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The Selection of Flocculants and other Solid-Liquid Separation Aids

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The use of chemical additives, such as flocculants, is a common step in solid-liquid separation operations. The correct selection of agent is an essential part of the design of such processes. Many excellent reviews and guides deal with this topic, and the interested reader is referred to works such as [l-4]. In particular the Harwell-Warren Spring Report “The Use and Selection of Flocculants" provides a good overview on the application of coagulants and flocculants. This section does not attempt to reproduce a detailed treatment of that kind; instead it is our intention to state a few general rules and principles concerning methods of choosing an additive, and to illustrate briefly their application in practice. The types of agents employed in solid-liquid separation fall into three principal classes:
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Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com GBH Enterprises, Ltd. Process Engineering Guide: GBHE SPG PEG 307 The Selection of Flocculants and other Solid-Liquid Separation Aids Process Disclaimer Information contained in this publication or as otherwise supplied to Users is believed to be accurate and correct at time of going to press, and is given in good faith, but it is for the User to satisfy itself of the suitability of the Product for its own particular purpose. GBHE gives no warranty as to the fitness of the Product for any particular purpose and any implied warranty or condition (statutory or otherwise) is excluded except to the extent that exclusion is prevented by law. GBHE accepts no liability for loss, damage or personnel injury caused or resulting from reliance on this information. Freedom under Patent, Copyright and Designs cannot be assumed.
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Page 1: The Selection of Flocculants and other Solid-Liquid Separation Aids

Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries

Web Site: www.GBHEnterprises.com

GBH Enterprises, Ltd.

Process Engineering Guide: GBHE SPG PEG 307

The Selection of Flocculants and other Solid-Liquid Separation Aids Process Disclaimer Information contained in this publication or as otherwise supplied to Users is believed to be accurate and correct at time of going to press, and is given in good faith, but it is for the User to satisfy itself of the suitability of the Product for its own particular purpose. GBHE gives no warranty as to the fitness of the Product for any particular purpose and any implied warranty or condition (statutory or otherwise) is excluded except to the extent that exclusion is prevented by law. GBHE accepts no liability for loss, damage or personnel injury caused or resulting from reliance on this information. Freedom under Patent, Copyright and Designs cannot be assumed.

Page 2: The Selection of Flocculants and other Solid-Liquid Separation Aids

Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries

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Process Engineering Guide: The Selection of Flocculants and other Solid-Liquid Separation Aids

CONTENTS 0 FLOCCULANTS: BASIC PRINCIPLES 0.1 Coagulants and Flocculants 0.2 Surfactants 0.3 Dispersants 1 FLOCCULANTS: TEST PROCEDURES AND METHODS 1.1 Those Applicable to the Original Suspension 1.2 Those Applicable to the Flocculant Solution 1.3 General Tests Applicable to the Flocculation Process 1.4 Specific Tests Applicable to the Flocculation Process

and to the Flocculation System 2 EXAMPLES 2.1 Formation of M729 Vermiculite / Glass Fibre

Refractory Paper 2.2 Harvesting and Concentration of a Filamentous

Organism Suspension before Isolation of a Pharmacologically-Active Compound

3 USE OF SURFACTANTS TO HELP REDUCE

FILTER-CAKE MOISTURE 4 REFERENCES

Page 3: The Selection of Flocculants and other Solid-Liquid Separation Aids

Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries

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TABLES 1 A SUMMARY OF THE CHARACTERISTICS OF SYSTEMS

FLOCCULATED WITH THE AID OF POLYMERIC AND SIMILAR AGENTS

2 THE IDEAL M729 FLOC STRUCTURE TO OPTIMIZE PARTICULAR PAPER FORMING CHARACTERISTICS AND FINAL PAPER PROPERTIES

3 SCHEMATIC REPRESENTATION OF THE EFFECTS ON PAPER

FORMING PROPERTIES OF THE PRINCIPAL VARIABLES IN THE SYSTEM, INVOLVING ACID DISPERSION OF GLASS AND USE OF NONIONIC OR ANIONIC FLOCCULANTS

4 THE MAJOR EFFECTS OF CHANGING PROCESS PARAMETERS ON

PAPER FORMING CHARACTERISTICS IN SYSTEMS INVOLVING USE OF CATIONICFLOCCULANTS

FIGURES 1 A SUGGESTED SEQUENCE OF OPERATIONS IN THE SELECTION /

OPTIMIZATIONOF A FLOCCULANT

2 TYPICAL PROCEDURE FORMAKEUP OF AN M729/GF SLURRY FORPAPER-MAKING

Page 4: The Selection of Flocculants and other Solid-Liquid Separation Aids

Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries

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INTRODUCTION The use of chemical additives, such as flocculants, is a common step in solid-liquid separation operations. The correct selection of agent is an essential part of the design of such processes. Many excellent reviews and guides deal with this topic, and the interested reader is referred to works such as [l-4]. In particular the Harwell-Warren Spring Report “The Use and Selection of Flocculants" provides a good overview on the application of coagulants and flocculants. This section does not attempt to reproduce a detailed treatment of that kind; instead it is our intention to state a few general rules and principles concerning methods of choosing an additive, and to illustrate briefly their application in practice. The types of agents employed in solid-liquid separation fall into three principal classes: (i) Coagulants and flocculants to facilitate the aggregation of fines to give

more easily separated species; (ii) Surfactants added to try to reduce filter-cake moisture content; (iii) Dispersants. Generally, dispersing agents ((iii) above) are used not so much in the separation operation itself but to help fluidize the concentrate obtained. We do not deal further with this subject at this point but the theme is taken up again in GBHE Suspension Processing Guides “Centrifugation” [GBHE SPG PEG 304], “Sedimentation [GBHE SPG PEG 303], and “Filtration” [GBHE SPG PEG 305] where examples of selection of dispersants to modify the rheological properties of concentrated suspensions and sludge’s are described. Of the remainder, coagulants and flocculants make up the preponderant part of processing aids employed. They are dealt with in Sections 3 Surfactants added to help reduce filter-cake moisture are less important but are considered in 3. A final class of agents, solid filtration or sedimentation aids, are considered in GBHE SPG PEG 309 “Clarification”.

Page 5: The Selection of Flocculants and other Solid-Liquid Separation Aids

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0 FLOCCULANTS: BASIC PRINCIPLES There are two main types of flocculating agents: (i) Inorganic salt coagulants such as Ca2+, Al3+ compounds. (ii) Various organic, particularly polymeric, flocculating materials. It is suggested that readers unfamiliar with the various mechanisms of flocculation refer to “Centrifugation” [GBHE SPG PEG 304] for a synopsis of the fundamentals of this topic. The characteristics of inorganic salt flocculating systems are very straightforward: Agents of this type (e.g. lime) can give a "cheap and cheerful" solution to problems, there being little difficulty with steps such as flocculant/suspensions mixing. For this reason such compounds ought to be given first consideration In any screening process for flocculants (see Section 3 below). The main limitation of inorganic coagulants is that, due to the particular mechanism of aggregate growth, natural forces of attraction being relied upon to give interparticle cohesion, the floes are usually rather open-textured and weak. This limits the extent to which they can be handled conveniently. As general rule coagulation efficiency increases rapidly with the charge on the species used, i.e. Na+ and other monovalent species are less effective than Ca2+, Mg2+ and so on. These, in turn, require higher dosages than Fe3+ or Al3+. With the latter a complication arises since these agents are subject to hydrolysis; iron or aluminium hydroxides are precipitated under certain conditions. In practice this is usually an advantage as the precipitated material “sweeps up" fines by entrainment or by surface adsorption. However, it does mean that careful attention must be paid to pH and other factors to ensure that optimum behavior is obtained. Further details on the use of Al3+ and other Inorganic salt coagulants are contained in references [1, 2 ,4-6]. A description of settling, flow and other characteristics of coagulated suspensions of this type are contained in references [7-9] and in “Centrifugation” [GBHE SPG PEG 304], “Sedimentation [GBHE SPG PEG 303]. With organic flocculants the position Is far more complicated. However, they offer further degrees of control over suspension properties owing to the possibilities they afford of differing flocculation mechanisms and differing floe structures from those observed in simple coagulated systems. Table 1 summarizes the characteristics of systems flocculated with various polymeric and similar agents, including liquid bridging flocculants dealt with in “Sedimentation [GBHE SPG PEG 303].

Page 6: The Selection of Flocculants and other Solid-Liquid Separation Aids

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It should be noted that our nomenclature for agent type and flocculation mechanism follows that defined in “Sedimentation [GBHE SPG PEG 303]. As should be evident from the Table, as a generality it is found that the higher molecular weight (say > 50,000) polymeric flocculants are most useful since in this size range phenomena such as polymer bridging between the surfaces of particles can greatly change floc morphology and, probably more importantly, robustness. Low molecular weight substances, in contrast, tend to give results not too different from Inorganic coagulants, except in special cases. More details on the fundamental properties of polymer flocculated systems are given in references [10] and [11]. 1 FLOCCULANTS: TEST PROCEDURES AND METHODS Test methods for flocculants, for example to determine optimum dose, have been dealt with in detail in many excellent texts and, indeed, in part elsewhere in this work (see, for example, Section 3.5.6(b)). What we will attempt to describe In this section are the kinds of techniques which should be used in an efficient screening/optimization program, and, even more importantly, the sequence in which they should be applied to arrive at a solution to a problem with minimum effort. The tests which may be applied during a process of selection of a flocculant divide conveniently into four classes:

(i) Those Applicable to the Original Suspension

This is to determine such factors as the chemical makeup of the particles, their surface charge, particle size distribution, and background electrolyte concentration. Usually certain of these data will already be known (e.g. particle composition), or can be assumed with reasonable certainty (e.g. the majority of colloidal suspensions contain negatively-charged particles. However it is wise to have at least some idea of particle size and background electrolyte concentration, whilst pH is an important variable if polymer flocculants or hydrolyzing metal salt coagulants (e.g. Al3+ or Fe3+) are to be used.

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(ii) Those Applicable to the Flocculant Solution

Fortunately sufficient information on factors such as flocculant chemical composition, charge or molecular weight will be available from sources such as manufacturers' data sheets. However, on occasion it may be necessary to characterize some aspect of the flocculant in detail if sensible progress is to be made. Details of certain of the methods are provided In reference [1].

(iii) General Tests Applicable to the Flocculation Process

These are methods such as the 'jar-test', flocculation kinetics and clarification measurements, which give an idea of how quickly and completely a particular agent aggregates the particles. These simple procedures also provide information on dose range, and the influence of co-factors such as pH and ambient electrolyte concentration. Tests for the concentration of (polymer) flocculant residues in the supernatant may be necessary in some cases both as a guide to efficiency of agent use and, in cases where a recycle loop is Involved, to check that flocculant build-up problems are not going to arise. (iv) Specific Tests Applicable to the Flocculation Process and to

the Flocculated Systems

These are the procedures necessary to find the correct flocculant type/dose for a particular unit operation (or operations) under consideration. They Include techniques which determine important fundamental characteristics (such as modulus or viscosity – see Section 3.21, or which directly measure the technological properties (e.g. filtration rate, sedimentation behaviour) in question.

Probably the most effective manner of screening for a flocculating agent is laid out in the flow diagram, Figure 1, We would make the following Important points concerning the sequence of operations:

(a) Checkout Coagulants First

Before searching for a suitable polymer flocculant, it is always worthwhile looking to see If the problem may be solved by use of an Inorganic coagulant, since agents of the latter type have such relative ease of use.

Page 8: The Selection of Flocculants and other Solid-Liquid Separation Aids

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(b) Pursue Simple Methods to Eliminate Options

To minimize the amount of work that has to be expended on fairly time consuming tests of filtration, centrifugation, sedimentation or flotation behavior, primary screening for candidate flocculants should be pursued with simple techniques such as the jar test. Ideas of the appropriate dose range and the effects of pH or added electrolyte (on polymer-particle interaction) can also usefully be found in this way. Any additional constraints (e.g. safety, toxicology) which could limit flocculant choice, should also be considered at as early a stage as possible to minimize experimental work. The first "sifting" for suitable polymer flocculants Is best performed “on paper" using the known characteristics of materials flocculated by different mechanisms (Table 1), combined with such constraints as have been introduced concerning the type of floes which must be obtained (e.g. robust, open-textured flocs for filtration). This will give some idea of the polymer characteristics, such as ionicity and molecular weight, required. However this goes only some way towards cutting down the myriad choice of synthetic and natural flocculant materials to manageable proportions for testing. To go further in selection it is necessary to make such judgment as to the likely strength of polymer-particle interaction. Ionicity is, of course, an excellent guide in many cases - species of opposite charge will tend to bind strongly - but in other instances one must simply use a mixture of common sense and experience. For example, polymers containing carboxyl groups may be expected, on the basis of simple chemistry, to bind strongly to inorganic surfaces containing, say, Ca2+ Ions. For inorganic particulates, such as metal oxides or carbonates, it should be borne in mind that polymer-particle adhesion Is usually quite sensitive to pH as the character of the solid surface can often be changed from predominantly negative to neutral and even positive on only limited acidification.

(c) Use the Approximate Test

Notwithstanding (b), final selection of flocculant type, dose and physical conditions of application, must be made on the basis of the tests appropriate to the unit operation (e.g. filtration, Section 3.5) for which optimization is required. This is because the floc properties which are best for one type of process will be entirely wrong for another separation operation. For example, in sedimentation fairly compact flocs, which will rapidly consolidate to a dense sediment, will usually be required.

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In contrast, rapid filtration demands that the aggregates be robust, open-textured and resistant to compaction.

(d) Optimize the Whole Process

It follows from (c) that if there are several different steps in the process train, one has to consider the flocculant and conditions which will give the optimum for the whole process. (NB this will be the case rare often than not. For example, even in a simple, one-step dewatering process one will be aiming to obtain a handleable concentrate. Thus factors such as concentrate solids content and rate of dewatering have to be played off against ease of handling of the final material. This can only be done by screening for the separate stages then determining the best compromise in a reconciliation stage. This matter will be dealt with in more detail in Section 3.10. The first of the examples in Section 3.7.3 deals with exactly this type of problem. In this instance it was very easy to find flocculation conditions which would be entirely suitable for a particular step In the process; finding ones which would satisfy all of the constraints simultaneously proved, however, much more difficult.

(e) Remember Kinetics

So far in it has tended to be tacitly assumed that sufficient residence times will be available for the equilibrium flocculated state to be obtained. Generally this will be the case for the kinds of concentrated systems typically encountered in solid isolation problems. However, this need not always be so and, if necessary, flocculation kinetics may need to be measured to determine optimum times of aggregation. This question will be returned to in “Centrifugation” where we deal with clarification operations. In the latter, which predominantly involve dilute suspensions, kinetic factors are a major concern.

(f) Do Not Forget That Specific Effects Can Occur

Specific Interactions between particle and flocculating agents occur with both polymers and inorganics. Examples include the precipitation of carboxylated biopolymers by Ca2+, and the tendency of Fe3+ to bind strongly to biological surfaces and polymers (see Example 2 in 3.7.3 below). Thus even with today's degree of systematization in selection of flocculants one must still be alive to special effects due to chemical interactions.

Page 10: The Selection of Flocculants and other Solid-Liquid Separation Aids

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Examples Below are details of two examples, taken from a European client’s experience, which illustrate selection of flocculants. One involves the extensive screening of polymer flocculants in order to fabricate a paper-like product from suspended ingredients; the other concerns use of Fe3+ coagulant In a specialist biological separation.

2.1 Formation of M729 Vermiculite / Glass Fibre Refractory Paper

A good example of the manner in which flocculants can be broadly selected, then screened in detail, is provided by the process of manufacture of a refractory paper from M729 vermiculite and glass fibre. The object was to develop a product, containing about a 50:50 ratio of clay (i.e. vermiculite) and glass fibre, which could be produced on conventional paper-making machinery. It had been shown that a material ("Fortress T"), consisting of vermiculite coated onto (preformed) glass fibre tissue, could retain its structural integrity at temperatures up to the order of 1000 oC. This made it technically suitable for a number of flame barrier/fire protection applications. However, preliminary studies indicated that a product of equally desirable characteristics could probably be manufactured at significantly lower cost by a paper-type route in which the vermiculite was mixed with the glass fibre stock prior to tissue manufacture. The clay was thus incorporated in the same operation as the glass tissue was made. Not only would this improve the competitive position in the then accessible markets but it would also probably open up possibilities for new applications requiring a lower cost material than 'Fortress T". New technical procedures were needed as formation of such highly-loaded papers in one operation was largely unknown territory.

The nature of the paper-making operation is not dissimilar to continuous filtration in action - the dispersed glass, or other fibrous feedstock, falls onto a moving wire mesh belt, the fibers forming a web as drainage proceeds - and uptake of other ingredients, such as clay suspended with the fibrous base, is dependent upon efficient capture in the interstices of the mat. As M729 vermiculite Is colloidal (substantially ( 1 micron in diameter) in scale, whereas typical glass had a diameter of ~ 10 microns and a length of several mm, retention of the clay was extremely poor when attempts were made to make paper from it and glass fibre alone.

Page 11: The Selection of Flocculants and other Solid-Liquid Separation Aids

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Accordingly, a flocculant (retention aid) was required which would give aggregates of vermiculite of sufficient size to be captured (with almost 100% efficiency) in the pores of the forming paper. However, the problem was not simply one of producing large, robust flocs of the clay: for example vermiculite aggregates of too large a size would give sheet with such a “lumpy” appearance that it would have been unsalable. Instead, a compromise had to be found which would best satisfy a number of (partly contradictory) product and process constraints, all of which were considered necessary for economic production of a customer-acceptable material. Details of these conditions, and of the paper structure likely to be best suited to their fulfillment, are provided in Table 2. Consideration of the relative Importance of the different factors suggested that the minimum requirements of M729 aggregate structure would seem likely to be as follows: (i) The flocs should be reasonably large and fairly robust so as to give

good retention. (ii) There must be high porosity in the forming M729/glass fibre we?

In order to give fast drainage and hence production rates. Ho: probably this could best be achieved by having M729 aggregates which have high individual porosity but structure having large interfloc pores might also be acceptable.

(iii) In order to give good paper texture and a manageable thermal

drying load the porosity of the floe structure in the formed (but not necessarily dried) paper had to be low. For this requirement to be compatible with (ii) either: (a) the flocs must be initially porous but be sufficiently compressible that they consolidate to a considerably denser structure during the last stages of drainage; or: (b) the flocs must initially be fairly dense but the interfloc structure must be such as to give enough large drainage pores.

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Turning to the initial sifting process outlined in the decision tree In Figure 1, one was quickly led to the following conclusions: (i) As the simplest mechanism of flocculation, viz destabilization of the

colloid by limited addition of electrolyte, usually gives flocs which are small, weak and often rather compact, the aggregate structure would be expected to satisfy (iii) but not conditions (i) and (ii) above. Experience supported this analysis and attempts to wet-form M729/GF paper using Ca2+ as coagulant gave material of excellent texture but with associated poor retention and drainage during production [16].

(II) Many of the mechanisms of flocculation (e.g. addition of oppositely

charged surfactants of low-medium molecular weight polymers) involve random aggregation of the particles to give relative open, voluminous structures. If these aggregates were sufficiently large and strong enough to give good retention, though draining well, they would tend to give high moisture retentions and (probably) a poor paper texture.

(iii) In this mechanism the particles are joined by flexible polymer bridges rather than there being Intimate contact between the surfaces of the particulate species. This (given appropriate process conditions) helps the formation of large, reasonably tough, open flocs. However, such aggregates are fairly compressible and will eventually settle or compact (as in the last stages of drainage during filtration) to quite a dense bed,

(IV) Optimum conditions for flocculation by polymer “bridging” require

that the Interaction between particle and polymers is not too strong. If particle-polymer Interaction is very strong an alternative mechanism of aggregation occurs. This is” heterogel” flocculation in which the particles initially thickly coat droplets of the added flocculant solution; subsequent agitation folds and stretches the initially-formed globular species to give highly characteristic “stringy” flocs (see Chapter 2 and reference [10]). Though these aggregates are large, and often remarkably tough, they are usually less porous, immediately after formation, than optimum polymer bridged species. In addition, beds of the flocs are much less compressible than those of aggregates induced by polymer bridging. Thus it was expected that such “heterogel” species would perform somewhat less well in practice than purely bridged aggregates owing to poorer drainage and less satisfactory paper texture. Subsequent studies largely bore out this prediction though,

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as shall be seen, it was possible to make efficient use of highly-interacting high molecular weight flocculants under special conditions when the basic aggregation mechanism was probably somewhat modified.

One final problem with “heterogel” flocculation is that the floc structure is notoriously sensitive to agitation history. This tends to present considerable problems as agitation regime usually proved to be one of the more difficult variables to control closely in any practical flocculation and pumping regime.

(V) Due to the likely crucial dependence of papermaking on M729 floc structure it was evident that the key process parameters for M729/GF paper production would probably be those which governed the basic mechanisms of flocculation.

The next stage in the research and development program was to confirm (or otherwise) the conclusions reached in the above paper exercise. It was known from early (empirical) scouting studies that (I) was certainly true, this lack of sufficient floc strength also extending to systems flocculated with medium molecular weight (~ 50,000) cationic flocculants (cf (II)). Bow the M729 vermiculite particles were known to be negatively-charged under most conditions and, accordingly, added high molecular weight cationic polymers were expected to give very tough, large aggregates. Simple screening of a few agents corroborated this supposition. As anticipated, such suspensions gave rise to lumpy, uneven papers of completely unacceptable quality (cf (IV)). Not only were the flocs generally too large but also mixing difficulties with the strongly interacting flocculant solution and particulate suspension gave rise to inhomgeneities which contributed to the poor texture. However, high molecular weight anionic or nonionic agents, acting probably by a bridging mechanism, showed promise. Thus, quite rapidly, an apparently unlimited choice of flocculation aids had been reduced to quite a modest selection of compounds to be screened. Final selection of polymers, and investigation of the effects of physical variables, such as mixing conditions, were now performed using a laboratory paper former (see reference [19] for details), with flocculated M729/glass fibre stock made up as shown in Figure 2. The flocculants examined were a range of commercially-available polyacrylamide-based agents of molecular weight greater than 1 million. Fortunately the basic rules which are known concerning the relationships between basic parameters and floc properties (Table 1) proved to hold and this rapidly guided the experimental program, especially in respect of isolation of key variables.

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In particular knowledge of the underlying principles enabled water hardness to be quickly identified as an important parameter (due to the sensitivity of polymer-particle interaction to added electrolyte). A relatively modest amount of experimental work allowed a picture to be sketched out of the effects on paper-forming properties of changing the main variables (Table 3). From this optimum flocculation "recipes" were determined to deal with various possible process conditions (e.g. hard and soft make-up water; changes In M729 feedstock type). Although In most circumstances anionic or nonionic (these materials tend to hydrolyze to give weakly anionic polymers on making up the flocculant solution) flocculants gave excellent results there was one version of the process in which they could not be used and alternatives had to be sought. This was when a cationic surfactant was employed to aid dispersion of (certain) glass fibre stocks. Coacervation (i.e. co-precipitation) of anionic or nonionic retention aid with the surfactant was observed, the problem being particularly acute under recycle conditions when process "white water" was employed for M729 suspension make-up: The surfactant would tend to strip out flocculant from solution with deleterious effects on process efficiency. Though It had been proved earlier that high molecular weight polymers of significant cationicity could not be used (owing to floc strength being excessive) it was found that weakly cationic materials could be employed, presumably because polymer-particle binding was sufficiently reduced to allow (as before) the formation of easily compressible flocs. A limited experimental program enabled key variables and trends to be Identified (Table 4), and workable formulations to be derived. The Important process parameters were CARL all the same as for the anionic /nonionic retention aid case. However, these changes were all readily predicted from the known characteristics of the flocculation mechanism (Table l), eliminating the need for much tedious, and ultimately pointless, empirical screening. Following the research phase the correctness of the conclusions reached was proved in two pilot-scale production campaigns In Germany [17, 18]. Subsequently it was found that near optimum process 'recipes" for paper-making plants of potential licensees, in several parts of the world, could be derived, with modest effort, based upon the principles determined in the work. Further technical details on the project are contained in references [19] and [20].

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2.2 Harvesting and Concentration of a Filamentous Organism Suspension

before Isolation of a Pharmacologically-Active Compound Our second example concerns one step in the Isolation of a fermentation product. The object of the process in question was to make a 25-30% solids paste from a fermenter broth (Initial concentration ~ 1-2% solids). This concentrate would be the intended feedstock for a solvent extraction operation by which the final product, a pharmacologically-active substance, would be isolated. Constraints on process options were at least three-fold viz:

(i) As the organisms in the broth were of colloidal size some kind of flocculation procedure was required for convenience of separation.

But

(ii) As the organism in question was filamentous (i.e. acicular) in nature, care had to be taken not to induce too strong a degree of flocculation otherwise the flocculated network in suspension would not readily consolidate to the required concentration. (This is a result of simple geometrical considerations which govern network strength - see Basic Principles & Test Methods [GBHE SPG PEG 302] for further details.)

And

(iii) Owing to the product use there were clear limitations on the kinds of flocculant which could be employed.

There were also likely to be the usual difficulties which attend separation of a biological (as opposed to inanimate) system – for example the propensity of flocculants to react with biopolymers in solution rather than flocculate the particles with optimum efficiency (see Section 3 for a discussion of this and other questions concerning bio-separations). All of the above Indicated that it was desirable to find a relatively simple flocculation aid, which would not give rise to safety concerns, and which would not give rise to too strong flocculation and a difficult-to-compress sludge. Screening of some possible agents indicated that species such as Ca2+ or cationic polymers were relatively ineffective.

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However Fe3+ gave much more promising results and the material could be thickened to the desired degree by centrifugation then filter-pressing. The high efficiency of the iron was probably not only attributable to its high charge, but also to some specific interaction between the Fe3+ and the biological species. Sufficient floc robustness was obtained to allow "balling up" of the aggregates into more compact structures, This Illustrates the important principle that one may expect specific (and not readily predicted) metal-polymer interactions when highly-charged ions and/or biological polymers are concerned. Whilst concentration could be carried out by the above process the position was not entirely satisfactory; there tended to be significant 'bleeding" of fines during mechanical dewatering, indicating either Insufficient floe robustness, or incomplete flocculation, with excess fines being only weakly entrained in (rather than strongly incorporated Into) the aggregates. The opposite trap, of producing an over-strong network had, though, been avoided. In the initial investigations flocculation had been performed at acid pH (due to the use of acidic FeCl3 as the source of iron) and attention now turned to control of this parameter as a means of modifying the flocculation behavior. Raising the pH in the process yielded some precipitation of Fe(OH)3, and a great improvement in fines retention owing to the "sweeping" action of the hydroxide. This clearly demonstrated the Importance of pH adjustment in the effective use of trivalent coagulants. It also showed what extra benefits, in terms of “sweeping” as well as coagulating action, may be obtained with such agents. As a final stage in development a small amount of high molecular weight cationic polymer flocculant was added after pH modification and Fe hydroxide precipitation. This helped to improve the aggregate robustness sufficiently that the problem of fines loss was completely eliminated. It was now also possible to simplify the process by removing the centrifugation step, concentration being achieved in one operation In the filter press. All that now remained was "fine-tuning" of conditions to give an overall optimum compromise between efficient harvesting and concentration, and ease of redispersion of the concentrate (at the later solvent extraction stage). Tests of cake strength, using the Shearometer (cf Section 3), provided an Important technique in the attempt to achieve maximum efficiency in filtration.

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3 USE OF SURFACTANTS TO HELP REDUCE FILTER-CAKE MOISTURE In addition to surface-active agents with overt flocculating action, on occasion other compounds are employed to try to reduce the moisture content of filter cakes obtained in various solid-liquid separation operations. Ostensibly such materials function by reduction of filtrate surface tension and/or by modifying the liquid/solid contact angle, allowing filter cake capillaries to drain more readily, thereby reducing cake moisture content. In the current stage of knowledge, however, there are no good rules for choosing such aids. Indeed, for most fine particle separations there must be some doubt whether the effects obtainable are worth the effort of screening and the cost of the agents. At present, probably the best guidance available is to refrain from trying to use such compounds except (perhaps) in special circumstances via: (a) When the primary particles are large (i.e. of the order of tens of microns in

size); and/or (b) When pressure blowing is being used to reduce filter cake water content. The theoretical background to the phenomenon to be exploited is very simple. It follows the analysis related in Section 3 for pressure required to cause expulsion of liquid from filter-cake pores by a blowing mechanism. Recapping:

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Thus If ƔL.V. can be reduced significantly, in principle the cake will drain at much lower applied pressures, perhaps even just on standing. The same applied, of course, to the COS ƟL.S. term. Note that these two factors may, or may not; move in concert with one another - ideally they should do so of course [12]. This makes surfactant selection even more of a problem since ƟL.S. depends specifically on the molecular interaction between the particular surfactant and the particular solids being filtered. In practice ƔL.V. reduction seems to be the main phenomenon exploited [14]. In reality, it appears fairly difficult to obtain effects of useful magnitude. As was shown in Sections 3), ΔP tends to be very high for colloidal-scale species and hence even if the interfacial tension is diminished by a perceptible factor the pressure required to give drainage may still be inaccessible. However, If the primary particles, and hence the pore size is relatively large, helpful results may be obtained:

(i) Pearse and Allen [12], in a paper written principally from the perspective of one European manufacturer ( who manufacturers surfactants for use as filter-cake deliquoring aids), have reported significant moisture content reductions with cakes of largish-sized materials such as sand, fine coal, and 11-12 micron silica spheres.

(ii) Significant reductions in filter-cake moisture content have been

noted in several instances (see, for example references [21] and [22]) on “oiling of fine coals”. Typically only ~1% of oil is used, far below that normally needed to give oil-bridged agglomerates (see Basic Principles & Test Methods [GBHE SPG PEG 302]).

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(iii) Drainage aids are often used to reduce moisture content in newly-laid paper (fibre size: typically mm x 10 micron or more diameters). Probably these agents mainly function by flocculating action but surface-active effects may also matter [13].

(iv) Moir and Read, in their review on reduction of the liquid content of

filter cakes [41, have reported variable results from surfactant use. Beneficial reductions in moisture content of up to 10% were observed in certain cases, but negligible effects in many others. Virtually all significant findings have been for coal filtration systems, and even in these cases the data suggests that in many instances positive effects can be attributed to flocculation rather than surface tension or contact angle phenomena (however cf (ii) above).

Thus, the balance of evidence is that for filter cakes composed of colloidal scale particles, surfactant addition will probably not be all that useful. For cakes containing larger size primaries there might be some beneficial effects, and screening of commercially available products (see e.g. reference [12]) may have merit. Two other points which we would make with respect to this topic are:

(I) Understanding of best practice in this area is somewhat confused due to lack of systematic research, or GBHE experience, on use of surfactants as aids to filter cake deliquoring. Better guidelines can only come from some thorough work In the area.

(II) Agents which are effective in reducing filter cake moisture content

will also tend to reduce (by the same mechanism) the excessive compaction, due to capillary forces, which sometimes attends drying of a filter cake or other porous structure. Use of processing aids of this type might be helpful where it is necessary to retain maximum porosity In a dried product (see reference [15] and Basic Principles & Test Methods [GBHE SPG PEG 302]).

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