RECOVERY OF SOLIDS FROM DISPERSIONS

Abstract: A process for the recovery of solid materials from dilute dispersion of finely divided solid material, such as animal wastes, sewage, asbestos and coal, comprising adding to the dispersion an agglomeration agent capable of inducing flocculation of the particles, mixing the agglomeration agent and the dispersion, conditioning the mixture to allow flocculation to take place, and passing the flocculated mixture, substantially without shearing the flocculated particles, to a rapped or vibrated sieve bend and recovering the flocculated particles from the overflow of the sieve bend.

Description: ESC07BRT OF SOLIDS FROM DISPERSIONS TECHNICAL FIELD The present invention relates to a process for the recovery of solid materials from dilute dispersions of finely divided particles of a solid material in a liquid medium. DISCLOSURE OF INVENTION The present invention consists in a process for the recovery of solid materials from dilute dispersions of finely divided particles of a solid material in a liquid medium, the process comprising adding to the dispersion an agglomeration agent capable of inducing flocculation and/or agglomeration of the particles, mixing the agglomeration agent and the dispersion, conditioning the mixture so formed to allow flocculation and/or agglomeration of the finely divided particles to take place until substantially all of the agglomeration agent has been utilized, and passing the flocculated and/or agglomerated mixture, substantially without shearing the flocculated and/or agglomerated particles, to a rapped or vibrated sieve bend such that the flocculated and/or agglomerated particles may be recovered as overflow from the sieve bend and at least part of the liquid medium may be recovered as an underflow therefrom. The present invention further consists in a circuit for the recovery of solid materials from dilute dispersions of finely divided particles of solid material in a liquid medium, the circuit comprising supply means for the dispersion, means to mix the dispersion with a flocculation and/or agglomeration agent, a condition vessel in which the particles of the dispersion may flocculate and/or agglomerate and a rapped or vibrated sieve bend over which the flocculated and/or agglomerated dispersion may be passed to produce an oversize stream or an undersize stream. Problems have been experienced in the past in the recovery of very finely divided solid particles from dispersions of those particles in a liquid, normally water. It has been known to flocculate these particles as an aid to OMPI - Z - their natural settlement or by their removal in some other way. The present invention is designed to flocculate such particles and remove them on a rapped or vibrated sieve bend. In view of the vibratory force applied to the particles during the sieving conventional wisdom is that such a sieve bend would have the effect of breaking the floccs and would not be effective in recovery of the flocculated and/or agglomerated particles. The present inventors have found, however, that the process according to this invention is extremely effective for the recovery of many types of fine particles from dispersions thereof. Dispersions which have been found to be capable of effective treatment by the method according to this invention include animal wastes such as piggery wastes and cattle effluent wastes, sewage, asbestos and coal. The process according to this invention may be utilised advantageously in respect of particles having a size down to 10 microns and in some cases even below that size. The selection of an appropriate agglomeration agent will depend on the solid particles being agglomerated and/or flocculated, however, the selection may be made according to the known parameters in the art. Conveniently polyelectrolytes may be used, however, any of the known agglomeration or flocculation agents may be used. The agglomeration agent must be mixed thoroughly with the dispersion being treated and this may be conveniently achieved by injection of the agglomeration agent into a stream of the dispersions immediately upstream from a pump. It has been found that the turbulence produced by the action of the pump is normally sufficient to adequately mix the agglomeration agent and the dispersion. Other conventional mixing means could, however, be used. Conditioning preferably takes place by passing the stream of the dispersion containing the agglomeration agent through a conditioning vessel in which the rate of flow of the dispersion is substantially reduced. Under conditions of low flow rate the formation of floccs or agglomerates is encouraged and maximum flocc or agglomerate size is achieved. It is essential that the conditioning process be allowed to proceed until substantially all of the agglomeration agent has been utilized in the formation of floccs. If conditioning does not proceed for a sufficient time then there is a danger of floccs being formed in the sieve screen of the sieve bend which will cause blinding of the screen. In one preferred embodiment of the invention the conditioning vessel is operated under a mild overpressure in order that the pump used to produce the turbulence at the time of agglomeration agent addition may be used to feed the dispersion through the conditioning vessel and to the sieve bend substantially without shearing of the floccs and/or agglomerates. It is obvious that shearing or other destruction of the floccs and/or agglomerates would be deleterious to the process. It is therefore desirable to avoid the presence of a pump between the conditioning vessel and the sieve bend. In another preferred embodiment of the invention the dispersion to which the flocculation and/or agglomeration agent has been added is pumped from a conditioning vessel to the sieve bend by a pump which has only a limited shearing effect on the floccs or agglomerates such as a screw type pump. In this embodiment of the invention the flocculation and/or agglomeration agent is preferably added to the dispersion in the conditioning vessel and the mixture slowly stirred as with a paddle stirrer. The dispersion is in one embodiment of the invention preferably fed to the sieve bend as an upward flow through an elutriation device such that a substantially uniform flow of the dispersion occurs across the full width of the sieve bend. In another embodiment of the invention the dispersion may be fed to the sieve bend through flow distribution device as described in Australian patent specification 80,859/82. While any rapped or vibrated sieve bend may be used in the process according to this invention it is highly desirable to use a sieve bend made according to Australian patent specification 90792/82. This specification describes a sieve bend in which the sieve screen is connected to a surrounding frame only at its end edges. This allows the sieve screen to vibrate more freely under an applied rapping or vibrating force. It has been found that this free vibration of the sieve screen substantially reduces the chance of the screen blinding under any given circumstances. The slot width of the sieve screen on the sieve deck is preferably from 4 millimeters down to 50 microns preferably 300 microns down to 75 microns. The exact size being selected to suit the particular floccs and agglomerates formed in the process. The process according to the present invention has particular advantage when treating sewage or animal effluence. These materials commonly have to be transported for disposal and are normally obtained in a form having a solids content of from 1 to 2% by weight. The cost of transporting these materials is high in relation to their solids content. By utilizing the process according to this invention the solids content may be raised from 10 to 15%. The water produced as underflow has been found to have a sufficiently low chemical and biological oxygen demand that it can be fed directly to conventional drains without adverse ecological effects. The process according to this invention may also be used for the recovery of minerals slimes. In this case agglomeration may be brought about by the use of an agglomeration agent such as oil or polyelectrolytes which will preferentially agglomerate the hydrophobic particles leaving the hydrophilic particles at the water in a dispersed phase. In this process not only is there a recovery of solids from the dispersion but also a conditioning of the minerals. BRIEF DESCRIPTION OF THE DRAWINGS Hereinafter given by way of example only is a preferred embodiment of the present invention described with reference to the accompanying drawings which shows schematically a circuit for carrying out the process according to the present invention. • BEST MODE OF CARRYING OUT THE INVENTION The circuit 10 is designed for the concentration of the solids content of piggery effluent. In the conduct of a modern piggery the pigs are maintained in buildings with a grid like floor through which urine and faecal matter may fall into a disposal channel below the floor. Water is used to clean the styes and wash the effluent along the disposal channels. The tank 11 receives this waste water in the circuit 10. Outflow from the tank 11 is controlled by valve 12 and flows through pipe 13 into a conditioning vessel 14. A flocculent supply tank 15 is provided in the circuit 10. The flocculent is pumped by pump 16 through lines 17 and 18 through valves 19 and 21 respectively into the pipe 13 and the conditioning vessel 14. A slowly moving paddle stirrer 22 is provided within the conditioning vessel 14. This stirrer 22 is driven by an electric motor 23 mounted on the top of the vessel 14 through a drive shaft 24. A low shear, screw type pump 25 pumps the flocculated dispersion from the conditioning vessel through pipe 26 to a rapped sieve bend 27. The sieve bend 27 includes a distribution box 28 to spread the flocculated dispersion evenly across the width of the sieve screen 29 of the sieve bend 27. The sieve screen 29 is so mounted on surrounding frame (not shown) that it can vibrate and flex freely relative to t h e frame. The sieve screen 29 is connected rigidly to the frame along a single end edge and is connected to the frame along the other end edge through a resilient attachment. The sieve screen 29 is provided on its underside with a rapping bar and means 31 are provided to cause the sieve screen 29 to vibrate. The undersize material which passes through the sieve screen 29 into a hopper 32 is discharged through pipe 33. The oversize material is fed off the lower end of the screen 29 into pipe 34. In use effluent is flowed from tank 11 down pipe 13. The flocculation agent is pumped from tank 15 into pipe 13 to mix with the effluent stream. This mixed stream is then flowed into the conditioning vessel 14 where the slow stirring ensures thorough mixing of the effluent and the flocculation agent. Additional flocculation agent may be added to the conditioning vessel 14 through line 18 if required. The conditioned and flocculated material is then pumped by pump 25, to the distribution means 28 where it is converted from a cylindrical flow into a flat stream flowing evenly onto the sieve bend 27. The sieve screen 29 of the sieve bend 27 divides the dispersion into oversize and undersize streams. The oversize stream largely comprises the flocculated particles of effluent and a proportion of the water while the undersize stream largely comprises water and dissolved organic and inorganic materials but very little solid material. The above circuit was use to treat piggery effluent with a high cationic flocculant sold under the registered trade mark "PRAESTOL". The results obtained were as follows:- Sample Suspended Solids B.O.D, Feed 6800 5800 Undersize material 3300 4700 obtained by sieving without flocculation Undersize material 1100 obtained by sieving after flocculation It can be seen that the flocculation reduced the suspended solids and B.O.D. content of the undersize material substantially and to an extent where it could be discharged as drainage effluent. The oversize material was increased from 1 to 2% solids to approximately 15% solids. This substantially reduced the volume of high strength waste which had to be transported for disposal as sewage waste.

Claims: THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS: 1. A process for the recovery of solid materials from dilute dispersions of finely divided particles of a solid material in a liquid medium, the process comprising adding to the dispersion an agglomeration agent capable of inducing flocculation and/or agglomeration of the particles, mixing the agglomeration agent and the dispersion, conditioning the mixture so formed to allow flocculation and/or agglomeration of the finely divided particles to take place until substantially all of the agglomeration agent has been utilized, and passing the flocculated and/or agglomerated mixture, substantially without shearing the flocculated and/or agglomerated particles, to a rapped or vibrated sieve bend such that the flocculated and/or agglomerated particles may be recovered as overflow from the sieve bend and at least part of the liquid medium may be recovered as an underflow therefrom. 2. A process as claimed in Claim 1 in which the liquid medium is selected from the group comprising piggery wastes, cattle effluent wastes, sewage, asbestos slurries and coal slurries. 3. A process as claimed in Claim 1 or Claim 2 in which the agglomeration agent is a polyelectrolyte. 4. A process as claimed in any one of Claims 1 to 3 in which the conditioning of the mixture is brought about by passing the stream of the dispersion containing the agglomeration agent through a conditioning vessel in which the rate of flow of the dispersion is substantially less than its rate of flow immediately prior to its entering the conditioning vessel. 5. A process as claimed in Claim 1 in which the sieve bend has a sieve screen having apertures with a width of from 200 microns to 75 microns. 6. A circuit for the recovery of solid materials from dilute dispersion of finely divided particles of solid material in a liquid medium, the circuit comprising supply means for the dispersion means to mix the dispersion with a flocculation and/or agglomeration agent, a condition vessel in which the particles of the dispersion may flocculate and/or agglomerate and a rapped or vibrated sieve bend over which the flocculated and/or agglomerated dispersion may be passed to produce an oversize stream and an undersize stream. 7. A circuit as claimed in Claim 6 in which the sieve bend has a sieve screen having apertures with a width of from 200 microns to 75 microns. 8. A circuit as claimed in Claim 6 in which stirring means are provided in the conditioning vessel to slowly mix the dispersion and the flocculating and/or agglomeration agent. 9. A process for the recovery of solid materials from dilute dispersions of finely divided particles of a solid material in a liquid medium substantially as hereinbefore described with reference to the accompanying drawing. 10. A circuit for the recovery of solid materials from dilute dispersions of firmly divided particles of a solid material in a liquid medium substantially as hereinbefore described with reference to the accompanying drawing. OMPr

Inventors: LEES JEREMY JAMES, LEES, JEREMY, JAMES
Applicants: ILECARD PTY LTD, ILECARD PTY. LIMITED
Application year: 1983
Main classification: B07B1
IPC codes: B01D21, B01D35, B01D37, B03B1, B03B7, B03B9, B03D3, C02F1, C02F11