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Abstract

Growing environmental concerns and economic necessities are leading all industries around the world to invest in developing suitable techniques for producing less waste and maximizing the recycling of natural resources. Water and solid waste pollutions are one of the most important problems in mining, mineral processing and metallurgical industries. In recent years, mineral processing techniques, especially flotation, has been used for solving some environmental problems. Flotation offers a simple and cheap solutions to waste recycling.

Froth flotation has been successfully used in mineral processing for fine particles (10-200 (m) since 1920’s. Today, it also provides solutions for pulp and paper, textile, food, plastic, petroleum, water purification, waste and sewage treatment etc. industries. A wide range of flotation machines for waste treatment are now available. Some of these machines are impeller agitated mechanical flotation cells. Flotation columns, dissolved air flotation, pneumatic flotation and electro flotation cells can also be used.

Key Words: Dispersed air flotation, dissolved air flotation, waste treatment, recycling, environment

1. Principles of Flotation for the Solutions of Environmental Problems

A wide range of flotation machine designs for waste treatment/recycling are available and most of these are mechanical type conventional flotation machines. They have impeller, which agitate the pulp and disperses air into it. Bubble diameter produced varies from 1 to 5 mm. But over the past 20 years, many new types of floatation machines have been introduced and started to use. According to bubble generation methods, four types of flotation machines can be used for waste treatment: dispersed air flotation (DPAF), dissolved air flotation (DAF), pneumatic flotation and electro-flotation. Table 1 summarizes the application areas of froth flotation process in environmental problems.

Table 1. Application areas of froth flotation for environmental problems.

1.1 Dispersed Air Flotation (DPAF)

Dispersed air flotation machines (mechanical cells) basically consist of a rectangular/circular flotation tank/cell and motor driven impeller immersed in it. A series of flotation cells produces a flotation bank. Contaminated waste enters at the feed end and the effluent is discharged at the opposite end. In Denver flotation machines compressed air and in Wemco depurator induced air (self aerating) are used as air phase for bubble generation. OK, Sala, Westpro etc. are some other major DPAF producers. A DPAF flotation machine has a cell capacity of 0.014 - 14.16 m3 (0.5 - 500 ft3). Cell motor power changes from 0.5 - 50 HP. Four, six or eight cell banks can be used. Figure 1 shows the cross-section of Westpro dispersed air flotation bank.

Wemco flotation machines (Depurator) remove emulsified oil, solids and other organic materials from fresh water and refinery or petroleum waste water. 200 to 5000 ppm of oil in waste water can be reduced down to 10 ppm oil after 4 minute cleaning cycle. Each unit is skid mounted and consists of a steel housing, float collection flumes and gas tight covers and inspection doors. Each unit has four cells individually equipped with flotation nozzle, water circulation valve, bearing stand, rotor, disperser, stand pipe, motor and V-belt drive. Machine capacities range from 2,000 to 170,000 bpd (50 - 5000 gpm), depending on the individual unit’s size. Depurators have 3 - 30 HP/cell motors. Wemco depurator can be widespreadly used for oil field production water, refinery process water, petro-chemical wastewater and ballast water (Kaya and Kurama, 1999).

1. Heavy duty tank construction, 2. Removable wear liner, 3. super structure, 4. Mechanism, 5. V-belt drive, 6. feed-box, 7. transition box, 8. discharge box, 9. froth paddle assembly.

Figure 1. Cross-section of Westpro dispersed air flotation bank.

1.2 Dissolved Air Flotation (DAF)

In dissolved air flotation, water is saturated with air under pressure and is passed into a flotation tank through a nozzle or valve. The resultant pressure drop releases air from solution as small bubbles (less than 0.1 mm) that collide with the dispersed phase and carry it to the surface. Hydrophobicity may be preexist due to impurities in water. While some turbulence is induced during the injection of the bubble swarm. DAF is very useful when loosely bound flocks are to be floated. There are three types of aeration in DAF: Full stream pressuration, split stream pressurization and recycled stream pressurization. Choice of aeration system depends on suspension type. Recycle-stream pressurization is most commonly used. In practice, industrial wastewater has required recycles of effluent varying from 5 - 50%. Floated product/sludge (2 - 20% solid) is usually removed by a mechanical chain, scraper or paddles. Main design parameters of DAF are air-to-solid ratio, hydraulic loading, saturator characteristics and injection nozzle performance (Kaya and Kurama, 1999).

Dissolved air flotation is a process for the removal of fine suspended/insoluble materials from an aqueous suspension/wastewater. The term "flotation" indicates something floated with air bubbles at the surface of a liquid. DAF provides the needed energy for effective flotation in the form of extremely fine air bubbles which become attached to the suspended material to be removed. The attraction between the air bubbles and particles is a result of adsorption forces that are a function of the characteristics of the particle surface, or physical entrapment (enmeshing) in the particle. This attachment of bubbles to the particle "reduces" the density of the particle resulting in increased buoyancy, thus effecting flotation. Chemical conditioning is often used to increase the effectiveness of the dissolved air flotation process. Pollutants are concentrated in the material that accumulates on the surface, called the float/skimming/sludge.

DAF has been applied for the clarification of wastewater to meet effluent discharge requirements; for recovering suspended material; for reclaiming water for re-use; for thickening of slurries and sludge’s prior to disposal, further treatment or dewatering. The most reliable and positive method of producing bubbles of the proper size is to dissolve air into water under pressure and to then reduce the pressure of the solution. As the pressure is reduced, the air comes out of solution in the form of micro bubbles. Feeding rate for DAF varies from 8-1200 gpm. A percentage of the clean effluent is recycled and super-saturated with air, mixed with the wastewater influent and injected into the DAF separation chamber (Figure 2) (http://www.panamenv.com). The dissolved air comes out of solution, producing millions of microscopic bubbles. These bubbles attach to the solids and float them to the surface where they are mechanically skimmed and removed from the tank. DAF systems are designed to remove fats, oils & grease (FOG), suspended solids, food/animal production/processing wastes, industrial wastes, hydrocarbon oils/emulsions and many other contaminants. Clarification rates as high as 97% or more can be achieved using our dissolved air flotation systems. Chemical pre-treatment can often help to improve the performance of contaminant removal.

Figure 2. Two different DAF operating systems: Conventional and Pan America

(www.panamenv.com.)

1.2.1 Conventional DAF 

Conventional DAF saturation design uses a recycle pump combined with a saturation vessel and air compressor to dissolve air into the water (Figure 2). This type of system, while effective, is expensive, labor intensive and can destabilize its point of equilibrium, creating burps due to incorrect, loss or creeping of EQ set-point in the saturation vessel. This design is slow to recover and can upset the flotation process in the DAF. Air transfer efficiency is approximately 9%-80% entrainment. This style of operation can increase chemical use, labor costs, downtime, effluent loadings, production schedules and other detrimental domino effects due to EQ loss.

1.2.2 Pan America's DAF 

Pan America’s DAF design simplifies the DAF process, requires less startup time, less capital cost, instrumentation, labor and maintenance. The design is process friendly, providing virtually instant saturation upon system startup without equalization and complex startup procedures. Once the system is adjusted the system can be shutdown and started up again without any readjustment or equalization. Higher air transfer efficiencies are also realized due to higher saturation pressures with 12%-93% entrainment. Pan America can provide both styles of DAF design depending on application and customer preferences.

1.2.3 DAF sizing

DAF sizing takes into consideration many criteria for sizing: Flow rate, water temperature, waste characteristics, chemical pre-treatment, solids loading (lbs/hr/ft2), hydraulic loading (gpm/ft2), air to solids ratio (lbs of air/lbs of solids). DAFs are designed on the basis of the peak flow rate expected. The flow can range from 1 to 5 gpm/ft2. Bench testing of waste stream samples is usually the preferred starting point when sizing equipment and determining proper chemical processes prior to the DAF. The chemical pretreatment will assist and improve the DAF separation process.

1.2.4 Chemical pretreatment 

Chemical pretreatment often improves DAF solids removal efficiencies. The use of chemical flocculants with DAF is based on system efficiency, application/use and cost. Commonly used chemicals include trivalent metallic salts of iron, such as FeCI2 or FeSO4 or aluminum, such as AISO4. Organic and inorganic polymers (cationic or anionic) are often used to enhance the DAF process. The most commonly used inorganic polymers are the polyacrylamides. Chemical flocculant concentrations used normally range from 100 to 500 mg/l. The wastewater pH may need to be adjusted between 4.5 and 5.5 for the ferric compounds or between 5.5 and 6.5 for the aluminum compounds using an acid such as H2SO4 or a base such as NaOH. In many applications, the DAF effluent requires pH adjustment utilizing a base such as NaOH to assure the DAF effluent pH is within the limits specified by the publicly owned treatment works (POTW) (6-9 typically).

Attachment of most of the bubbles to solid particles can be effected through surface energies while others are trapped by the solids or by hydrous oxide flocs as the floc spreads out in the water column. Colloidal solids are normally too small to allow formation of sufficient air-particle bonding. They must first be coagulated by a chemical such as the Al or Fe compounds mentioned above and then absorbed by the hydrous metal oxide floc generated by these compounds. Frequently, a coagulant aid is required in combination with the flocculants to agglomerate the hydrous oxide floc, increase particle size and improve the rate of flotation. Mechanical/chemical emulsions can also be broken through pH and polymer reactions. Where the float is to be used to feed animals used for human consumption, organic compounds such as chitosan, carrageenan, and lignosulfonic acid, or their derivatives can be used. Use only compounds approved by the Food and Drug Administration (FDA) Office of Veterinary Medicine.

1.2.5 Float dewatering 

DAF float often contains 2 to l0% solids. The solids may need to be dewatered before disposal to reduce the sludge volume by reducing water content. Float dewatering is usually performed by using one of the following technologies: Filter press, belt filter press, centrifuge, drying bed, vacuum precoat filter.

1.2.6 DAF applications

Table 2 shows the DAF application and utilization areas. Krofta Eng. Corp. (Supracell), USA, Komline-Sanderson (K-S)-Canada, Envirotech Canada Ltd (Emico Div.) are some DAF producers. Supracell clarifiers are suitable for Pulp&Paper Ind. and K-S and Emico DAF clarifiers for municipal and other industrial wastewater. K-S clarifiers are deeper systems. Supracell combines DAF and sedimentation in shallow cylindrical tank with a retention time of 3 min. Diameters ranges from 2.4 to 21.3 m and capacities from 0.45 to 45 m3/min. Higher throughputs are possible by stacking clarifiers on top of each other.

Table 2: Typical Applications of DAF (http://www.komline.com and http://www.iwwsllc.com/)

2. Industrial Applications of Flotation for Environmental Problem Solutions

Industrial applications and utilizations of dissolved and dispersed air flotation for solving environmental problems are rewieved here in detail.

2.1 Pulp and Paper Industry

2.1.1 White Water

During paper production fibers and auxiliary constituents of the paper pulp such as rosin size, glue, casein, clay, mineral fillers and other mineral pigments pass into the white water. Discarding this water creates not only a large scale nuisance but also a loss of valuable papermaking ingredients. Flotation helps resolve this problem.

2.1.2 Scrap/Waste Paper

Ink and ink-stained fibers and pigments have been floated from pulps made from scrap/used papers. Scrap paper, especially the better grades of writing and printing papers, consists primarily of clay, calcium carbonate or mineral pigments and adhesives over a reinforcement of cellulose. In deinking flotation of waste paper, the aim is to recover cellulose (Kaya and Öz, 1995).

2.2 Textile Industry

2.2.1 Fiber Recovery/Recycle

Floatability of various types of fibers is different. 0.5 mm fibers can be floated without any reagent. Vinyon and wool float well independent of pH and without reagents. Rayon does not float at any pH, while silk, casein and nylon floats in acid medium. Floatability of fibers is related to the structure of the fibers (Gaudin, 1957).

2.2.2 Rayon Spinning Baths

Production of viscose rayon involves squirting with spinnerets of highly viscous liquid treads of viscose in an acid bath. Reaction causes sulfur impurities in the bath. Flotation may be used for continuous removal of impurities from the spinning bath. Impurity content can be reduced from 20 ppm down to about 1 ppm.

2.3 Water Purification

Flotation can be used to purify water, the froth part becoming the reject. 90-95% of the turbidity of the water and tenfold bacteria in water reduction are possible. Flotation removes not only clay and other suspended minerals but also a large portion of bacteria. DAF is extensively used for drinking water treatment in Europe, US and Africa. The Table Rock and North Saluda Water Treatment Plant went online in Greenville, S.C., becoming the largest U.S. treatment plant to apply DAF to the treatment of potable water. With a capacity of 75 million gallons per day, the plant treats an average of 50 mgd and serves 350,000 customers (www.americancityand county.com).

2.4 Industrial Wastes and Sewage

Flotation can be applied to concentration of specific wastes such as high in greasy matters (packing houses), soaps (laundries), oils and detergents (machine shops), or to general or domestic sewage. From industrial wastes and sewages valuable fertilizers can be created. Laundry wastes containing 1500 ppm total solid at a pH 10 can be treated by flotation after the addition of 500 ppm ferric chloride floc formers. Grease can be removed 95% and biological oxygen demand (BOD) is reduced 85%. For general sewage, flotation with dodecylamine reduces suspended solids by 95-99%, bacteria 99-99.9%, dissolved solids by 25-35%, biological oxygen demand by perhaps 50%. The cost of the operation is high due to relatively large quantities of reagent usage. Flotation is suitable for the flotation of industrial wastes and sewage. Flotation of sewage produces denser sludge’s than the gravity-settled sludge’s (Gaudin, 1957). Table 3 shows DAF results for untreated and treated water.

Table 3. DAF results for untreated and treated water (www.martintenvironmental.com).

Parameter Untreated (mg/L) Treated (mg/L)

BOD 450 110

COD 1260 236

TSS 713 15

Oil&Grease 650 45

pH 11.5 7.5

Zinc 5 <0.03

Copper 2.0 <0.03

Nickel 0.36 <0.1

Cadmium 0.03 <0.02

Chromium 0.19 <0.02

2.5. Food Industry

The food processing industry seeks effective technologies to remove fats, oils and greases from food processing waste water at acceptable costs. The baking, dairy, oil extraction (e.g. olive, soybean, cottonseed oil), fish processing and meat and poultry industries as well as manufacturers of oil-containing foods (e.g. margarine and salad dressing) face the problem of reducing the oil contaminant load to downstream waste water systems. The recovery of valuable by-products, such as proteins and milk fat in the dairy industry, while at the same time reducing the biochemical oxygen demand (BOD) and total suspended solids (TSS) charges from the publicly owned treatment works (POTW) make systems that can remove fat, oil, and grease (FOG) increasingly economical.

The removal of emulsified oils, which is the major contributor to high oil concentrations in the effluent stream, is more difficult. In DAF and/or electro-coagulation systems, FOG coagulates and floats to the surface in a separator tank, where it is being removed by scrapers. In membrane systems, FOG is held back because of physical separation at the sub-micron level. Typically, the size of oil droplets in emulsions is between 0.1 µm and 0.5 µm. Ultra filtration membranes have pore sizes below 0.01 µm and work very well on filtering out FOG. Micro filtration membranes with pore sizes above 0.05 µm can also be effective in oil separation but there is a potential for oil breakthrough, especially at high emulsion concentrations (www.foodsci. unl.edu/fmc/need-09.htm).

Tubular UF membranes have been used successfully in the separation of proteins, fats, oil and greases in salad dressing, margarine and bakery plants, achieving FOG reduction in the waste water on the order of 99%. Oil in water from can-washing operations also has been removed by membranes. DAF is a well accepted method of FOG removal in the dairy, meat and poultry industries. Successful applications have also been found in the baking industry, particularly when doughnuts are being processed. Through the addition of coagulants and flocculants, very effective separation is achievable. The DAF treatment has become the standard for treating waste water in the meat and poultry industry. However, the coagulants and flocculants are fairly expensive and alter the composition of the retained fats and solids so that re-use as animal feed is sometimes not possible. A very high retention of FOG (on the order of 99%) has been obtained with a relatively new technology called electro-coagulation. While electro-coagulation has been successfully applied for oil removal in the automotive and metal treating industry, only recently have its capabilities been realized for the food industry. This is an area of interesting development work.

2.5.1 DAF systems for bakeries

Many food processing plants, such as poultry and meat processors, use DAFs as pretreatment systems. The experience of these plants may be useful for bakery managers as they consider DAFs for pretreatment. However, wastewater characteristics vary widely, depending on the products produced and the time of day. Also, bakeries use considerably less water than large meat and poultry plants (Carawan and Valentine, 1996). Several bakeries are currently using DAFs as pretreatment systems. These systems often have removal efficiencies similar to those in Figure 3. Note that these DAF removal efficiencies were obtained from a multiproduct bakery. Bread bakeries have substantially less biochemical oxygen demand and fats, oils, and grease contents. Therefore, opportunities for a DAF in bread plants would be more limited.

Figure 3. DAF removal for multiproduct bakery.

Bakeries are facing increasingly stringent restrictions on their wastewater discharges. In order to help reduce the concentration of contaminants in their wastewater and to avoid sewer surcharges, some plant managers have installed systems to pretreat the wastewater before it is discharged to municipal sewers or other disposal outlets. Many other managers are considering pretreatment to reduce surcharges or have been asked by their publicly owned treatment works to consider waste reduction. One way to pretreat wastewater is with a DAF system, which can remove insoluble materials such as fats, oils, and grease (www.bae.ncsu.edu/bae). Note that soluble contaminants such as sugar are not usually removed by the system, although they are occasionally trapped in the float along with other particles. Similarly, bakery wastewater often contains a large amount of settleable solids—materials such as dough pieces, bits of fruit, nuts, raisins, and product. These materials will settle because they are heavier than water. Thus, a DAF tends to negate the settling of these materials. Heavier materials will settle if given enough time, but some will be trapped with the float. Therefore, the nature of these materials must be considered in DAF designs for bakeries. Decisions about the use of chemical flocculants in DAFs are based on cost, system efficiency, and the intended use of the DAF float. Advantages of use of DAF for bakeries are: reduce grease (FOGs), reduce suspended solids (TSS) and settleable solids (but only if equipped with a bottom sweep), may help bakeries meet POTW permit restrictions; do not require excessive maintenance or management may reduce surcharges, especially for larger bakeries. Disadvantages of use of DAF for bakeries are: do not remove soluble materials such as sugars, do not remove the BODs associated with soluble materials, only concentrate the pollutants; the float must still be disposed of properly, are costly to buy and are expensive to operate. (If high chemical use is required, operational costs will be high.), they rarely cost less than surcharges, especially for smaller bakeries.

2.6. Plastic Recycling Industry

Recycling of plastics that used to end up only at city landfills or incinerators is increasing around the world. Discarded plastic products and packaging make up growing portion of Municipal Solid Waste (MSW). The Environmental Protection Agency (EPA) states that in 2000, the amount of plastics throw away were 50% greater than at the beginning of the 1990s. Plastics made up 7% of MSW by weight and 20% by volume.

2.6.1 Waste plastic mixture sorting flotation

Currently used density separation methods are unable to sort used plastic of different composition; but, similar density for example PET and PVC. Flotation can be used for PET/PVC separation before recycling based on the differences in wettability. Since plastics are hydrophobic, it is necessary to selectively render one of them hydrophilic (Guern et al., 1997). Gamma flotation of PET/PVC mixture in pneumatic flotation cells after conventional wet-chemical or dry-physical conditioning at coarse particle size (i.e. 2-6 mm) is performed at a grade of 99% and recovery of 80-90% (Stückard et al., 1997).

The plastics to be separated by gamma flotation, which depends on critical surface tension (Kaya, 2002), can be treated in a high pH NaOH solution prior to flotation. The modification of polymer wettability is due to the hydrolysis of the plasticizers in the polymer. The best separation between PET and PVC was achieved at 190 ppm MIBC, 20% CH3OH, pH:11 and two minutes flotation time (Buchan and Yarar, 1996). Under these conditions the best selectivity was obtained at a PVC recovery of 92% and PET recovery of 0%. Flotation separation of glass, reinforced PP, PS, PC, PMMA, PA6, PVC and POM were also carried out on a bench-scale in a specially designed low turbulence flotation cell (Dalmijn, 1996).

Schut (2001) stated that Daimler Chrysler Corp. supported R&D on froth flotation for recovering ABS, PP, PU foam from auto shredder residue (ASR). Car bumpers, instrument panels, tail lights, telephones, computers, other electronic devices, disposable cameras and CDs are reclaimed by heavy media separation, hydrocyclones, flotation and combinations of these methods. Recovery Plastics has developed ASR recovery technology and build five flotation plant for plastic separation since 1983. The first line was build to separate PVC from PET bottle and the rest for ASR. The recycling cost was 24 cent/lb. The Argonne Process has been applied to plastic waste stream (ABS, HIPS, PP etc.) generated from shredding obsolete appliances. For this application, the plastic stream is feed through granulator, then to a three-step separation process. In the first and second stages, the ABS and HIPS are separated from other plastics in gravity separation tanks. In the third stage, froth flotation is used to float HIPS. ABS with purity greater than 99% meets the recycle market specifications for auto headlamps (Argonne, 2001).

3. Conclusions

Froth flotation is one of the most commonly used mineral processing technology and has a significant place in the waste management and waste recycling/recovery industry. It provides simple, relatively inexpensive solutions to environmental problems. DAF/DPAF systems are designed for removal of many types of contaminants/wastes via flotation. Products removed are suspended/insoluble solids, animal/vegetable fats, oils, greases, fuels, organic materials, inks, waxes, animal/vegetable by-products and other many products. Today, flotation is also extensively employed for drinking water, municipal and industrial waste water treatment/use, and waste chemical processing around the world. It is believed that flotation will find new potential application areas and gain more importance in waste management and recycling/recovery industries in the near future.

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