CHAPTER 1 GENERAL CONCEPTS OF FLOTATION

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1 CHAPTER 1 GENERAL CONCEPTS OF FLOTATION 1. Importance of Flotation to Industry Flotation is now of immense industrial importance. Without the development of flotation there would be no mining industry as we know it today. Virtually all of the world s supply of lead, zinc and silver is produced by flotation. The majority of copper is produced by flotation. In addition other metals that are made using flotation are manganese, chromium, columbium, nickel, cobalt, vanadium, germanium, antimony, bismuth and tungsten. But its use also extends to non-metallic minerals. Phosphate, fluorite, feldspar, mica, barite and other industrial minerals are flotation products. Coal is now froth-floated on a large scale. Then outside the minerals industry flotation has application in the paper, food, water and waste industries. Flotation of inks, fats, oils, grease and fibres is widespread. The introduction of flotation has meant that more difficult ores can be processed. Previous methods such as gravity separation were much less efficient. The feed grade of ores that can be treated has decreased significantly since flotation was developed. In 1935 the average copper content of ores treated in USA was 1.57%. By 1960 it had dropped to 0.72% and the cost fell also appreciably. This trend has continued. It is now possible to treat ores that previously could not be processed. Flotation has been critical in these industrial developments. The world s economic ore reserves have been greatly increased by flotation. 2. Brief History of the Flotation Process Development Some of the early benchmarks in the historical development of flotation were tabulate by Gaudin (1957) as illustrated in Table 1.1. Flotation was introduced into industrial practice about 100 years ago. Early patents (William Haynes, England, 1860) were granted based on the use of oils which preferentially trapped particles of mineral or gangue of different types.

2 2 Table 1.1. Important contributions to early flotation technology (Gaudin, 1957) Year Contributor Contribution 1860 Haynes Bulk-oil process 1877 Bessel Boiling process for graphite 1885 Bessel Chemical-generation gas process for graphite 1886 Everson Acidulated pulps desirable 1902 Froment, Potter and Delprat Gas as a buoyant medium for sulphide ores 1905 Schwarz Sodium sulphide to recover oxidized base metal minerals 1913 Bradford Sulphur dioxide to depress sphalerite 1921 Perkins and Sayre Specific organic collectors Alkaline circuits 1922 Sheridan, Griswold Cyanides to depress sphalerite and pyrite 1924 Sulman and Edser Soaps for flotation of oxides 1925 Keller Xanthates as collectors 1929 Gaudin ph control 1929 Jeanprost Flotation of highly soluble salines 1933 Nessler Flotation separation of water-soluble chemical salt mixtures 1934 Chapman and Littleford Agglomeration Alkyl sulphates as collectors Cationic collectors They were then floated to the surface with the oil. This type of process is still proposed for the oil agglomeration and recovery of fine coal. Later variations added acid to increase the differentiation of valuable mineral and gangue. The Bessel brothers (Dresden, 1877) patented a true flotation process for graphite. The ore was pulverised then oils added and the mixture boiled for gas bubble generation. Graphite floated and the gangue remained in the water. These processes used large amounts of oil. This

3 3 is probably the precursor to froth flotation as we know it today. They later patented the use of carbonate minerals in acid medium to generate gas bubbles. Potter (Brocken Hill, Australia, 1901) developed the first process that operated on a large scale. It was used to produce over 6 million tonnes of zinc assaying up to 42% Zn. Waste dumps at about 20% Zn were fed dry into hot acid solution. The acid attacked the ore liberating gas bubbles that attached themselves to the zinc blend and carried them to the surface and they were collected. A great deal of industrial research followed aimed at improving the basic process. The vacuum process was used to generate gas by lowering the pressure (this is still used today for special applications) to separate sulphides from gangue using added oils. Then gas air was added by violent agitation and froth flotation was established. Some modern cells induce air through the agitator others have an air blower to provide air directly. Sub aeration was introduced by Owen in Selective flotation was an important advance. There was much galena in the zinc dumps and this floated with the zinc causing smelting problems. Lyster (1912) introduced the first differential flotation process by using the fact that in alkaline solution galena floated much better than zinc blende. Lowry & Greenway (1912) depressed the lead using dichromate. Bradford (1913) depressed the zinc using sulphur dioxide and also made the important discovery that a small amount of copper sulphate increased the floatability of zinc. This was the first use of activation. A major advance was the development of modern collectors. Soluble organic compounds with tri-valent nitrogen or bi-valent sulphur were substituted for oils. A very important discovery was the use of xanthate (Keller and Lewis, 1925). They are still the basic sulphide collector. For non-sulphides soaps, alcohols and amine derivatives were used but good results needed depressants. The move to selective flotation was important. It was easy to float all the sulphide minerals in an ore it is often difficult to prevent one from floating while a second floats. Selective depressants were needed. Alkalis were used first, to depress pyrite while sphalerite floats. A very valuable depressant used was sodium cyanide (Sheridan and Griswold 1922). In alkaline solution it depresses both pyrite and sphalerite but not galena. This enabled purer lead

4 4 concentrates to be produced. Then activation by copper gives a zinc concentrate and finally a pyrite concentrate can be obtained if required after neutralisation. Bartsch (1924) was the first to systematically study the action of frothers. They were found to be important to maintain a stable froth and large surface area for particle capture. Cell geometry was established quite early. Many patents were lodged claiming improved operation, but the basic stirred cell with induced air or supplied air by a blower. Incremental improvements were made to improve agitation, bubble generation, froth removal, etc. The major change was the increase in scale from a few cubic metres to a few hundred cubic metres. Control systems were developed also. A major advance was the introduction of on-line instrumentation. These were developed in the 70s to enable assay data to be measured on a continuous basis. This was a huge advance from an operational viewpoint. Computer control, mainly stabilizing control was introduced about the same time and has been steadily improved. Most recent changes have been the use of columns as alternative devices. 3. Place of Flotation in Mineral Processing Circuits The primary mineral processing circuit involves size reduction followed by some separation step. Flotation is one such separation steps along with gravity, magnetic, electrostatic, etc. The whole circuit is used to carryout the mineral concentration; hence flotation must be considered part of the overall circuit and not in isolation. The feed preparation must be suitable for flotation separation. Then the flotation itself must be optimised for this feed stream to get the best overall performance. We are always dealing with mixtures of minerals intimately bound together in the ore. The crushing and grinding stages are in part designed to break the mineral grains apart so that flotation becomes possible. If this feed preparation is not satisfactory then no amount of optimisation of the flotation step can solve the problem.

5 Liberation and Concentration The key features of the preparation of feed to a flotation plant are: Liberation of the minerals Adjusting particle size so that it is appropriate for flotation. Often this involves a balance. We shall see later that even with pure particles of a single mineral they can be too large to stick to bubbles and float. Also, by grinding very fine to achieve good liberation, the particles may be too small to be captured by the bubbles and float efficiently. Liberation of the minerals in an ore is achieved by a reduction in particle size. Breakage of an ore is largely random which means that it is necessary to grind to a particle size well below the size of the mineral grains in the ore. As the particle size is reduced the liberation increases. Liberation must be referred to a particular mineral since the grain size of minerals in an ore generally varies, e.g. in a Pb/Zn ore the grain size of galena is generally smaller so that the lead mineral is the first to liberate. It may be possible to liberate one mineral, remove it by flotation, then grind again to liberate another mineral. In this way the problem of overgrinding the first mineral is avoided. Particles that are not fully liberated are termed locked, i.e. each particle contains more than one mineral. Since flotation is basically a physical separation (though controlled by chemistry), the minerals within locked particles cannot be separated. These particles must either go to the concentrate or the tailing stream. They dilute the purity of the concentrate and they may contribute to the losses in the tails. The overall operating strategy, governed by economics will determine the best definition for these locked particles. Developments in grinding technology (Stirred Mills, Vertimills, Isamills) now make ultra-fine grinding economic. Further, the predominance of large, complex low-grade deposits make ultra-fine grinding increasingly necessary. Therefore, ultra-fine flotation needs to be developed and considerable research is ongoing in ultra-fine flotation.

6 Mineral Texture The mineral texture is the geometrical arrangement of the different mineral phases in the ore. It includes such features as mineral proportions, mineral grain sizes and mineral associations. The texture of an ore will determine how the minerals will liberate during comminution and the structure of the particles that are formed. The structure of the particles determines how flotation or other separation processes will perform (Figure 1.1). 4. Complexity of the Process Flotation is a very complex process. Chemical engineering problems often contain only one or two phases, but here there are solid, liquid and gas with generally a large number of solid phases. Particle shapes and structures are complex; the fluid flow regime in the agitated cells is poorly defined; then added to this are the chemical aspects of the interfacial action of flotation reagents. The resulting overall process is difficult to define and though a general understanding is now possible complete descriptions and predictions are not yet possible. It is not surprising that the early developments of the technology were largely empirical and the tremendous amount of research that has been undertaken in the last 100 years has been trying to explain what happens. It is rather remarkable that so complex a process can operate so effectively on a wide range of materials and at so large a scale. Some of the complexities of the system are as follows: Relative movement of the phases affected by the geometry and the phase properties Dynamics of this movement collision of particle and bubble and drainage of froths Hydrolysis reactions and ph important as generally float from water slurry Flotation gas is generally air so that oxidation/reduction reactions are important Chemical composition of aqueous medium function of solubility of mineral components under ph oxidation conditions Recycle of mill water can add further solution species Surface condition of the minerals and surface charge affects hydrophobicity Chemicals are added in an attempt to control the surface properties Etc, etc, etc!

7 7 In consequence it is not possible to define all these processes. Much of the research work has of necessity simplified the problem, e.g. floated single minerals so that the interactions of the other minerals and ions are removed or looked at very simple flotation equipment with isolated bubbles and only a few particles so that physical effects of the flotation cell are excluded. 5. Scale of Operations One of the major challenges has been the development of large scale processing equipment. Many early circuits were expanded by adding parallel banks to duplicate the equipment. Plants might have 6 or 8 lines. This was partly due to the capacity of the grinding mills, but was also thought to give flexibility to the plant and permit maintenance. More recently the economic trends have moved towards larger and larger tonnages and larger and larger equipment. Expense of maintenance of multiple units and control of parallel lines support the moves to fewer lines and simpler equipment. AG and SAG mills have grown to almost 40ft diameter and flotation cells up to 200m 3. Dorr-Oliver cells 200 m 3 Outotec cells 500 m 3 Eimco cells 200 m 3 In recently, Outotec launched the largest Tank cells in the world TankCell 300 and TankCell 500 with over 300 m 3 of active volume and 500 m 3 of active volume in 2007 and 2011, respectively. Massive tonnages are possible. Escondida Cu plant in Chile About 1 million tonne/year of Cu in concentrates (0.5 1 millions tonnes per day of earth moved) Chuquicumata Cu plant in Chile 65 million tonnes of ore per year El Teniente Cu plant in Chile 35 million tonnes of ore per year Ok Tedi Cu plant in PNG 29 million tonnes of ore per year

8 8 Basic flotation cells are tanks with: A stirrer to keep the slurry in suspension Some means of aerating the pulp induced air or air blown in through a sparge system A feed and exit arrangement for the pulp A launder to collect the froth and transport it away Various control systems Cells are connected together into banks or stages. This promotes plug flow and minimises short circuiting of the pulp. Banks are connected in circuits roughing, cleaning, scavenging, etc There have been many, many attempts to design newer and better flotation equipment. The most important development in recent years has been the flotation column. This approach follows chemical engineering practice of counter-current processing. It has gained considerable use, particularly in cleaning circuits. Other recent variants are the Jameson Cell, Microcell, TurboFlotation. Applications: Sulphide minerals copper, lead, zinc, silver Non sulphide minerals Feldspar, mica, quartz, phosphates, etc. References Gaudin, A.M., 1957, Flotation, McGraw-Hill Book Co., New York.

9 9 Figure 1.1. Effect of particle structure on treatment processes