Controlled flocculating additive

Utilize controlled flocculation (addition of electrolyte) that allows repeatable redispersion by shaking. 

Control philosophies for clarifiers are based on the idea that the overflow is the most important performance criterion. Underflow density or suspended sohds content is a consideration, as is optimal use of flocculation and pH control reagents. Automated controls are of three basic types (I) control loops that optimize coagulant, flocculant, and pH control reagent additions (2) those that regulate underflow removal and (3) rake drive controls. Equahzation of the feed is provided in some installations, but the clarifier feed is usually not a controlled variable with respect to the clarifier operation. 

Automated controls for flocciJating reagents can use a feedforward mode based on feed turbidity and feed volumetric rate, or a feed-back mode incorporating a streaming current detector on the flocculated feed. Attempts to control coagulant addition on the basis of overflow turbidity generally have been less successful. Control for pH has been accomplished by feed-forward modes on the feed pH and by feed-back modes on the basis of clarifier feedwell or external reaction tank pH. Control loops based on measurement of feedwell pH are useful for control in apphcations in which flocculated sohds are internaUy recirculated within the clarifier feedwell. 

Flowmeters These are used to measure flocculant addition, underflow, and feed flow rates. For automatic control, the more commonly used devices are magnetic flowmeters and Doppler effect flowmeters. 

Flocculating agents can be simple electrolytes that are capable of reducing the zeta potential of suspended charged particles. Examples include small concentrations (0.01-1%) of monovalent ions (e.g., sodium chloride, potassium chloride) and di- or trivalent ions (e.g., calcium salts, alums, sulfates, citrates or phosphates) [80-83], These salts are often used jointly in the formulations as pH buffers and flocculating agents. Controlled flocculation of suspensions can also be achieved by the addition of polymeric colloids or alteration of the pH of the preparation. 

The controlled flocculation method may be used in conjunction with the addition of a polymeric material to form a structured vehicle. After the formation of the floes, an aqueous solution of polymeric material, usually negatively charged, such as carboxy-methylcellulose or carbopol, is added. The concentration employed depends on the consistency desired for the suspension, which also relates to the size and density of the dispersed phase. Care must be taken to ensure the absence of any incompatibility between the flocculating agent and the polymer used for the formation of the structured vehicle. 

Although the production of highly deflocculated suspensions is a primary objective for formulation of suspension concentrates, these systems tend to settle under gravity forming dilatant sediments (clays). The latter must be prevented either by controlled flocculation or by the addition of a second disperse phase to the continuous medium (1). One method which may be applied to sterlcally stabilised dispersions, is to add a free (ie. non-adsorbing) polymer to the continuous medium. 

Figure 7.31 Diagrammatic drawing of flocculation and controlled flocculation in a sulfamerazine suspension the effect of the addition of sodium dioctyl sulfosuccinate (docusate sodium) and aluminium chloride is shown and two interpretations of the results are outlined.

For a structured suspension, obtained by controlled flocculation or the addition of thickeners (such polysaccharides, clays or oxides), the floes sediment at a rate depending on their size and porosity of the aggregated mass. After this... 

During extended storage, the particles in (pharmaceutical) suspensions often form agglomerates that can be no longer destroyed by shaking the preparation. This is of particular concern in, for example, eye drops. The problem can be avoided by controlled flocculation of the solids. After the addition of an electrolyte, the fine particles aggregate to loose floes that can be easily redispersed by shaking the dispenser prior to application. 

In the USA several companies market more efficient and compact thickeners, fuUy equipped with instrumentation and sensing devices. Some claims are made for improved equipment design but the higher thickening rates are mostly due to better use and control of flocculant addition. One example is the Dorr-Oliver Hi-rate Thickening System which uses a Fitch feed well (ref. 19, Figure 5.24). This is a new type of feed well consisting of three horizontal... 

Bridging and depletion flocculation are two of the major mechanisms under the family called sensitisation , i.e. controlled flocculation of dispersions by addition of small quantities of materials which, if used in larger amounts, would act as stabilizing agents. Steric forces can also be attractive in some cases (at short separation) and then bridging can... 

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