GB2450797A

GB2450797A - Dewatering solids

Info

Publication number: GB2450797A
Application number: GB0811939A
Authority: GB
Inventors: Stephen John Adkins
Original assignee: Ciba Holding AG
Current assignee: BASF Schweiz AG
Priority date: 2007-07-06
Filing date: 2008-07-01
Publication date: 2009-01-07
Anticipated expiration: 2028-07-01
Also published as: GB0811939D0; GB2450797B; GB0713024D0

Abstract

A process of providing dewatered solids derived from effluent resulting from the processing of granite or sand and gravel by concentrating an aqueous suspension of solid particles derived from the effluent, comprises the steps of adding to the aqueous suspension at least one organic polymeric flocculent to the suspension thereby forming flocculated solids, allowing the flocculated solids to settle in a gravimetric thickener to form a layer of suspended concentrated solids as a thickener underflow, and passing the thickener underflow to a dewatering stage, in which the process comprises the application of an effective amount of an agent, and in which the agent is selected from the group consisting of free radical agents, oxidising agents, enzymes and radiation. In particular, flocculated solids can be settled to form a bed in which higher solids and/or reduced yield stress can be achieved. Furthermore, when the concentrated suspension is dewatered on a filter press the process can provide a significant reduction in press cycle time.

Description

Concentration and Dewatering of Suspensions The present invention

relates to an improved flocculation process for the concentration of suspensions of solid particles consisting essentially of effluent resulting from the processing of granite or sand and gravel. In particular.

flocculated solids can be settled to form a bed in which higher solids and/or reduced yield stress can be achieved. Furthermore, when the concentrated suspension is dewatered on a filter press the process can provide a significant reduction in press cycle time.

It is known to concentrate suspensions of solids in aqueous liquids by use of flocculants resulting in flocculation of the solids which facilitates the separation of the solids from the liquid. In many processes the flocculated solids settle to form a bed by sedimentation. In other processes separation can be facilitated by mechanical dewatering, for instance in pressure filtration, centrifugation, by belt thickeners and belt presses.

The types of flocculant added to the suspension will often depend upon the substrate. Generally suspensions tend to be flocculated by high molecular weight polymers. Examples of this are described in WO-A-9314852 and US3975496 regarding the flocculation of mineral suspensions such as red mud.

Other disclosures of high molecular weight polymeric flocculants include US 6447687, WO-A-0216495 and WO-A-02083258 dealing with the flocculation of sewage sludge. It is known to sometimes add other chemical additives to condition the suspension. For instance suspensions may be first coagulated by a high charge density polymeric coagulant such as poIyDADMAC or inorganic coagulants including ferric chloride.

It is known to control the operation of the dewatering process in response to some parameter of the suspension to be flocculated or the resulting underflow.

This may be the control of flocculating agent or other chemical additive or some other operational control feature. US 4178243 describes the dewatering of an aqueous slurry of fine solid particles, particularly tailings of a coal treatment underilow. A radiation beam from a radiation source, typically of gamma radiation, is impinged on the flocculated solids transverse to the direction of travel of the solids and adjacent to the discharge means for the dewatered solids from the thickener. The impinged radiation beam is detected by detection means to continually measure the density of the flocculated solids on which the radiation beam impinges. The detector means in turn produces an electrical signal proportional to the impinged radiation detected and to the density of the flocculated solids. This electrical signal is used to automatically regulate and control a valve means adjacent to the discharge means and in turn the rate of discharge of the flocculated solids from the thickener. The radiation beam does not have a material effect on the dewatered solids and is only used to measure density of the under-flow suspension.

Other additives are also use in the conditioning of suspensions. For example peroxides are sometimes added to suspensions such as sewage sludges or other suspensions containing organic material in order to remove reducing agents, in order to reduced odours, gas formation or prevent putrefaction. In general the peroxides or oxidising agents tend to be added in order to remove harmful or unwanted substances or other materials contained in the suspension.

Generally the amount of peroxides added is only sufficient to remove the unwanted substances and materials and generally peroxides or other oxidising agents are included in relatively small amounts.

Examples of adding peroxides to sewage sludge are described in JP561 50481.

Peroxides or oxidising agents may also be added to other suspensions for similar reasons including treating dredged material to remove contaminants as described in Us 2003 121863 and JP 10109100. JP 11156397 describes a process for flocculating mud using non-ionic and anionic polymers in which the mud has been pretreated with an oxidising agent.

U.S. 6733674 describes a method of dewatering sludge by adding an effective amount of one or more cellulolytic enzymes and one or more oxidants and one or more flocculants to form a mixture in water which is coagulated and flocculated followed by separation of solids from the water. The examples seem to indicate a significant time elapsed between oxidant addition and flocculation.

The enzymes appeared to be present in order to degrade material contained in the sludge.

Suspensions are frequently concentrated in a gravity thickener vessel. A continual flow of the suspension is typically fed into the thickener and treated with a flocculant. The flocculated solids thus formed settle to form a bed of solid underflow and supernatant aqueous liquid flows upwards and is usually removed from the thickener vessel through a perimeter trough at the water surface. Normally the thickener vessel has a conical base such that the underflow can easily be removed from the centre of the base. In addition a rotating rake assists the removal of the underfiow solids. A typical process for concentrating suspensions in a gravity thickener is described in US4226714.

Various suspensions can be concentrated in gravity thickeners, including suspensions of organic solids such as wastewater, sewage and sewage sludges. It is also commonplace to thicken or dewater mineral suspensions using gravity thickeners.

In a typical mineral processing operation, waste solids are separated from solids that contain mineral values in an aqueous process. The aqueous suspension of waste solids often contains clays and other minerals, and are usually referred to as tailings. These solids are often concentrated by a flocculation process in a thickener and settle to form a bed. Generally it is desirable to remove as much water from the solids or bed in order to give a higher density underflow and to recover a maximum of the process water. It is usual to pump the underflow to a surface holding area, often referred to as a tailings pit or dam, or alternatively the underfiow may be mechanically dewatered further by. for example, vacuum filtration, pressure filtration or centrifugation.

US 5685900 describes a selective flocculation process for beneficiating a low brightness fine particle size kaolin in order to produce a higher brightness kaolin clay. The process involves a classification step to recover the kaolin fraction wherein the particles are at least 90% by weight below O.5pm. The recovered fraction is then subjected to a bleaching step to partially bleach organic discolorants. The resulting slurry is selectively flocculated using a high molecular weight anionic polyacrylamide or acrylate acrylamide copolymer.

This flocculation step forms a supernatant phase which is highly concentrated with contaminant titania and a flocced clay phase which is devoid of titania that contains the discolorants. The flocs are then treated with gaseous ozone in order to oxidise the remaining discolouring organics and also destroy the flocculant polymer in order to restore the kaolin to a dispersed state. This is said to be achieved by passing the flocculated solids through an ozonation step, preferably using a high shear pump.

Similar disclosures are made in WO 2004 071 989 and US 2006 0131243.

WO 2005 021129 discloses controlling the condition of a suspension of solid particles within a liquid including applying 1 or more stimuli to the suspension.

In this disclosure conditioning is preferably reversible and involves flocculation and/or coagulation in which inter particle forces may be attractive or repulsive between the solid particles within the liquid. The stimulus may be one or more chemical additives and may for instance be a stimulus sensitive polyelectrolyte which can be absorbed on the surface of the suspended particles in sufficient quantity to create steric or electrostatic repulsion between the particles. In one instance a polyelectrolyte may be substantially insoluble at pH values where it is substantially uncharged thereby to effect flocculation of the suspension.

Polyelectrolytes that are responsive to a temperature stimulus are also described. Reference is also given to a method of controlling the consolidation of a bed of solid particles within a liquid by applying one or more stimuli to the bed. Each stimulus effects reversibly operable conditioning between an initial state prevailing prior to said applying one or more stimuli and a conditioned state resultant from said one or more stimuli. The processes described bring about improvements in certain solids liquids separation activities.

JP 11-46541 describes a temperature sensitive hydrophilic polymer added to a suspension of particles below a transition temperature whereupon flocs are formed by absorbing and cross-linking particles as a conventional flocculant.

The mixture is heated to above the transition temperature and the absorbed polymer becomes hydrophobic and the suspended particles are rendered hydrophobic and form flocs by hydrophobic interaction. Appropriate external pressure is applied at this time and the particles are readily realigned and water between the particles is expelled by the hydrophobicity of the particles.

JP 2001 232104 describes a process similar to JP 11-46541 but using improved temperature sensitive flocculants that are ionic temperature sensitive polymers as opposed to non-ionic polymers which adsorb onto suspended particles and when the polymer becomes hydrophobic at temperatures about the transition point there are strong hydrate layers around the ionic groups but hydrated layer adhesion between the polymers is prevented by hydrophobic interaction.

WO 2007/082797, unpublished at the date of filing of the present application.

describes a process of concentrating an aqueous suspension of solid particles comprising the steps of adding at least one organic polymeric flocculant to the suspension thereby forming flocculated solids. The flocculated solids are allowed to form a layer of solids and thereby forming a more concentrated suspension in which the process comprises the addition of an effective amount of an agent which is added to the suspension prior to or substantially simultaneously with adding the organic polymeric flocculant. Alternatively or in addition the organic polymeric flocculant can be added to the suspension in the same vessel into which the agent is applied. The agent can be any of free radical agents. oxidising agents, enzymes or radiation. This process is particularly suitable for solids liquid separation in which the flocculated solids are allowed to settle by sedimentation in a gravity thickener.

Bertini, V.et. al. Particulate Science and Technology (1991). 9(3-4). 191-9 describes the use of multifunctional polymers for the pH controlled flocculation of titanium minerals. The polymers are radical vinyl copolymers containing catechol functions and acrylic acid units. The polymers can change their effect from flocculating to dispersing or inert and vice versa by changing pH.

The pH or temperature sensitive flocculants in principle provide control over the flocculation state of a suspension. However, the choice of flocculant would need to be appropriate for the particular suspension or bed that is to be flocculated and at the same time be responsive to a particular stimulus to bring about the reversibly operable conditioning. In some cases it may be difficult to find the right choice of flocculant.

Frequently some water will be trapped in the flocculated solids and this water is often difficult to release and therefore held in the bed. Whilst pH and temperature responsive flocculants may assist with this problem it is often difficult to achieve satisfactory flocculation across a wide range of substrates.

In processes involving gravity thickeners it is desirable to operate such that the bed has the highest possible solids capable of being removed from the thickener as an underfiow. Normally the limiting factor is the ability of the rake in the thickener to move the sedimented solids, It would therefore be desirable to provide a process which increases the rate of separation of the solids from the suspension and removal of the underflow.

In the processing of granite or sand and gravel it is normal practice to wash the process material in order to remove clays and other unwanted waste materials.

The effluent resulting from this washing process will contain solids removed from the granite or sand and gravel in aqueous suspension. It is normal practice to concentrate and dewater the suspended solids. In the process of concentrating and then dewatering these solids suspensions consisting essentially of effluent resulting from the processing of granite or sand and gravel, it is common practice to first of all concentrate the suspension, preferably in a gravimetric thickener vessel, to form a thickened or concentrated layer which is passed to a dewatering stage. such as a filter press. However.

the cycle time for dewatering tends to be limited by the solids of the thickened or concentrated suspension. It would be desirable to increase the solids of the thickened or concentrated suspension. However, in doing so this tends to increase the yield stress of the thickened or concentrated suspension that can lead to problems of moving the rake in the thickener vessel and/or pumping the thickened or concentrated suspension to the dewatering stage. It would also be desirable to decrease the cycle time of the dewatering stage, e.g. press cycle time, in order to increase the rate of providing dewatered solids.

According to the present invention we provide a process of providing dewatered solids derived from effluent resulting from the processing of granite or sand and gravel by concentrating an aqueous suspension of solid particles derived from the effluent, comprising the steps of adding to the aqueous suspension at least one organic polymeric flocculant to the suspension thereby forming flocculated solids, allowing the flocculated solids to settle in a gravimetric thickener to form a layer of suspended concentrated solids as a thickener underfiow, and passing the thickener underilow to a dewatering stage, in which the process comprises the application of an effective amount of an agent, and in which the agent is selected from the group consisting of free radical agents. oxidising agents, enzymes and radiation.

Unexpectedly, we have found that the application of the agent brings about a reduction in the yield stress of the underflow for a given concentration by comparison with the same solids concentration in the absence of applying the agent. Furthermore, this means that for a given yield stress the concentration of solids in the under-flow can be greater in this process in which the agent is applied. Surprisingly, we also find that the process of the present invention results in a significant reduction in dewatering cycle time (e.g. press cycle time) by comparison to the equivalent process in the absence of the agent.

The flocculated solids should settle in a gravimetric thickener. By gravimetric thickener we mean any suitable container including vessels traditionally known as gravimetric thickeners, decanters, clarifiers, settlers or other vessels suitable for allowing the flocculated solids to settle by sedimentation, or other containers including pits.

Preferably the organic polymeric flocculant is added to the aqueous suspension, that is derived from the granite or sand and gravel processing, in a gravimetric thickener vessel. The agent may be applied at any convenient stage, for instance prior to the organic flocculant being added or substantially simultaneously with the addition of the organic flocculant or alternatively subsequently to the flocculant being added. In one form of the invention it may be desirable for the agent to be applied in the same vessel into which the flocculant is added and typically this will be into the gravimetric thickener vessel.

Preferably, the agent is applied subsequently to the addition of the flocculant and more preferably the agent is applied to the underflow after it has passed from the thickener.

In the process of dewatering the thickener underflow it may be passed to a separate vessel in which the thickener underfiow is collected before being passed to the dewatering stage. Desirably this will be underflow from the gravimetric thickener vessel. This may be necessary in systems where the dewatering stage is the rate limiting factor of the process. This is especially so where the dewatering stage is a batch process. for instance a filter press. In one preferred process the agent is applied in said separate vessel.

In some cases it may be desirable to dilute the thickener underfiow from the thickener once it is in the separate vessel. Suitably this may be achieved by the addition of water or other aqueous liquid, It may be desirable to add further flocculants although usually this will not be necessary since flocculant present in the underfiow will tend to adequately reflocculate the solids to form a settled layer in a similar manner to the thickener. In some cases it will not be necessary to further settle the solids and instead passed the diluted thickener underfiow to the dewatering stage.

The agent may be applied to thickener underfiow when it is in the separate vessel. This may be carried out at any stage, for instance before any dilution, simultaneously or subsequently.

Typically the process includes the flocculation of slurry from a granite quarrying operation or other sand and gravel operations, where the slurry is treated in a thickener vessel. The thickener underfiow from the vessel would be subsequently dewatered by a suitable dewatering stage.

The dewatering stage will generally form a cake, for instance a filter cake.

Preferably according to present invention the dewatering stage will involve filtration. The application of the agent has been found to enhance the filtration cycle. Preferably the dewatering stage is a filter press.

Preferably the flocculated solids are allowed to settle to form a bed of solids which may also be termed a sediment. More preferably the process involves sedimentation in a gravity thickener and a sediment or bed is removed from the thickener as a thickener underfiow.

Surprisingly contacting the layer or bed of solids with the agent enables a significant increase in aqueous liquid released.

Desirably the agent brings about fragmentation of the flocculated structure.

Preferably, we find that the flocculated network can collapse and often occupy a smaller volume than the settled solids would have occupied in the absence of the agent.

In one form the agent may bring about degradation of the organic polymeric flocculant. It is believed that the chemical interaction between the flocculant and the solids is permanently altered as a result of this degradation of polymeric flocculant. The polymer may be degraded such that the solids have a reduced flocculated network. In one aspect the polymer chain may break down into smaller chains which induces a dispersant effect on the solids. In some cases the polymer may be degraded to the extent that it no longer has a flocculating effect on the solids. The degradation of the organic polymeric flocculant preferably is in conjunction with a degradation or size reduction of the flocculated structure. In a more preferred form we find that the flocculated network collapses thereby increasing the solids content for a given volume.

In one preferred form the agent brings about a reduction in the yield stress of a layer of solids formed from the action of the organic flocculant. More preferably the layer of solids should be at least 30% below the yield stress of a layer of solids at an equivalent solids content without the addition of the agent. Thus the agent desirably brings about a reduction in the yield stress of the layer or bed of solids and it enables higher solids to be achieved and facilitates the transfer of the thickener underfiow to the dewatering stage. This can be both in terms of the removal of the thickener underflow from the thickener and pumping of the particular underflow to the dewatering stage. Preferably the reduction in yield stress will be at least 50% below the yield stress of a layer of solids at an equivalent solids content without the addition of the agent. More preferably the reduction in yield stress will be at least 60 or 70% and often at the least 80 or 90%.

Unexpectedly we have also found that the yield stress can be reduced below the yield stress of a layer of solids at an equivalent solids content that had not been flocculated and without the addition of the agent. Until now it has been a generally accepted view that sedimentation of solids in the absence of flocculation would achieve the lowest yield stress. It was generally believed that a process involving flocculation would always result in a higher yield stress than in the absence of the flocculant because the flocculant would tend to hold the sedimented solids in a structure that would tend to increase the yield stress.

Consequently it is particularly surprising that such a process involving the use of the flocculant may result in a yield stress below the yield stress than a settled suspension without the use of flocculant.

Preferably the above-mentioned reductions in yield stress will be in combination with either a degradation or fragmentation of the flocculated structure and/or alternatively in combination with a degradation of the organic polymeric flocculant. It is especially preferred that the degradation of the organic polymeric flocculant is responsible for a degradation or size reduction of the flocculated structure which in turn brings about a reduction of the yield stress of the layer or bed of solids.

In a preferred form of the process the flocculated solids settle to form a bed and water is released from the suspension and in which we have found that the exposure of the flocculated solids to the agent brings about an increase in the water released from the suspension. Consequently, we find that this increase in water released is also accompanied by an increase in the solids.

In a further aspect the agent may evolve a gas. We find that the release of gas into the layer or bed of solids can enhance the release of water. In a preferred form the flocculated solids settle to form a bed and the agent is in contact with the flocculated solids or the bed and this brings about a further release of for and an increase in the solids. Preferably, the gas is released and forms gas bubbles. Without being limited to theory it is believed that the gas may for instance cause the formation of channels or fissures in the bed of solids and facilitate the release of water from the suspension.

Agents that can evolve a gas include carbonates. bicarbonates and peroxides.

The process of the present invention has been found to enhance the concentration of a suspension, especially by gravity sedimentation. In this sense the rate of consolidation of separated solids is increased. In addition the mobility of concentrated phase, i.e. settled or sedimented solids, can be significantly improved.

The agent may be one or more chemical compounds selected from the group consisting of free radical agents, oxidising agents and enzymes. Alternatively or additionally when the agent brings about degradation or fragmentation of the flocculated structure and/or degradation of the organic polymeric flocculant and/or brings about a reduction in the yield stress of the layer or bed solids the agent may also include radiation. The radiation may be for instance ultrasound, ionising radiation or electromagnetic radiation. When the agent evolves gas it may alternatively be a mechanical apparatus that releases gas bubbles from within the layer of solids, especially where the layer is a sediment or bed.

The agent is preferably selected from the group consisting of free radical agents, oxidising agents and enzymes.

It has been found that the incorporation of a free radical agent or oxidising agent into the flocculation process has resulted in a more rapid compaction phase, and/or reduced viscosity of the layer or bed of solids e.g. sediment at corresponding solids contents such that a higher solids content can be achieved without exceeding the maximum viscosity that the equipment carrying out the removal process can tolerate. In further embodiment enzymes have also been found to provide a similar effect. This is particularly the case when the polymer is a natural polymer or semi natural polymer, for instance polysaccharide which may have been modified, and the selected enzyme is known to degrade the natural or semi natural polymer.

Suitable free radical agents include chemical compounds selected from the group consisting of ferrous ammonium sulphate, ceric ammonium nitrate etc. It may also be desirable to use activators in conjunction with the free radical agents which in some cases may accelerate the radical generation. Typically such activators include amino carboxylates and diamines, cupric EDTA (ethylene dia mine tetra acetic acid) and reducing sugars such as fructose and lactose.

Any conventional oxidising agent may be used. Oxidising agents may be chemical substances selected from the group consisting of chlorine, transition metal or other metal compounds in a high oxidation state, such as chromium, manganese, iron and copper compounds each of which include substances that are powerful oxidizing agents, tBHP (tertiary butyl hydro peroxide), sodium sulphite, bi-suiphite compounds, ammonium per sulphate, sodium perborate, sodium hypopchlorite and ozone.

The use of ozone. peracetic, perborates. percarbonate and persulphates have been found to be particularly effective for oxidizing purposes.

Any suitable enzyme capable of acting on the organic polymeric flocculant.

particularly as a natural polymer, may be used. Typically such enzymes include hydrolases. Suitable enzymes include proteases. which will break down proteins: glycosylases which break down sugars: pectinases which break down pectin; amylases which degrade starch: esterases which degrade any ester bonds: cellulases which break down cellulose and cellulose compounds; glucosidases and galactosidases or degrading sugars. Other hydrolases may modify the surface of other polymeric flocculants, including for instance polyamides and polyesters. Other enzymes include depolymerases which will break down a polymer. particularly microbiologically generated polymers such as polyesters. Preferred enzymes include amylases, cellulases, galactomannanases. In general these are suitable for use with natural or semi natural polymers for example starch. CMC (carboxy methyl cellulose), guar, alginates, pectinates and suiphated gum carrageenan. Other specific enzymes are known to act on chitosan.

Preferred agents for use in present invention are peroxides. A particular preferred peroxide is hydrogen peroxide.

In certain situations it is preferred that the agent is applied to the suspension prior to or substantially simultaneously with adding the organic polymeric flocculant. Without being limited theory it is thought that adding the agent before any substantial formation of the flocculant structure will ensure that the agent is distributed throughout the flocculated solids. Addition of the agent and the flocculant simultaneously may also provide the advantage of a single addition point especially if the agent and the flocculant are premixed. However, with mixtures of agent and flocculant it may be necessary to ensure that the mixture is applied to the suspension prior to any significant deleterious effects of the agent on the flocculant.

The agent should be applied in an effective amount. Preferably a sufficient quantity should be added in order to ensure that it brings about at least one of: i) fragmentation of the flocculated structure; and/or ii) degradation of the organic polymeric flocculant; and/or iii)brings about a reduction in the yield stress of the layer of solids below the yield stress of a layer of solids at an equivalent solids content that had not been flocculated and without the addition of the agent and/or: iv) enables an increase in the solids content of at least 5% by weight of the layer having a given yield stress compared to a layer having the same yield stress from an equivalent process but in the absence of the agent.

The amount of agent will vary according to the specific process conditions, the type of substrate and flocculant. The agent preferably should be present in an amount of at least 1 ppm based on weight of agent on volume of the suspension. The agent can be effective at low levels for example between 1 and ppm. Generally the agent will be added in an amount of from at least 25 ppm and in some cases may be at least 50 ppm based on the weight of the solids in the suspension. In such cases it may be desirable to add as much as 1000 ppm, and preferred doses will be between 50 and 300 ppm and more preferably between 75 or 100 and 200 ppm. In some cases it may be desirable to add significantly higher levels of the agent, for instance at least 200 ppm as much as 40,000 or 50,000 ppm or higher. In some cases effective doses usually can be in the range between 150 and 20,000 ppm, such as between 1000 and 15.000 ppm.

More preferably the increase in water released from the layer or bed and the increased solids of the layer or bed is also accompanied by a decrease in yield stress. Preferably we find that the yield stress of the layer or bed is less than a layer or bed at equivalent solids content in which the flocculated solids are not exposed to the agent.

It is known that in general solids in suspensions will often settle without the addition of flocculant. The flocculant brings about bridging flocculation of the solids and increases the rate at which the solids settle to form a bed. Thus in conventional gravity thickening situations, improved rate of free settlement and initial compaction are achieved by the use of polymeric flocculants and optionally coagulants. In such a process the individual solid particles tend to gather together to form aggregates which have a more favourable density to surface area ratio. These aggregates can settle to form a compacted bed from which water can be further removed by upward percolation. In this way the bed progressively increases in solids content over an extensive period of time until the desired solids concentration in the bed is reached and material in the bed can be removed.

Unfortunately, in general the viscosity or yield stress of the flocculated settled solids in conventional processes tends to be significantly higher than the settled solids in the absence of the flocculant. This tends to make the removal process of raking and pumping progressively more difficult. On the other hand it would not be practical to concentrate a suspension in the absence of flocculant since this would take an extremely long time, especially in a gravimetric thickener which relies upon free sedimentation.

In the process according to the invention we have found that a more rapid compaction phase can be achieved. In addition it has been found that the present process tends to result in a significantly reduced viscosity or yield stress of the layer of solids or bed as a result of treatment by the agent. In particular we find that the yield stress is not only lower than the equivalent process in the absence of the agent, but the yield stress can be as low as or lower than settled solids in the absence of the flocculant. In some cases we find that the process results in a layer or bed of solids having a yield stress significantly below that of settled solids in the absence of flocculant. This unexpected property of the settled solids facilitates the ease of removal of a solids underilow whilst at the same time ensuring rapid settling of the solids. Furthermore, it is preferred that the process is operated by allowing the solids content of the consolidated bed to increase significantly above that which can be tolerated by the equipment in the absence of the agent. In this sense the consolidated bed may still be operated at the maximum yield stress for the equipment but in which the solids content is significantly higher than the bed in a process without the agent.

The yield stress of the layer of solids including sedimented bed will vary according to the substrate. Typically the maximum yield stress of a sedimented bed that can be tolerated by conventional equipment is usually no more than 250 Pa. Within capabilities of the existing equipment it would not be possible to increase the solids using the conventional process since the yield stress would be too high. The process of the invention employing the agent has been found to reduce the yield stress by at least 10% and usually at least 50% and in some cases as much as 80 or 90% or higher. On the other hand the solids content of the layer or bed produced according to the invention can be allowed to increase by at least 5% and sometimes more than 10% without exceeding the maximum yield stress that can be tolerated by the equipment. In some cases it may be possible to increase the solids by up to 15 or 20% or more in comparison to a layer or bed having the same yield stress obtaining by the equivalent process but in the absence of the agent.

The actual weight percent thickener underflow solids that can be achieved with acceptable yield stress varies considerably dependent upon the constituent and particle size of the suspended solids, and also the age and sophistication of the settling equipment. It may be as low as around 12% but is usually between around 20% and 50%.

The Yield Stress is measured by Brookfield R/S SST Rheometer at an ambient laboratory temperature of 25 C using the RHEO V2.7 software programme in a Controlled Shear Rate mode. Rotation of a Vane spindal (50_25 vane at a 3 to 1 vessel sizing) in 120 equal step increases of 0.025 rpm generate a progressive application of increased Shear Rate.

Yield Stress is defined as the maximum shear stress before the onset of shear.

The Yield Stress is calculated by linear regression of the 4 measurement points with Shear Rate > 0.1 1/s and subsequent calculation of the intercept of the axis of Tau (Pa) for Shear Rate = 0.

Preferably suspended solids in the suspension should be at least 90% by weight greater than 0.5 microns. Frequently the particles in suspension will be at least 90% by weight at least 0.75 microns and preferably at least 90% by weight at least one or two microns. Typically suspended particles may have a particle size at least 90% by weight up to 2mm and usually at least 90% by weight within the range above 0.5 microns to 2 mm. Preferably suspended particles will be at least 90% by weight up to 1 mm or more preferably at least 90% by weight up to 750 microns, especially at least 90% by weight within the range of between one or two microns and one or two millimetres.

The suspensions will often contain at least 5% by weight suspended solids particles and may contain as much as 30% or higher. Preferably suspensions will contain at least 0.25% more preferably at least 0.5% Usually the suspensions will contain between 1% and 20% by weight suspended solids.

Suitable doses of organic polymeric flocculant range from 5 grams to 10,000 grams per tonne of material solids. Generally the appropriate dose can vary according to the particular material and material solids content. Preferred doses are in the range 10 to 3,000 grams per tonne, especially between 10 and 1000 grams per tonne, while more preferred doses are in the range of from 60 to 200 or 400 grams per tonne.

The aqueous polymer solution may be added in any suitable concentration. It may be desirable to employ a relatively concentrated solution. for instance up to % or more based on weight of polymer. Usually though it will be desirable to add the polymer solution at a lower concentration to minimise problems resulting from the high viscosity of the polymer solution and to facilitate distribution of the polymer throughout the suspension. The polymer solution can be added at a relatively dilute concentration, for instance as low as 0.01% by weight of polymer. Typically the polymer solution will normally be used at a concentration between 0.05 and 5% by weight of polymer. Preferably the polymer concentration will be the range 0.1% to 2 or 3%. More preferably the concentration will range from 0.25% to about 1 or 1.5%. Alternatively the organic polymeric flocculant may be added to the suspension in the form of dry particles or instead as a reverse phase emulsion or dispersion. The dry polymer particles would dissolve in the aqueous suspension and the reverse phase emulsion or dispersion should invert directly into the aqueous suspension into which the polymer would then dissolve.

The process according to the invention exhibits improved sedimentation rates.

It has been found that sedimentation rate is between 2 and 30 rn/hour can be achieved. In addition we find that the process enables greater than 99% by weight of the suspended solids to be removed from a suspension. In addition the process enables an increase in solids sediment concentrations of greater than 10% by weight in comparison to conventional processes operating in the absence of the agent. More preferably reduced sediment yield stress is obtaining compared to the best conventional processes.

The organic polymeric flocculant may include high molecular weight polymers that are cationic, non-ionic, anionic or amphoteric. Typically if the polymer is synthetic it should exhibit an intrinsic viscosity of at least 4 dug. Preferably though, the polymer will have significantly higher intrinsic viscosity. For instance the intrinsic viscosity may be as high as 25 or 30 dug or higher.

Typically the intrinsic viscosity will be at least 7 and usually at least 10 or 12 dl/g and could be as high as 18 or 20 dl/g.

Intrinsic viscosity of polymers may be determined by preparing an aqueous solution of the polymer (0.5-1 % w/w) based on the active content of the polymer. 2 g of this 0.5-1% polymer solution is diluted to 100 ml in a volumetric flask with 50 ml of 2M sodium chloride solution that is buffered to pH 7.0 (using 1.56 g sodium dihydrogen phosphate and 32.26 g disodium hydrogen phosphate per litre of deionised water) and the whole is diluted to the 100 ml mark with deionised water. The intrinsic viscosity of the polymers are measured using a Number 1 suspended level viscometer at 25 C in 1 M buffered salt solution.

Alternatively, the organic polymeric flocculant may be a natural polymer or semi natural polymer. Typical natural or semi natural polymers include polysaccharides. This will include cationic starch, anionic starch, amphoteric starch, chitosan One preferred class of polymers includes for instance polysaccharides such as starch. guar gum or dextran, or a semi-natural polymer such as carboxymethyl cellulose or hydroxyethyl cellulose.

One preferred class of synthetic polymers includes polyethers such as polyalkylene oxides. Typically these are polymers with alkylene oxy repeating units in the polymer backbone. Particularly suitable polyalkylene oxides include polyethylene oxides and polypropylene oxides. Generally these polymers will have a molecular weight of at least 500,000 and often at least one million. The molecular weight of the polyethers may be as high as 15 million of 20 million or higher.

Another preferred class of synthetic polymers include vinyl addition polymers.

These polymers are formed from an ethylenically unsaturated water-soluble monomer or blend of monomers.

The water soluble polymer may be cationic, non-ionic, amphoteric. or anionic.

The polymers may be formed from any suitable water-soluble monomers.

Typically the water soluble monomers have a solubility in water of at least 5g/lOOcc at 25 C. Preferably the organic polymeric flocculant is anionic. More preferably it will have and anionic content of at least 10% by weight based on the total weight of monomers. More preferably still are anionic polymers with an anionic content of between 10 and 60% by weight based on weight of total monomers. Suitable anionic polymers are formed from monomers selected from ethylenically unsaturated carboxylic acid and sulphonic acid monomers, preferably selected from (meth) acrylic acid, allyl sulphonic acid and 2-acrylamido-2-methyl propane sulphonic acid, and their salts, optionally in combination with non-ionic co-monomers, preferably selected from (meth) acrylamide, hydroxy alkyl esters of (meth) acrylic acid and N-vinyl pyrrolidone.

Especially preferred polymers include the in which the organic polymeric flocculant is selected from the group consisting of the homopolymer of sodium acrylate, the homopolymer of acrylamide, the homopolymer of 2-acrylamido-2-methyl propane sulphonic acid as the sodium salt, the copolymer of acrylamide and sodium acrylate and the copolymer of acrylamide with 2-acrylamido-2-methyl propane suiphonic acid as the sodium salt. In general these especially preferred polymers will have and anionic content of at least 10% by weight of total monomers, and more preferably and anionic component content of between 10 and 60% by weight. Typically preferably these polymers were facilitate an intrinsic viscosity of at least 4 dl/g and more preferably still at least dl/g.

Preferred non-ionic polymers are formed from ethylenically unsaturated monomers selected from (meth) acrylamide, hydroxy alkyl esters of (meth) acrylic acid and N-vinyl pyrrolidone.

Preferred cationic polymers are formed from ethylenically unsaturated monomers selected from dimethyl amino ethyl (meth) acrylate -methyl chloride, (DMAEA.MeCI) quat. diallyl dimethyl ammonium chloride (DADMAC). trimethyl amino propyl (meth) acrylamide chloride (ATPAC) optionally in combination with non-ionic co-monomers, preferably selected from (meth) acrylamide, hydroxy alkyl esters of (meth) acrylic acid and N-vinyl pyrrolidone.

In the invention, the polymer may be formed by any suitable polymerisation process. The polymers may be prepared for instance as gel polymers by solution polymerisation. water-in-oil suspension polymerisation or by water-in-oil emulsion polymerisation. When preparing gel polymers by solution polymerisation the initiators are generally introduced into the monomer solution.

Optionally a thermal initiator system may be included. Typically a thermal initiator would include any suitable initiator compound that releases radicals at an elevated temperature. for instance azo compounds, such as azo-bis-isobutyronitrile. The temperature during polymerisation should rise to at least 70 C but preferably below 95 C. Alternatively polymerisation may be effected by irradiation (ultra violet light, microwave energy. heat etc.) optionally also using suitable radiation initiators. Once the polymerisation is complete and the polymer gel has been allowed to cool sufficiently the gel can be processed in a standard way by first comminuting the gel into smaller pieces. drying to the substantially dehydrated polymer followed by grinding to a powder.

Such polymer gels may be prepared by suitable polymerisation techniques as described above, for instance by irradiation. The gels may be chopped to an appropriate size as required and then on application mixed with the material as partially hydrated water soluble polymer particles.

The polymers may be produced as beads by suspension polymerisation or as a water-in-oil emulsion or dispersion by water-in- oil emulsion polymerisation. for example according to a process defined by EP-A-i 50933, EP-A-1 02760 or EP-A-i 26528.

Alternatively the water soluble polymer may be provided as a dispersion in an aqueous medium. This may for instance be a dispersion of polymer particles of at least 20 microns in an aqueous medium containing an equilibrating agent as given in EP-A-1 70394. This may for example also include aqueous dispersions of polymer particles prepared by the polymerisation of aqueous monomers in the presence of an aqueous medium containing dissolved low IV polymers such as poly diallyl dimethyl ammonium chloride and optionally other dissolved materials for instance electrolyte and/or multi-hydroxy compounds e. g.

polyalkylene glycols, as given in WO-A-983i 749 or WO-A-983i 748.

The aqueous solution of water-soluble polymer is typically obtained by dissolving the polymer in water or by diluting a more concentrated solution of the polymer. Generally solid particulate polymer, for instance in the form of powder or beads, is dispersed in water and allowed to dissolve with agitation.

This may be achieved using conventional make up equipment. Desirably, the polymer solution can be prepared using the Auto Jet Wet (trademark) supplied by Ciba Specialty Chemicals. Alternatively, the polymer may be supplied in the form of a reverse phase emulsion or dispersion which can then be inverted into water.

The following examples indicate ways in which the invention can be employed without in any way intending to be limiting.

Examrles ExamDle 1 A granite quarry passes the washing slurry to a thickener. Anionic flocculant is added to settle the solids into an underflow. The anionic flocculant is a copolymer of sodium acrylate and acrylamide (30/70 weight weight) exhibiting an intrinsic viscosity of 13 dl/g. The underfiow slurry is pumped to a buffer tank and then dewatered further using a filter press.

Hydrogen peroxide was added at approximately 25ppm into the thickener to bring about degradation of the floc structure. The resultant filter press cycle was reduced by 46% when compared to when there was no addition of hydrogen peroxide.

Claims

Claims 1. A process of providing dewatered solids derived from effluent

resulting from the processing of granite or sand and gravel by concentrating an aqueous suspension of solid particles derived from the effluent, comprising the steps of adding to the aqueous suspension at least one organic polymeric flocculant to the suspension thereby forming flocculated solids.

allowing the flocculated solids to settle in a gravimetric thickener to form a layer of suspended concentrated solids as a thickener underflow. and passing the thickener underfiow to a dewatering stage, in which the process comprises the application of an effective amount of an agent, and in which the agent is selected from the group consisting of free radical agents, oxidising agents. enzymes and radiation.
2. A process according to claim 1 in which the organic polymeric flocculant is added to the aqueous suspension in a gravimetric thickener vessel.

and in which the agent is added to the thickener underilow after it has passed from the thickener.
3. A process according to claim I or claim 2 in which the thickener underflow is passed to a separate vessel in which the thickener underfiow is collected before being passed to the dewatering stage, preferably in which the agent is added in said separate vessel.
4. A process according to any preceding claim in which the dewatering stage is a filter press.
5. A process according to any preceding claim in which the agent is selected from perborates, percarbonates, carbonates. persulphates, ozone and peroxides.
6. A process according to any preceding claim which the agent is hydrogen peroxide.
7. A process according to any preceding claim in which the agent brings about a reduction in the yield stress of the thickener underfiow at least 30% below the yield stress of a thickener underilow at an equivalent solids content without the addition of the agent.
8. A process according to any preceding claim in which the addition of the agent brings about an increase in the solids content of at least 5% by weight of the thickener under-flow having a given yield stress compared to a thickener underflow having the same yield stress from an equivalent process but in the absence of the agent.
9. A process according to any preceding claim in which the organic polymeric flocculant is a nonionic or anionic polymer that is either a synthetic polymer of intrinsic viscosity of at least 4 dl/g or a natural polymer.
10. A process according to any preceding claim in which the organic polymeric flocculant is selected from the group consisting of the homopolymer of sodium acrylate. the homopolymer of acrylamide, the homopolymer of 2-acrylamido-2-methyl propane sulphonic acid as the sodium salt, the copolymer of acrylamide and sodium acrylate and the copolymer of acrylamide with 2-acrylamido-2-methyl propane sulphonic acid as the sodium salt.