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     COAGULATION

The colloidal particles are normally less than one micron in size and undergo Brownian motion. The energy of this motion is sufficient to prevent the particles from settling under gravity and particles remain suspended for long periods of time. Colloidal suspensions can be stable or unstable.

COAGULATION is the process in which destabilization is achieved by the addition of salts which reduce, neutralize or invert the electrical repulsion between particles. Most common coagulants are mineral salts: aluminium sulfate, ferric chloride, lime, calcium chloride. magnesium chloride.

     FLOCCULATION

FLOCCULATION is used to describe the action of polymeric materials which form bridges between individual particles. Bridging occurs when segments of a polymer chain adsorb on different particles and help particles aggregate. Flocculants carry active groups with a charge which will counterbalance the charge of the particles. Flocculants adsorb on particles and cause destabilization either by bridging or charge neutralization.

An anionic flocculant will usually react against a positively charged suspension (positive zeta potential). That is the case of salts and metallic hydroxides.

A cationic flocculant will react against a negatively charged suspension (negative zeta potential) like silica or organic substances.

However the rule is not general. For example, anionic flocculants agglomerate clays which are electronegative.

Three groups of flocculants are currently used

     1.1 – MINERAL FLOCCULANTS

They are colloidal substances. Adsorption and charge neutralization play some part in the flocculation mechanism. They are:

• activated silica.
• certain colloidal clays (such as bentonite),
• certain metallic hydroxides with a polymeric structure (alum, ferric hydroxide)

     1.2- NATURAL FLOCCULANTS

They are water soluble anionic, cationic or nonionic polymers. Nonionic polymers adsorb on the suspended particles. The most common natural flocculants are:

• the starch derivatives: mostly pregelatinized hence water-soluble. They are corn or potato-starches. They can be natural starches, anionic oxidized starches or amine treated cationic starches. The use of this class of products has decreased in water treatment but remains important in the paper industry.
• the polysaccharides: usually guar gums and mostly used in acid medium.
• the alginates: anionic and used in potable water treatment.

     1.3- SYNTHETIC FLOCCULANTS

Polyacrylamides

The most common polymers are those based on polyacrylamide, which is a nonionic polymer. Their effect is due to bridging between particles by polymer chains.

Polymers can be given anionic character by copolymerizing acrylamide with acrylic acid. Cationic polymers are prepared by copolymerizing acrylamide with a cationic monomer. All available acrylamide based polymers have a specific amount of ionic monomer giving a certain degree of ionic character.

They have a specific average molecular weight (i.e. chain length) and a given molecular distribution.

For each suspension, a certain degree of anionic, cationic or nonionic character is beneficial. Usually, the intrinsic flocculating power increases with the molecular weight.

Polyacrylamides have the highest molecular weight among the synthesized industrial chemicals in the range of 10-20 millions. Other polymers display specific properties and are used under specific conditions.

They are mostly:

• Polyethylene-imines
• Polyamides-amines
• Polyamines
• Polyethylene-oxide
• Sulfonated compounds

     2.1- HOW TO DISSOLVE POLYACRYLAMIDE FLOCCULANTS

Flocculant solutions are highly viscous and it is difficult to prepare highly concentrated solutions. Flocculant solutions tend to degrade after a while.

In the laboratory, 0.5 % is the recommended concentration of stock solution which is then stable for two weeks. A 0.1 % solution is stable for six days. Polyacrylamides can be dispersed and dissolved in cold water. The water should be gently stirred using a magnetic or propeller stirrer. The powder is added at a rate which gives a good dispersion of the flocculant particles in the water.

Each flocculant particle should be wetted separately to prevent agglomeration increasing the normal dissolution time of under two hours. High shear can deteriorate the polymer chains so avoid the use of high speed mixers, disintegrators or centrifugal pumps.

     2.2 – SETTLING TESTS: HIGH SOLID SUSPENSION

In the case of such suspensions the boundary line between solid phase and liquid phase is clear cut. The settling speed can be measured in a cylinder by observing the rate of change of solid phase height with time.

     METHOD

• fill a one liter measuring cylinder with the suspension
• add the requisite amount of flocculant by pipetting a one gram per liter solution of flocculant
• close the cylinder and slowly invert it four times or stir the contents by slowly plunging a perforated disc on a rod four times to the bottom of the cylinder
• measure the height of the solid phase at regular intervals
• plot the settling curve of height against time.

Repeat the procedure with all the flocculants under tests and select the best one. Repeat the procedure with different dosages of this flocculant and thus determine the optimum dose rate. The flocculation of concentrated suspensions is very sensitive to stirring and it is therefore vital to employ uniform stirring throughout.

     2.3- SETTLING TESTS: LOW SOLID SUSPENSION

In the case of low solid suspensions, we observe low settling speeds. The flocs are dispersed and it is necessary to induce a velocity to the suspended solids in order to obtain bigger flocs.

The obtained results are compared in terms of floc size and clarity of the supernatant.

The most practical device for such evaluations is a jar test. One proceeds as follows:

• fill the 5 beakers with one liter suspension
• add the flocculant at a high revolving speed of the propellers (100 rpm) during 10 seconds in order to obtain a good dispersion of the polymer
• stir three minutes at 40 rpm.

Then compare the different flocculants and the various dosages in terms of floc size supernatant clarification and settling speeds.

     2.4- FLOCCULATION TESTS AFTER COAGULATION

All suspensions which contain a high proportion of colloidal organic substances cannot be directly flocculated. They first have to be destabilized through addition of a di or trivalent metallic salt: lime, ferrous sulphate, ferric chloride, aluminum sulphate or sodium aluminate.

The use of synthetic flocculants makes coagulation less pH sensitive and makes possible the use of:

• calcium salts           at pH between 4 and 14
• iron salts                at pH between 4 and 13
• aluminum salts       at pH between 4.5 and 10

With each suspension, there is an optimum pH which has to be found in order to reach the optimum results. Evaluations are made with a jar test.

Firstly, the amount of coagulant necessary to destabilize the suspension has to be determined:

• add 10, 30, 50, 100, 200 ppm (or mg/l) of coagulant in 1 per cent solution,
• adjust the pH when very acidic back to pH 6 by addition of caustic,
• stir 1 minute at 200 rpm,
• add 2 ppm of synthetic flocculant usually of an anionic grade over 2 minutes at 50 rpm.

The beaker which gives first a clear supernatant contains the optimum amount of coagulant sufficient to destabilize the colloidal suspension. More than 200 ppm of coagulant may be required for optimum destabilization.

Secondly, the amount of flocculant necessary in order to obtain the desired settling speed is determined:

• fill the 5 beakers with the suspension. Add the amount of coagulant determined during the first step and stir for two minutes at 100 rpm,
• compare the available flocculants in terms of floc size, supernatant clarity and settling speed.

In many cases, the best result is obtained with a combination of lime and iron salts, especially in effluent treatment, when the optimum pH lies between 7 and 9.5.

     2.5- FILTRATION TESTS

          2.5.1- Buchner test

• Mix with a glass rod a suspension with the flocculant
• Then pour the flocculated mixture on a Buchner filter at a given pressure
• Measure the amount of filtrate after a time interval (30 seconds or 1 minute)
• A washing test can be made with a measure of the amount of clear water passing through the filter cake during a given time period (30 or 60 seconds).

           2.5.2- Tests with test filter leaf

A leaf is mounted on a funnel which is connected to a vacuum pump with a pipe. The funnel is immersed in the suspension for a known time after which the thickness of the filter cake, the moisture content of the cake, the quality of the filtrate and the washing speed can be measured.

     2.6- CENTRIFUGING TESTS

The test on a laboratory centrifuge has only relative significance:

• fill the bowls of the laboratory centrifuge with flocculated suspensions at various dosages.
• test at 1000 “g” during 2 minutes,
• measure the amount of settled material and the supernatant clarity in each case.

In most applications, the amount of flocculant necessary to obtain a good solid/liquid separation is
very small. The average range of dosage is:

• 0.5 to 3 gram per cubic meter of diluted mineral suspension,
• 2 to 20 gram per cubic meter of concentrated mineral suspension,
• filtration or centrifugation of a mineral slurry: 25 to 300 grams of flocculant per ton of solid,
• in the case of clarification of an organic effluent: 10 to 200 ppm coagulant – 0.25 to 2 ppm flocculant,
• filtration or centrifuging of organic sludge after coagulation: 1 to 5 kg per ton of dry solid,
• retention on a paper machine: 50 to 250 grams per ton of finished paper,
• viscosity increase of solutions: 4 to 10 grams per liter.

Industrial scale dissolution of flocculants requires a procedure which has to be adapted to the flocculant specifications:

• concentration: solutions of flocculant, even diluted, are very viscous
• high shear should be avoided during mixing
• flocculant particles if not properly dispersed tend to agglomerate lumps of flocculant do not dissolve easily
• if flocculant is spread on the earth, when wetted, it becomes slippery.

A dissolution plant has the following components:

• a disperser system to ensure a proper wetting of the powder without agglomeration,
• a dissolving tank,
• a transfer pump,
• a stock tank,
• a metering pump and dilution system.

It is recommended that the flocculant is dissolved at the highest possible concentration and diluted after the metering pump.

     4.1- THE DISPERSER SYSTEMS

          4.1 .1 – Disperser for flocculant

The disperser operates on an aspiration principle. It facilitates dissolving powder flocculants.

Up to an amount of 5 kg per batch, water projections on the disperser can clog the inlet tube.

          4.1.2- Direct addition into the vortex of the dissolution tank

The flocculant is poured in the vortex of the dissolving tank either directly or through a funnel or through a vibrating device. This method is adapted to low concentrations of the flocculant solution. Above a certain concentration, the viscosity increase of the solution tends to prevent the dispersion of the powder when it reaches the solution.

          4.1.3- Addition of the powder in a fresh water cyclone

As above, an optimal dispersion is obtained with this process.

     4.2- DISSOLVING TANK

Flocculant solutions are non-corrosive. It is impossible to use mild steel or plastic equipment (polyester, glass fiber, polyethylene, polypropylene, PVC).

Agitation must be sufficient to keep the flocculant particles in suspension and not too violent in order to prevent mechanical degradation of the polymer.

     4.3- TRANSFER PUMPS

Positive displacement pumps can be used or low pressure centrifugal pumps. The dissolving tank can be put above the stock tank. The transfer pump can then be omitted.

     4.4- THE STOCK TANK

The stock tank will have a higher capacity than the make up tank. The solution in the tank need not be agitated.

The flocculant has to be used properly to ensure maximum efficiency. Its use depends on numerous physico-chemical factors which can alter the obtained result.

At the industrial stage, efficiency lies at 60 to 120 per cent of laboratory efficiency.

Most factors which will influence the final result are:

• the location of the injection point which has to be such that turbulence will ensure a good dispersion of the flocculant but will not break the flocs,
• a multiple point addition usually improves the contact of the flocculant with the system,
• dilute solutions very often give better results.

On most settling units, it is possible to check efficiency through sample observation at the inlet part of the unit.

In the case of low solid suspensions, sludge recirculation to the inlet of the settling unit improves the settling rate and the clarity of the supernatant.

When flocculant and coagulant are used together the following equipment improves the efficiency:

• a coagulating tank with mild stirring and about 5 minutes retention time,
• a flocculating tank which can be the inner part of the settling tank where the chemical reaction will take place. In the case of vacuum filters or centrifuges, the flocculant is introduced into the inlet pipe.

The tendency to use compact liquid solid separation equipment corresponds to the use of high efficiency polymers.

     6.1 – In the mineral industry

• Ore leaching (uranium, zinc, gold, bauxite, copper, etc…)
• Treatment after flotation
• Treatment of the tailings to prevent pollution and allow water reuse

     6.2- In the chemical industry

At the clarification stage of the following processes: phosphoric acid, dicalcium phosphate, brine electrolysis, magnesia production, titanium dioxide.

     6.3- Industrial waste treatment

• Blast-furnace gas washing
• Surface treatment industry
• Petroleum refinery effluent

     6.4- Sewage and municipal waste

• In the case of physico-chemical treatment
• Prior sludge dewatering

     6.5- Paper industries

• Retention of fines and fillers
• Drainage improvement

Other applications are found in most water consuming industries:

• Raw water treatment
• Potable water treatment
• Decarbonation
• Sugar industry
• Secondary oil recovery

Polyacrylamides are high molecular weight water soluble polymers, being flocculants their main use. When mixed with water, they dissolve slowly giving a viscous solution, generally through a 60-120 minute process.

To increase the speed of dissolution it is possible to grind the polymer to a size of over 750 µm, particles of under 750 µm tend to agglomerate together when added to water, resulting in “fish eyes”. For quick dissolution time and “fish eyes” free solutions, it is possible to use polyacrylamides in emulsion form.

To produce these emulsions the manufacturing process is given below:

• prepare a water solution of the monomer,
• obtain a water in oil emulsion (particle size approx. 1 µm) with the monomer solution using a stabilizer,
• polymerize the emulsion using catalyst.

This emulsion, when mixed with water, will not disperse/dissolve.

Therefore, it is necessary to:

• add a hydrophilic surfactant to the water which inverts the emulsions, i.e., the oil is dispersed by the surfactant and the polymer dissolves rapidly, or
• add the same surfactant to the original emulsion, resulting in a “self inverting” emulsion. In either case the same effect is obtained.

Tramfloc produces “self inverting” emulsions only. However, in certain cases as mentioned below, it is necessary to add some surfactant to the water.

To obtain a good inversion of the emulsion it is necessary for the surfactant to be present at its minimum effective concentration.

In water of standard hardness, the level has been determined to be 5 g/l to give a good inversion.

Standard emulsions contain between 28 and 46 % active content. Optimum dilution is between 50 and100 fold.

If this concentration is not achieved:

• greater than 100 fold, part of the emulsion does not dissolve,
• less 50 fold, the viscosity of the emulsion is too great.

To obtain a good inversion it is necessary for each polymer particle to be dispersed separately in water, otherwise the particles will agglomerate.

To ensure efficient dispersion it is necessary to use high shear mixing at the point of emulsion water contact.

     Equipment used:

• venturi eductors,
• two speed agitator in the dissolution tank, high speed during addition of emulsion followed by slower speed,
• high shear static mixer,
• valve discharge at a fixed pressure.

If the emulsion is efficiently dispersed, there is no need for further agitation to obtain dissolution, only a contact time of 3-15 minutes is imperative. In practice if the inversions are not perfect, a stirrer is necessary in the storage tank mixing for 3-15 minutes.

Water quality affects inversion seriously.

Hard water : 30-40 grains of hardness gives poor dispersion with severe difficulties over 50 grains.

Calcium is reducing the efficiency of the surfactant, i.e., sea water and brines 30 g/l are at the limit of acceptance.

In such cases it is possible:

• to adjust the surfactant level in emulsion production to a level previously determined by testwork on the water to be used.
• to make small additions of surfactant to the dissolution water.

For cationic emulsions it is necessary to select a nonionic surfactant to avoid precipitation with the polymer.

By nature, the emulsion is an unstable compound. The continuous phase, oil having a specific gravity of 0.85 and the water over 1.05. Over extended periods of time, this water phase has tendency to settle to the bottom.

Over a 6 month period at an average temperature of 20⁰C, this settled phase is composed of agglomerates which are easily redispersed using mechanical or air agitation.

At higher temperatures, 35⁰C, the effect is accelerated and becomes more difficult to redisperse. The shelf life is reduced to 3 months.

A similar effect is observed if the product is frozen, the emulsion remains pourable down to -10⁰C, below this temperature the product becomes immobile, due to increased viscosity of the oil.

If the emulsion has been frozen

• the redispersion of the settled layer is more difficult,
• the dissolution in water is more difficult. In addition, if the emulsion is subjected to a freeze-thaw cycle, the additional use of surfactant in the dissolution water is required to obtain effective dissolution. However, the efficiency of the polymer is unaffected.

     6.1 – Compatibility

The carrier oil is an aliphatic hydrocarbon containing less than 0.1 % aromatic.

Plastics, rubber and metal compatibility:

Avoid the use of copper, cast iron, galvanized iron.

Polyethylene is used for the transport of emulsions but the oil has a tendency to dissolve in low density polyethylene decreasing the drum strength.

     6.2 – Emulsion Storage

Emulsion storage tanks are normally stainless steel (304), fiberglass or high density polyethylene.

To inhibit layering:

• nitrogen is fed into the bottom of the tank, for 10 minute periods every two days,
• slow speed stirring 10 minutes per day,
• recirculation using mechanical pump.

This method imports high shear which reduces emulsion stability

The tank has to be vertical with a conical bottom to help redispersion, it should also be fitted with an inspection cover to facilitate cleaning using high pressure water, once per year.

     6.3 – Pumps and filters

The emulsion if not properly mixed may contain some agglomerates which can plug filters and pumps. These agglomerates being water soluble can be pumped to the dissolution tank, except in paper applications.

Pump types

• Gear pumps should be constructed in stainless steel or Teflon. Steel pumps can be used and changed every one or two years.

  Pump seals have to be consistent with the compatibility chart.
  

• Mono pumps with nitrile stator.
  
• Electro-magnetic pumps especially adapted for viscous liquids (large pipe for suction).
  
• Piston and membrane pumps are not very efficient for emulsions, especially if the output is low. In this case, a filter is placed between the emulsion tank and the pumps, suitably sized to avoid frequent cleaning. Filters of 300 – 500 µm size are suitable.

     6.4 – Filtration and storage of the finished products

When the dissolved emulsion is used in paper production it is normally filtered at 50 microns. The dissolved emulsion can be stored in polyethylene, polypropylene, PVC, fiberglass, stainless steel and coated steel. Iron and galvanized iron should be avoided, and all pipes should be plastic, copper or stainless steel.

Problems arising with the oil

• Extended storage with diluted emulsions can lead to separation of the oil, giving problems when pumped, especially in paper production.
• The oil can be absorbed selectively by flexible pumps (Jabsco pumps if correct rotors are not used).

It is very difficult to weigh small amounts of emulsion. The use of a syringe is more general. The specific gravity of emulsions is between 1.02 /1.05. Glass syringes are used due to the incompatibility of plastics.

Following its use, it should be washed with mineral spirits or hot water.

As for polyacrylamide powders, the solutions are more stable at high concentrations, preferably 5 g/l active content and also preferably inverted at this concentration. This solution is diluted before use.

Given the same initial composition, powders and emulsions have varying efficiency differences depending on the application.

Generally:

• cationic emulsions have higher molecular weight than powders, giving improved sludge dewatering.
• powders have a broader molecular weight distribution giving improved clarification.
• the surfactants contained in emulsions can give improved results on sludges containing oils and fats, but can have a tendency to redispersion due to foaming,
• the hydrolysis during polymerization is lower in emulsions and therefore are slightly more cationic than powders.

The polymer contained in the emulsions has the same toxicity (very low) as powders.

The free acrylamide content of industrial emulsions is around 1,000 ppm (0.1 %).

For special uses it is possible to produce products with less than 500 ppm (0.05 %) of free acrylamide.

The oil is a dearomatized aliphatic hydrocarbon.

The flash point of the solvent, alone, is over 105⁰C, the emulsion has a flash point of over 100⁰C but when settled it decreases to 100⁰C.

The surfactants used have very low toxicity, obtained from esters of sorbitol and oxyethylated nonyl phenol.

The cationic emulsions are eye irritants. Emulsion spills should be absorbed with saw dust and burned. Any remaining should be washed with high pressure water.