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Historical use and future development of chemicals for solid–liquid separation in the mineral processing industry
Minerals Engineering 16 (2003) 103–108 This article is also available online at: www.elsevier.com/locate/mineng
Historical use and future development of chemicals for solid–liquid separation in the mineral processing industry M.J. Pearse Ciba Speciality Chemicals, P.O. Box 38, Bradford, West Yorkshire, BD12 0JZ, United Kingdom Received 4 September 2002; accepted 22 October 2002
Abstract From their ancient beginnings to the present day, coagulants and flocculants, used to enhance solid–liquid separation by particle aggregation, are described; particular reference is given to developments in the mineral processing industry. The impact of polyacrylamide-based flocculants on technical effectiveness, disposal possibilities and environmental control, over the last 40 years, has had a profound effect on the efficiency and design of solid–liquid separation equipment. Recent developments, in terms of molecular architecture, offer new horizons for the future at a time when requirements for separating solid particles from liquid are becoming more demanding in the minerals industry. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Dewatering; Filtration; Flocculation; Tailings disposal; Thickening
1. Particle aggregation The process of particle aggregation to assist solid– liquid separation in the minerals industry is conventionally described by two mechanisms and these involve the use of different types of chemicals. Coagulation by coagulants, derived from the Latin word coagulare, meaning to be driven together, involves neutralisation of particle surface charge or potential by the action of inorganic counter-ion adsorption or electrostatic screening. Flocculation by flocculants, derived from L. flocculus, meaning a loose fibrous structure pertaining to a tuft of wool, involves the bridging together of particles by long organic polymer chains, where charge neutralisation may or may not be involved. A hybrid case involves the use of synthetic organic coagulants, sometimes referred to as Ôprimary coagulantsÕ. Slightly different definitions are employed in other industries, but broadly speaking, the terms above are adhered to for the purpose of this publication. 2. Historical development Far back in geological time, the first chemically assisted solid–liquid separation could conceivably have E-mail address: [email protected] (M.J. Pearse).
been the destabilisation and coagulation of fresh water colloidal clays borne by rivers from areas of erosion to the saline oceans. Present day seawater has an ionic strength of 0.7 M and its coagulation power plays a major role in the formation of the fine fractions of deltas. This process would have commenced with the evolution of river systems and oceans; the most ancient sedimentary rocks are 3.5 billion years old. The surface forces at work there laid the foundations of many of our present day techniques involving inorganic coagulants and to the whole science of the electrical double-layer surrounding particulate matter in aqueous media. 2.1. Inorganic coagulants Human intervention for intentional coagulation of water is reported as early as 2000 BC by Romans and Egyptians using alum (Faust and Aly, 1998). Pliny, 77AD, reported the use of a mixture of lime and alum for water clarification. Alum became an important item of trade in these early years mainly for leather treatment and as a dye mordant. Alum Bay on the Isle of Wight and Boulby, North Yorkshire were manufacturing sites for alum in the UK commencing in the 1500s and 1600s, respectively. By 1757, muddy water was being clarified in the UK using alum––2–3 grains of alum per quart (approximately 150 mg/l)––followed by filtration of the supernatant. These early reports most likely refer to
0892-6875/03/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0892-6875(02)00288-1
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M.J. Pearse / Minerals Engineering 16 (2003) 103–108
potash or ammonium alum, the former utilising the potassium from clays and the latter using urine in the manufacture. The manufacture of aluminium sulphate was eventually patented in the USA in 1884 and was found to be a superior coagulant to the double salts. The term ÔalumÕ as used as a coagulant in solid–liquid separation is invariably reserved for aluminium sulphate. In the same year, ferric salts were patented for water clarification. An excellent account of early metallurgical practice has been given by Burger and Wendland (2001). The early publication of Georgius Agricola (1556) illustrates washing and settling of ores being carried out in a single operation (Agricola, 1950). The use of lime in mineral processing is a panacea. It coagulates, raises pH, depresses iron sulphides, keeps cyanide in solution and causticises by removing carbonate ions. Early hard rock base metal tailings relied on lime as the coagulant during thickening and the early thickener sizing methods, like that of Coe and Clevenger, were developed using limed gold pulps (Pearse, 1977). Whereas the use of lime has dominated high solids systems, aluminium and ferric salts have dominated the inorganic coagulant market for low solids clarification, as in mine site run-off, due to their inherent hydrolysis yielding highly cationic oligomers. A simplified version of their chemistry is seen in the speciation diagrams of Fig. 1, based mainly on the work of Amirtharajah and OÕMelia (1990). 2.2. Pre-polymerised inorganic coagulants Intentionally pre-polymerised versions––polyaluminium chloride (PAC) and polyferric sulphate (PFS) are important developments from the simple salts and were patented in Japan in 1972 and 1976, respectively. The latter was patented by a Japanese mining company. Both have the advantage of being less pH dependent than their simple salts. Their development resulted from
the chemical study of the simple salts and the highly charged active species involved in effective coagulation. The two well known pre-polymerised aluminium coagulants can be represented by the general formula: Aln ðOHÞm Clð3nmÞ Polyaluminium chloride (PAC) n ¼ 2, m ¼ 3 Aluminium chlorohydrate (ACH) n ¼ 2, m ¼ 5 Coagulation characteristics are similar for both chemicals. As the pre-polymerised versions have undergone hydrolysis, there is no reduction in pH of the effluent stream as with simple salts, an important aspect for environmental discharge of mine site run-off clarification. 2.3. Naturally derived organic flocculants Whereas inorganic salts promote coagulation by charge neutralisation and double-layer compression, organic polymers promote solid–liquid separation by the bridging mechanism sometimes in combination with charge patch effects. The conventional view of bridging flocculation of particles by long-chain polymers is shown in Fig. 2. Polymers derived from plant-based materials were the first flocculants. Sanskrit writings in India dating from several centuries BC make reference to seeds of the Nirmali nut tree, Strychnos potatorum, as a clarification aid. Peruvian texts from the 16th and 17th centuries describe the use by sailors of powdered, roasted grains of Zea mays (corn starch) as a means of settling impurities. More recently, Chilean folklore texts from the 19th century refer to water clarification using the sap from the ÔtunaÕ cactus (Opuntia fiscus indica). Isinglass, produced from fish swim bladders, has been used as finings for centuries for clarifying beer. The principal reactive constituent of isinglass is collagen. Collagen is a unique molecule that exists as a triple helix with three chains of amino acids wound around each other and
Fig. 1. Simplified solubility equilibria for (a) alum and (b) ferric salts.
M.J. Pearse / Minerals Engineering 16 (2003) 103–108
105
Fig. 2. Conventional view of polymer bridging in flocculation. Fig. 4. Structure of guar gum.
held in place by complex hydrogen bonding. It is amphoteric but functions as a cationic polymer when removing yeast particles. Naturally derived polymers that were used extensively as flocculants in mineral processing included animal glue, gelatin, starch and guar gum. Glue and gelatin are derived from thermal denaturing of collagen-bearing materials like bones and hides; glue is said to contain less hydrolysis products than gelatin. As described for isinglass, collagen is a proteinaceous material composed mainly of amino acid units––glycine, proline and hydroxyproline, which impart an amphoteric nature to glue and gelatin by virtue of the presence of both amino and carboxyl groups. Both materials were used extensively in acidic hydrometallurgical operations like uranium extraction and were particularly effective at removing colloidal silica. The structure of gelatin is shown in Fig. 3. Guar is derived from the seeds of the leguminous shrub Cyamopsis tetragonoloba, grown mainly in India, Pakistan and South Africa. The process of extracting guar gum was commercialised in the USA in 1953. Guar is a galactomannan with approximately two mannose units for each galactose, as shown in Fig. 4. The guar chain is somewhat rigid in solution and nonionic in character (Mackenzie, 1986). The hydroxyl
groups of the mannose chain are in cis-position which may give a reinforced hydrogen bonding to the surface of particles or at least some unique properties. Starch is obtained from wheat, maize (corn), rice and potatoes; it consists of two components. Straight chain amylose makes up approximately 20–30% and branched amylopectin, the remainder. Both polymers are composed of glucose units with hydroxyl groups in transposition. Starch is naturally nonionic or slightly anionic and can be solubilised by causticising or heating. The amylopectin component contains small amounts of phosphate, especially in potato starch, giving an affinity to iron minerals. Representations of starch structure are shown in Fig. 5. Both starch and guar remain in niche applications as flocculants, the former in alkaline environments such as the Bayer process and the latter in acidic operations like uranium filtration; they were used more extensively in the past. Both starch and guar still find considerable application as flotation depressants. The major limitation to the performance of naturally derived flocculants is their ceiling in average molecular mass or effective chain length. For gelatin/glue the maximum is approximately 300,000 Da, for guar approximately 250,000 Da and amylose starch 65,000 Da, although amylopectin fractions are higher. Potato and tapioca starches have higher molecular masses than corn starch. 2.4. Synthetic polyacrylamide-based flocculants
Fig. 3. Structure of gelatin.
A major advancement was seen in the 1950s when polyacrylamide flocculants were introduced. Early reference is made to application in water treatment in 1958 and they were certainly being used in the mineral processing industry in the early 1960s. The polyacrylamide molecule could be tailored to virtually every mineral processing situation and the next four decades saw massive expansion in its use. By substitution to the polyacrylamide chain, the addition of cationic and anionic functional groups gave polyelectrolytes
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M.J. Pearse / Minerals Engineering 16 (2003) 103–108
Fig. 5. Structure of starch components.
which covered all slurry environments from monomineralic to multi-mineralic, low to high suspended solids, low to high dissolved solids and low to high pH. Manipulation of molecular mass from 5 to 25 million Da allowed successful application to be made on all solid–liquid separation equipment––clarifiers, thickeners, filters of all types and centrifuges. Along with the polyamine and polyDADMAC (diallyldimethyl ammonium chloride) primary coagulants discussed below, polyacrylamide-based flocculants account for over 90% of the mineral processing flocculant market. The entities of acrylic chemistry used in common types of flocculant are shown in Fig. 6. Acrylamide is polymerised by vinyl addition to give nonionic polyacrylamide. Sodium acrylate is copolymerised with acrylamide to give anionic flocculants, while dimethylaminoethyl acrylate is used with acrylamide to give cationic products. In relation to some of the natural polymers, acrylicbased flocculants have a relatively simple structure. It is interesting to note that according to Kirk (1991), acrylic acid, a component of anionic flocculants, was first pro-
Fig. 6. Monomer entities which link together to form acrylamidebased flocculants.
duced in 1847 by oxidation of acrolein. Present day manufacture is by catalytic oxidation of propylene. 2.5. Synthetic coagulants The main coagulants used in the minerals industry today are based on polyDADMAC (diallyldimethyl ammonium chloride) and quaternised polyamines. Structures are shown in Fig. 7. These products are often known as primary coagulants as they are usually used prior to the addition of a polyacrylamide-based flocculant. Both types are highly cationic and of low molecular mass (<1 MDa). The primary mechanism involved with these products is charge neutralisation, although bridging occurs in a secondary mode. Many of the inorganic coagulants used in the minerals industry previously have been replaced by synthetic coagulants. Both types of synthetic coagulant were developed in the 1970s.
3. Parallel development of equipment Solid–liquid separation equipment, separation science and flocculants have showed parallel development and some of the milestones in the interlinked technologies are summarised in Table 1. Notable synergy is seen between high molecular mass flocculants and high rate thickeners, especially with the development of flocculating feedwells and the addition of dilution water to feedwells (clarified overflow) allowing the most to be made of the powerful polymers. As flocculants have developed in their effectiveness, the footprint of thickeners have shrunk from the massive diameter conven-
M.J. Pearse / Minerals Engineering 16 (2003) 103–108
107
Fig. 8. The changing profile of clarifiers and thickeners.
Fig. 7. Structures of synthetic coagulants.
tional types using lime or low molecular weight polymers and utilising settling rates of around 1–5 m/h to the compact inclined plate and high rate types which use high polymers and have settling rates often in excess of 20 m/h. The other notable development is the increase in height of thickeners, making use of the compressible nature of highly flocculated slurries to produce a thick underflow, in many cases suitable for direct disposal. The deep cone, deep thickener and paste thickener make use of this technology. This is illustrated in Fig. 8. As tank sizes have become smaller, less buffering capacity is evident and a rapid dose response is seen with high molecular mass flocculants. This has been accompanied by the development of control equipment, which has progressed rapidly since the 1980s. Filter belt presses have gained increasing popularity in the mineral pro-
cessing industry over the past 20 years, and their operation is also dependent on high molecular mass flocculants.
4. Present and future developments The average molecular mass ceiling of polyacrylamide-based flocculants has been pushed to its virtual limit using presently known technology. In the mineral processing industry, and others, higher molecular mass of conventional, linear type polymers does not necessarily produce the best performance. This has led to the molecular architecture approach, for example, as in Cibaâ UMAâ (Unique Molecular Architecture) technology. The UMAâ approach is a departure from conventional thinking in polymer design and several facets are involved in the overall picture (Pearse et al., 2001). For example, highly branched and interactive polymer chains produce flocculant solutions containing a proportion of
Table 1 Some milestones in solid–liquid separation Milestone
Significance
Milestone
Significance
Stokes, 1851 Darcy, 1856 Helmholtz, 1879 Schulze-Hardy, 1882–1900
Particle settling Filtration EDL structure Empirical coagulation concentrations High solids filter cakes Clarification of dilute slurries
DLVO Theory, 1941–1948 Kynch, 1952 Polyacrylamide, 1950s Ruehrwein and Ward, 1952
Particle stability Mathematics of sedimentation Quantum leap in flocculation Bridging model of flocculation
Talmage and Fitch, 1955 LaMer, 1958
Thickener sizing Quantitative bridging model of flocculation Polymer conformation Deep cone thickener Alum chemistry Charge patch model Deep thickener Efficient use of flocculants
Filter Press, 1900 Hazen, 1904 Dorr, 1905 Mishler, 1912 Gouy-Chapman, 1913–1917 Stern-Grahame, 1924–1947 Coe and Clevenger, 1916 Rotary Vacuum and Belt Filters, 1920–1930 Carman-Kozeny, 1937
Continuous thickener Settling of thick slurries EDL structure EDL structure Thickener sizing Continuous filtration Filtration equation
Silberberg, 1962 Abbott, 1970s OÕMelia, 1972 Gregory, 1973 Chandler, 1980s High rate and high capacity thickeners, 1980s Filter Belt Presses, 1980s
High throughput filtration
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M.J. Pearse / Minerals Engineering 16 (2003) 103–108
5. Conclusions
Fig. 9. Flocculation with structured polymer.
Fig. 10. Comparison of conventional and UMAâ flocculants.
semi-particulate entities and polymer chains reticulated in three dimensions which produce flocs of different characteristics than those formed by conventional flocculants. Denser and stronger flocs are produced containing less intrafloccular water. This modified bridging mechanism is shown in Fig. 9 and can be compared to the conventional model of Fig. 2. Another facet of UMAâ technology is manipulation or skewing of the molecular mass distribution to produce fractions that have a greater activity in efficient flocculation. Benefits to be seen from this approach are better overall dosage efficiency, better clarification at a given settling rate and better rheological properties of settled solids than shown with conventional flocculants, in terms of higher solids concentrations for a given yield stress. In terms of settling rate, better dose efficiency can be seen in Fig. 10, where typical UMAâ polymer performance is compared to low (10 MDa) and high (20 MDa) molecular mass conventional polyacrylamidebased flocculants. Molecular architecture is a relatively new technology and will play an increasing role in the future of flocculant development for the mineral processing industry. UMAâ is a registered trademark of Ciba Specialty Chemicals.
The original inorganic coagulants, alum and ferric salts, are now used to a small extent in the minerals industry, mainly for environmental control of site runoff water in the clarification of low solids effluents. Their developments, the pre-polymerised aluminium and iron products, similarly find small use in the same application. Lime is used extensively in mineral processing, but its coagulating effect is often secondary to the prime reason for use, particularly in flotation plants. Synthetic polyamine and polyDADMAC dominate present day coagulant use at mineral processing operations. Naturally derived flocculants like glue, gelatin, starch and guar have declined in use although starch and guar are used as flocculants in niche applications. More extensive use of starch and guar is seen as flotation depressants rather than as flocculants. The advent of high molecular mass polyacrylamidebased flocculants was revolutionary to solid–liquid separation in the mineral processing industry. Equipment developments that have centred around this are the high rate thickener, the deep or paste thickener and the filter belt press. The shape of the ubiquitous thickener has changed dramatically over the past 20 years on account of the separation efficiency afforded by polyacrylamide-based flocculants. Present day developments for polyacrylamide-based flocculants and those of the future are likely to be based on the Ômolecular architectureÕ concept. This will enable flocculant technology to produce the performance required to meet the increasing and changing demands for solid–liquid separation in the mineral processing industry.
References Agricola, G., 1950. De Re Metallica. Translated from Latin by H.C. Hoover. Dover Publications Inc., New York. Amirtharajah, A., OÕMelia, C., 1990. Water Quality and Treatment, fourth ed. American Water Works Association. Burger, R., Wendland, W.L., 2001. Sedimentation and suspension flows: Historical perspective and some recent developments. Journal of Engineering Mathematics 41, 101–116. Faust, S.D., Aly, O.M., 1998. Chemistry of Water Treatment, second ed. Ann Arbor Press Inc., Michigan, ISBN 1-57504-011-5. Kirk-Othmer, O., 1991. Encyclopedia of Chemical Technology, fourth ed. John Wiley and Sons, New York. Mackenzie, M., 1986. Chemical Reagents in the Mineral Processing Industry. Society of Mining Engineers, Inc., Jostens Publications, Kansas, ISBN 0-97335-062-6,139-145. Pearse, M.J., 1977. Gravity Thickening Theories: A Review. Warren Spring Laboratory, ISBN 0-85624-110-5. Pearse, M.J., Weir, S., Adkins, S.J., Moody, G.M., 2001. Advances in mineral flocculation. Minerals Engineering 14 (11), 1505–1511.
Historical use and future development of chemicals for solid–liquid separation in the mineral processing industry M.J. Pearse Ciba Speciality Chemicals, P.O. Box 38, Bradford, West Yorkshire, BD12 0JZ, United Kingdom Received 4 September 2002; accepted 22 October 2002
Abstract From their ancient beginnings to the present day, coagulants and flocculants, used to enhance solid–liquid separation by particle aggregation, are described; particular reference is given to developments in the mineral processing industry. The impact of polyacrylamide-based flocculants on technical effectiveness, disposal possibilities and environmental control, over the last 40 years, has had a profound effect on the efficiency and design of solid–liquid separation equipment. Recent developments, in terms of molecular architecture, offer new horizons for the future at a time when requirements for separating solid particles from liquid are becoming more demanding in the minerals industry. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Dewatering; Filtration; Flocculation; Tailings disposal; Thickening
1. Particle aggregation The process of particle aggregation to assist solid– liquid separation in the minerals industry is conventionally described by two mechanisms and these involve the use of different types of chemicals. Coagulation by coagulants, derived from the Latin word coagulare, meaning to be driven together, involves neutralisation of particle surface charge or potential by the action of inorganic counter-ion adsorption or electrostatic screening. Flocculation by flocculants, derived from L. flocculus, meaning a loose fibrous structure pertaining to a tuft of wool, involves the bridging together of particles by long organic polymer chains, where charge neutralisation may or may not be involved. A hybrid case involves the use of synthetic organic coagulants, sometimes referred to as Ôprimary coagulantsÕ. Slightly different definitions are employed in other industries, but broadly speaking, the terms above are adhered to for the purpose of this publication. 2. Historical development Far back in geological time, the first chemically assisted solid–liquid separation could conceivably have E-mail address: [email protected] (M.J. Pearse).
been the destabilisation and coagulation of fresh water colloidal clays borne by rivers from areas of erosion to the saline oceans. Present day seawater has an ionic strength of 0.7 M and its coagulation power plays a major role in the formation of the fine fractions of deltas. This process would have commenced with the evolution of river systems and oceans; the most ancient sedimentary rocks are 3.5 billion years old. The surface forces at work there laid the foundations of many of our present day techniques involving inorganic coagulants and to the whole science of the electrical double-layer surrounding particulate matter in aqueous media. 2.1. Inorganic coagulants Human intervention for intentional coagulation of water is reported as early as 2000 BC by Romans and Egyptians using alum (Faust and Aly, 1998). Pliny, 77AD, reported the use of a mixture of lime and alum for water clarification. Alum became an important item of trade in these early years mainly for leather treatment and as a dye mordant. Alum Bay on the Isle of Wight and Boulby, North Yorkshire were manufacturing sites for alum in the UK commencing in the 1500s and 1600s, respectively. By 1757, muddy water was being clarified in the UK using alum––2–3 grains of alum per quart (approximately 150 mg/l)––followed by filtration of the supernatant. These early reports most likely refer to
0892-6875/03/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0892-6875(02)00288-1
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M.J. Pearse / Minerals Engineering 16 (2003) 103–108
potash or ammonium alum, the former utilising the potassium from clays and the latter using urine in the manufacture. The manufacture of aluminium sulphate was eventually patented in the USA in 1884 and was found to be a superior coagulant to the double salts. The term ÔalumÕ as used as a coagulant in solid–liquid separation is invariably reserved for aluminium sulphate. In the same year, ferric salts were patented for water clarification. An excellent account of early metallurgical practice has been given by Burger and Wendland (2001). The early publication of Georgius Agricola (1556) illustrates washing and settling of ores being carried out in a single operation (Agricola, 1950). The use of lime in mineral processing is a panacea. It coagulates, raises pH, depresses iron sulphides, keeps cyanide in solution and causticises by removing carbonate ions. Early hard rock base metal tailings relied on lime as the coagulant during thickening and the early thickener sizing methods, like that of Coe and Clevenger, were developed using limed gold pulps (Pearse, 1977). Whereas the use of lime has dominated high solids systems, aluminium and ferric salts have dominated the inorganic coagulant market for low solids clarification, as in mine site run-off, due to their inherent hydrolysis yielding highly cationic oligomers. A simplified version of their chemistry is seen in the speciation diagrams of Fig. 1, based mainly on the work of Amirtharajah and OÕMelia (1990). 2.2. Pre-polymerised inorganic coagulants Intentionally pre-polymerised versions––polyaluminium chloride (PAC) and polyferric sulphate (PFS) are important developments from the simple salts and were patented in Japan in 1972 and 1976, respectively. The latter was patented by a Japanese mining company. Both have the advantage of being less pH dependent than their simple salts. Their development resulted from
the chemical study of the simple salts and the highly charged active species involved in effective coagulation. The two well known pre-polymerised aluminium coagulants can be represented by the general formula: Aln ðOHÞm Clð3nmÞ Polyaluminium chloride (PAC) n ¼ 2, m ¼ 3 Aluminium chlorohydrate (ACH) n ¼ 2, m ¼ 5 Coagulation characteristics are similar for both chemicals. As the pre-polymerised versions have undergone hydrolysis, there is no reduction in pH of the effluent stream as with simple salts, an important aspect for environmental discharge of mine site run-off clarification. 2.3. Naturally derived organic flocculants Whereas inorganic salts promote coagulation by charge neutralisation and double-layer compression, organic polymers promote solid–liquid separation by the bridging mechanism sometimes in combination with charge patch effects. The conventional view of bridging flocculation of particles by long-chain polymers is shown in Fig. 2. Polymers derived from plant-based materials were the first flocculants. Sanskrit writings in India dating from several centuries BC make reference to seeds of the Nirmali nut tree, Strychnos potatorum, as a clarification aid. Peruvian texts from the 16th and 17th centuries describe the use by sailors of powdered, roasted grains of Zea mays (corn starch) as a means of settling impurities. More recently, Chilean folklore texts from the 19th century refer to water clarification using the sap from the ÔtunaÕ cactus (Opuntia fiscus indica). Isinglass, produced from fish swim bladders, has been used as finings for centuries for clarifying beer. The principal reactive constituent of isinglass is collagen. Collagen is a unique molecule that exists as a triple helix with three chains of amino acids wound around each other and
Fig. 1. Simplified solubility equilibria for (a) alum and (b) ferric salts.
M.J. Pearse / Minerals Engineering 16 (2003) 103–108
105
Fig. 2. Conventional view of polymer bridging in flocculation. Fig. 4. Structure of guar gum.
held in place by complex hydrogen bonding. It is amphoteric but functions as a cationic polymer when removing yeast particles. Naturally derived polymers that were used extensively as flocculants in mineral processing included animal glue, gelatin, starch and guar gum. Glue and gelatin are derived from thermal denaturing of collagen-bearing materials like bones and hides; glue is said to contain less hydrolysis products than gelatin. As described for isinglass, collagen is a proteinaceous material composed mainly of amino acid units––glycine, proline and hydroxyproline, which impart an amphoteric nature to glue and gelatin by virtue of the presence of both amino and carboxyl groups. Both materials were used extensively in acidic hydrometallurgical operations like uranium extraction and were particularly effective at removing colloidal silica. The structure of gelatin is shown in Fig. 3. Guar is derived from the seeds of the leguminous shrub Cyamopsis tetragonoloba, grown mainly in India, Pakistan and South Africa. The process of extracting guar gum was commercialised in the USA in 1953. Guar is a galactomannan with approximately two mannose units for each galactose, as shown in Fig. 4. The guar chain is somewhat rigid in solution and nonionic in character (Mackenzie, 1986). The hydroxyl
groups of the mannose chain are in cis-position which may give a reinforced hydrogen bonding to the surface of particles or at least some unique properties. Starch is obtained from wheat, maize (corn), rice and potatoes; it consists of two components. Straight chain amylose makes up approximately 20–30% and branched amylopectin, the remainder. Both polymers are composed of glucose units with hydroxyl groups in transposition. Starch is naturally nonionic or slightly anionic and can be solubilised by causticising or heating. The amylopectin component contains small amounts of phosphate, especially in potato starch, giving an affinity to iron minerals. Representations of starch structure are shown in Fig. 5. Both starch and guar remain in niche applications as flocculants, the former in alkaline environments such as the Bayer process and the latter in acidic operations like uranium filtration; they were used more extensively in the past. Both starch and guar still find considerable application as flotation depressants. The major limitation to the performance of naturally derived flocculants is their ceiling in average molecular mass or effective chain length. For gelatin/glue the maximum is approximately 300,000 Da, for guar approximately 250,000 Da and amylose starch 65,000 Da, although amylopectin fractions are higher. Potato and tapioca starches have higher molecular masses than corn starch. 2.4. Synthetic polyacrylamide-based flocculants
Fig. 3. Structure of gelatin.
A major advancement was seen in the 1950s when polyacrylamide flocculants were introduced. Early reference is made to application in water treatment in 1958 and they were certainly being used in the mineral processing industry in the early 1960s. The polyacrylamide molecule could be tailored to virtually every mineral processing situation and the next four decades saw massive expansion in its use. By substitution to the polyacrylamide chain, the addition of cationic and anionic functional groups gave polyelectrolytes
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M.J. Pearse / Minerals Engineering 16 (2003) 103–108
Fig. 5. Structure of starch components.
which covered all slurry environments from monomineralic to multi-mineralic, low to high suspended solids, low to high dissolved solids and low to high pH. Manipulation of molecular mass from 5 to 25 million Da allowed successful application to be made on all solid–liquid separation equipment––clarifiers, thickeners, filters of all types and centrifuges. Along with the polyamine and polyDADMAC (diallyldimethyl ammonium chloride) primary coagulants discussed below, polyacrylamide-based flocculants account for over 90% of the mineral processing flocculant market. The entities of acrylic chemistry used in common types of flocculant are shown in Fig. 6. Acrylamide is polymerised by vinyl addition to give nonionic polyacrylamide. Sodium acrylate is copolymerised with acrylamide to give anionic flocculants, while dimethylaminoethyl acrylate is used with acrylamide to give cationic products. In relation to some of the natural polymers, acrylicbased flocculants have a relatively simple structure. It is interesting to note that according to Kirk (1991), acrylic acid, a component of anionic flocculants, was first pro-
Fig. 6. Monomer entities which link together to form acrylamidebased flocculants.
duced in 1847 by oxidation of acrolein. Present day manufacture is by catalytic oxidation of propylene. 2.5. Synthetic coagulants The main coagulants used in the minerals industry today are based on polyDADMAC (diallyldimethyl ammonium chloride) and quaternised polyamines. Structures are shown in Fig. 7. These products are often known as primary coagulants as they are usually used prior to the addition of a polyacrylamide-based flocculant. Both types are highly cationic and of low molecular mass (<1 MDa). The primary mechanism involved with these products is charge neutralisation, although bridging occurs in a secondary mode. Many of the inorganic coagulants used in the minerals industry previously have been replaced by synthetic coagulants. Both types of synthetic coagulant were developed in the 1970s.
3. Parallel development of equipment Solid–liquid separation equipment, separation science and flocculants have showed parallel development and some of the milestones in the interlinked technologies are summarised in Table 1. Notable synergy is seen between high molecular mass flocculants and high rate thickeners, especially with the development of flocculating feedwells and the addition of dilution water to feedwells (clarified overflow) allowing the most to be made of the powerful polymers. As flocculants have developed in their effectiveness, the footprint of thickeners have shrunk from the massive diameter conven-
M.J. Pearse / Minerals Engineering 16 (2003) 103–108
107
Fig. 8. The changing profile of clarifiers and thickeners.
Fig. 7. Structures of synthetic coagulants.
tional types using lime or low molecular weight polymers and utilising settling rates of around 1–5 m/h to the compact inclined plate and high rate types which use high polymers and have settling rates often in excess of 20 m/h. The other notable development is the increase in height of thickeners, making use of the compressible nature of highly flocculated slurries to produce a thick underflow, in many cases suitable for direct disposal. The deep cone, deep thickener and paste thickener make use of this technology. This is illustrated in Fig. 8. As tank sizes have become smaller, less buffering capacity is evident and a rapid dose response is seen with high molecular mass flocculants. This has been accompanied by the development of control equipment, which has progressed rapidly since the 1980s. Filter belt presses have gained increasing popularity in the mineral pro-
cessing industry over the past 20 years, and their operation is also dependent on high molecular mass flocculants.
4. Present and future developments The average molecular mass ceiling of polyacrylamide-based flocculants has been pushed to its virtual limit using presently known technology. In the mineral processing industry, and others, higher molecular mass of conventional, linear type polymers does not necessarily produce the best performance. This has led to the molecular architecture approach, for example, as in Cibaâ UMAâ (Unique Molecular Architecture) technology. The UMAâ approach is a departure from conventional thinking in polymer design and several facets are involved in the overall picture (Pearse et al., 2001). For example, highly branched and interactive polymer chains produce flocculant solutions containing a proportion of
Table 1 Some milestones in solid–liquid separation Milestone
Significance
Milestone
Significance
Stokes, 1851 Darcy, 1856 Helmholtz, 1879 Schulze-Hardy, 1882–1900
Particle settling Filtration EDL structure Empirical coagulation concentrations High solids filter cakes Clarification of dilute slurries
DLVO Theory, 1941–1948 Kynch, 1952 Polyacrylamide, 1950s Ruehrwein and Ward, 1952
Particle stability Mathematics of sedimentation Quantum leap in flocculation Bridging model of flocculation
Talmage and Fitch, 1955 LaMer, 1958
Thickener sizing Quantitative bridging model of flocculation Polymer conformation Deep cone thickener Alum chemistry Charge patch model Deep thickener Efficient use of flocculants
Filter Press, 1900 Hazen, 1904 Dorr, 1905 Mishler, 1912 Gouy-Chapman, 1913–1917 Stern-Grahame, 1924–1947 Coe and Clevenger, 1916 Rotary Vacuum and Belt Filters, 1920–1930 Carman-Kozeny, 1937
Continuous thickener Settling of thick slurries EDL structure EDL structure Thickener sizing Continuous filtration Filtration equation
Silberberg, 1962 Abbott, 1970s OÕMelia, 1972 Gregory, 1973 Chandler, 1980s High rate and high capacity thickeners, 1980s Filter Belt Presses, 1980s
High throughput filtration
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5. Conclusions
Fig. 9. Flocculation with structured polymer.
Fig. 10. Comparison of conventional and UMAâ flocculants.
semi-particulate entities and polymer chains reticulated in three dimensions which produce flocs of different characteristics than those formed by conventional flocculants. Denser and stronger flocs are produced containing less intrafloccular water. This modified bridging mechanism is shown in Fig. 9 and can be compared to the conventional model of Fig. 2. Another facet of UMAâ technology is manipulation or skewing of the molecular mass distribution to produce fractions that have a greater activity in efficient flocculation. Benefits to be seen from this approach are better overall dosage efficiency, better clarification at a given settling rate and better rheological properties of settled solids than shown with conventional flocculants, in terms of higher solids concentrations for a given yield stress. In terms of settling rate, better dose efficiency can be seen in Fig. 10, where typical UMAâ polymer performance is compared to low (10 MDa) and high (20 MDa) molecular mass conventional polyacrylamidebased flocculants. Molecular architecture is a relatively new technology and will play an increasing role in the future of flocculant development for the mineral processing industry. UMAâ is a registered trademark of Ciba Specialty Chemicals.
The original inorganic coagulants, alum and ferric salts, are now used to a small extent in the minerals industry, mainly for environmental control of site runoff water in the clarification of low solids effluents. Their developments, the pre-polymerised aluminium and iron products, similarly find small use in the same application. Lime is used extensively in mineral processing, but its coagulating effect is often secondary to the prime reason for use, particularly in flotation plants. Synthetic polyamine and polyDADMAC dominate present day coagulant use at mineral processing operations. Naturally derived flocculants like glue, gelatin, starch and guar have declined in use although starch and guar are used as flocculants in niche applications. More extensive use of starch and guar is seen as flotation depressants rather than as flocculants. The advent of high molecular mass polyacrylamidebased flocculants was revolutionary to solid–liquid separation in the mineral processing industry. Equipment developments that have centred around this are the high rate thickener, the deep or paste thickener and the filter belt press. The shape of the ubiquitous thickener has changed dramatically over the past 20 years on account of the separation efficiency afforded by polyacrylamide-based flocculants. Present day developments for polyacrylamide-based flocculants and those of the future are likely to be based on the Ômolecular architectureÕ concept. This will enable flocculant technology to produce the performance required to meet the increasing and changing demands for solid–liquid separation in the mineral processing industry.
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