatus of the invention has been devised to provide the above conditions. The liquid containing the particles and the coagulant chemicals is introduced into the inlet (bottom) end of a tapered conduit of circular cross-section which increases in size in a direction away from the inlet. The entry of the liquid into the conduit is arranged to provide a vortex flow in the liquid which rotates about the axis of the conduit, which axis is usually vertical. It is to be understood however that the method of the invention can be performed where the axis of the apparatus and the vortex flow through the apparatus is not vertical. The mixing of the chemicals and the liquid is very vigorous and the velocity gradients are very high. As the liquid flows through the conduit the cross-sections size of the conduit increases and accordingly the velocity of the liquid and the velocity gradients decrease, and correspondingly the equilibrium size of the floes will increase. It is essential that the taper of the conduit is not such that the flow will leave the conduit wall and cause turbulence as this will interfere with the uniform decrease of the velocity gradients as the liquid passes through the conduit. The conduit may be conical in shape, or it may be of varying rate of change in diameter along its length, such as is to found in a trumpet. The shape can be calculated so that the diffusing vortex flow does not separate. In a representative apparatus the conduit is conical and is usually vertically disposed with the large open end of the cone uppermost. There is a vortex forming means at or adjacent the small end of the cone. The vortex forming means can be a tangential discharge liquid feed pipe discharging liquid under pressure into the conduit, it can be a feed pipe axially or otherwise entered into the conduit from below or above fitted with a head from which the feed liquid is discharged under pressure at an angle to the axis of the conduit thereby to form a vortex flow in the conduit. On discharge from the conduit the fluid will have been prepared so that the suspended solids as floes will settle rapidly in a settling basin which may be circular in shape and surround the apparatus. In a variation of the above method dissolved air flotation is added. Dissolved air flotation involves saturating the liquid with air or gas at pressure and then expanding the mixture to a lower pressure so the air or gas becomes supersaturate s at the lower pressure and micro bubbles are formed in the liquid around the solid particles which act as nuclei. The micro bubbles increase in size as more air comes out of solution and the bubbles with their attached solid particles rise to the surface of the liquid and form a floating layer. The floating layer is then scraped from the surface of the liquid. The above process can be enhanced by the addition of coagulants or flocculants. Liquid treated as above is introduced into the inlet end of a conical conduit, as before described. The centrifugal forces acting in the vortex cause the lighter materials to tend to float towards the centre of the vortex as well as upwardly. In this way the rising floes will move away from the walls of the cone an proceed upwardly leaving substantially clear liquid adjacent the conduit walls. This tendency increases as the flow rises in the cone so that at the top surface of the conical vessel the floating floes will congregate towards the centre leaving a margin of clear liquid around the congregation of floes. In a proposed arrangement for the collection of the floes, there would be a collecting/settling tank around the outlet end of the conduit and spaced therefrom so that water discharging over the outlet end rim will flow into the tank. Around the outlet end of the conduit and spaced therefrom but inside the tank there would be a retaining baffle with an upper edge above the outlet end of the conduit and a lower edge below the outlet end of the conduit. The water flow to the tank from the conduit would pass under the baffle and the floating floes would be retained within the baffle. The floating layer of floes can be removed either by suction or by some mechanical means as they accumulate. In another arrangement the water from the conduit with its floes would be discharged into a settling tank where the floes would float or sink, depending upon the relative densities of the water and the floes and the particles and whether the liquid had been aerated. The floes would be collected from the surface of the tank or from the bottom of the tank as required by conventional means.

Claims: Claims : 1. A method of promoting the formation of floes comprising the steps of passing a liquid having suspended particles and an added flocculant as a vortex flow through a tapered conduit where the conduit is circular in cross-section and increases in cross-sectional size in the direction of liquid flow through the conduit. 2. The method of claim 1 where the flocculant is added to the liquid prior to-the admission of the liquid to the conduit. 3. A method of promoting the formation of floes comprising the steps of mixing a liquid having suspended particles with gas under pressure and expanding the gas/liquid mixture to a lower pressure and passing the mixture as a vortex flow through a tapered conduit where the conduit is circular in cross-section and increases in cross-sectional size in the direction of liquid flow through the conduit. 4. A method as claimed in claim 3 including the additional step of mixing a flocculent with said liquid. 5. Apparatus for carrying out the method of any one of claims 1 to 4 comprising a conduit with an inlet for liquid adjacent a first conduit end and provided with an outlet end, said conduit being circular in cross-section and increasing in diameter from said first end to said outlet end, and means at said liquid inlet to promote vortex flow of the liquid through the conduit. 6. Apparatus as claimed in claim 5 where the conduit has a constant angle of taper between the conduit first and outlet ends. 7. Apparatus as claimed in claim 5 where the angle of taper of the conduit is not constant. 8. Apparatus as claimed in any one of claims 5 to 7 where the vortex flow is created by introducing the liquid into the conduit through a tangential inlet through the wall of the conduit adjacent said first conduit end. 9. Apparatus as claimed in any one of claims 5 to 7 where the vortex flow is created by tangential discharge of liquid from a discharge head disposed within the conduit adjacent said first conduit end.

Inventors: RODGERS WILLIAM, RODGERS, WILLIAM
Applicants: RODGERS WILLIAM, RODGERS, WILLIAM
Application year: 1993
Main classification: B04C3
IPC codes: B01D21, B04C3, C02F1