Flotation as currently practised is a process in which one or more specific particulate constituents of a suspension of finely dispersed particles in a liquid become attached to gas bubbles so that they can be separated from other constituents of the slurry. The buoyancy of the bubble-particle aggregate is such that it rises to the surface of the flotation vessel where it is separated from the remaining constituents which remain suspended in the liquid phase.
The success of the flotation process depends crucially on the control of the chemical conditions in the suspension so that the particles which it is desired to float are rendered hydrophobic or non-wetting so that they will attach readily to air bubbles with which they collide in the flotation vessel, while the particles which are not to be floated remain hydrophilic or wetted by the suspension liquid. Chemicals which increase the hydrophobicity of the particles are known as "collectors", and other reagents may also be added, such as "promoters" which tend to improve the performance of the collectors, "depressants" which tend to reduce the hydrophobicity of the "gangue" or material which is not to be floated, and "frothers" which improve the quality of the froth layer formed on the surface of the liquid layer.
In flotation cells in common use, the bubbles are removed from the vessel in the form of a layer of froth or foam on the surface of the suspension liquid. The froth layer overflows into a launder, while the liquid layer is taken from the vessel through an exit pipe in the bottom. The liquid level in the tank is maintained by a suitable control valve acting on the liquid exit stream. The material leaving in the froth, which consists of the hydrophobic particles attached to the bubbles as well as some particulates suspended in the liquid between the bubbles, is known as the "concentrate", while the remaining particulates which leave the vessel in the liquid slurry through the exit port are known as the "tailings".
Two types of flotation equipment are in common use. The older form is the "mechanical cell", in which the slurry is held in a tank which is stirred by a rotating impeller. Air is introduced into the slurry and is broken up into bubbles by the action of the impeller. More recently another form has come into use, in which the feed slurry is introduced to the top of a tower or column, typically 8 to 15 meters in height, and air bubbles are formed in the base of the column, either through a porous medium such as a fabric or rubber sheeting pierced with a large concentration of fine holes; a sintered metal or ceramic material; or a gas liquid mixing device such as a venturi. The bubbles rise up the tall column, colliding with the hydrophobic particles which are descending toward the tails exit pipe in the base of the tower.
In both the mechanical cell and the flotation column, the bubbles form a froth layer on the surface of the underlying slurry, and the material to be floated is removed in the froth. It is a feature of the flotation column, that clean water is introduced to the froth, either by a shower or spray of droplets on the upper surface, or through a piped distribution system immersed in the froth. The purpose of the washing system is to provide a countercurrent flow of clean water which has the effect of flushing the unwanted gangue particles which are normally entrained in the water carried upward in the froth, back into the slurry from which they came. In this way, it is possible to increase the purity of the froth concentrate, relative to the purity which is achievable without froth washing.
In the design of flotation equipment it is advantageous to take steps to effect a rapid contact between the bubbles and the flotable particles, in order to minimize the residence time of the slurry and hence the size of the equipment. In mechanical cells, which are customarily arranged in banks of 2 to 20 machines in series, the overall residence time is typically in the range 5 to 60 minutes, while in column, the residence time is typically 10 to 60 minutes. One way to improve the rate of flotation which has been tried, would be to increase the volumetric flowrate of air passing through the cell or column. This method has been found difficult to put into practise however, for several reasons. In mechanical cells for example, it has been found difficult to design impellers which can disperse large flowrates of gas into fine bubbles. As the air flowrate increase, the "hold-up" or fraction of cell volume occupied by bubbles also increases, and the impeller finds itself rotating not in a liquid but in a gas-liquid mixture. Accordingly, the impeller becomes flooded and cannot impart shear or momentum to the surrounding fluid, and so the sizes of the bubbles formed by the impeller become larger and larger, with increasing flowrate. A similar effect happens with the porous spargers commonly used in flotation columns, although in principle this problem could be overcome by the use of a greater area of sparger surface.
The diameters of bubbles in mechanical and column flotation cells are customarily in the range 0.5 to 3 mm, although when an impeller or sparger is overloaded with air, the diameter can be up to 5 to 10 cm.
Because of practical limitations to do with the distribution of air by rotating impellers or in spargers, the ratio of the air volumetric flowrate to the volumetric flowrate of feed in individual flotation cells or columns is generally in the range of 1 to 4 volumes of air per volume of feed. Increased recoveries could be expected from such cells if the air-feed ratio could be substantially increased without at the same time causing corresponding increases in the amount of entrained gangue.
Another limitation of conventional flotation machines concerns the behaviour of the froth phase, especially in relation to the superficial air velocity J.sub.G, which is defined as the volumetric air flowrate (e.g. cubic centimeters per second) divided by the area of cross-section of the cell at right angles to the mean direction of flow of the rising air (e.g. square centimeters).
At low values of the superficial air velocity, a well-defined interface between the froth and the underlying pulp is observed. In such cases liquid is entrained into the froth in the wakes of the bubbles passing through the interface, but the rate of drainage under gravity of the liquid in the thin films between the bubbles just above the interface, is equal to the rate of entrainment of liquid in the bubble's wakes just below the interface, so a stable equilibrium is established. In effect, the void fraction in the froth adjusts itself so that the liquid films between the bubbles are sufficiently large to give the required flow area for the liquid to drain properly. In such cases the position of the interface, i.e. the liquid level, is clear and well-defined and can be sensed by suitable instrumentation, enabling the level to be controlled automatically.
As the superficial air velocity is increased however, the interface between froth and slurry broadens, and it becomes increasingly difficult to distinguish between the phases. The flowrate of entrained material increases as the air velocity increases, but the force under which the froth is draining, i.e. that of gravity, remains the same. For an equilibrium to be established the thickness of the films between the bubbles in the froth has to increase, and eventually, a point is reached when no matter how the void fraction in the froth adjusts itself, it is not possible for the downward draining rate of the liquid to equal the rate of entrainment into the froth phase. When this happens in column flotation, the column is said to be "flooded", and the phenomenon is accompanied by the disappearance of the clear interface between the slurry and the froth and a marked deterioration in performance, occasioned by the flow of massive amounts of entrained material into the froth phase with corresponding reduction in the grade or purity of the concentrate.
Mechanical cells and flotation columns both customarily operate with superficial velocities J.sub.G in the range 0.5 to 3.0 cm/s, and flooding is often observed when J.sub.G exceeds 4 to 5 cm/s. For a given cell of specified area, the possibility of flooding places a practical limit on the allowable air flowrate.
The superficial gas flowrate is important because it is a measure of the amount of gas-liquid interfacial area provided, per unit time. For flotation to occur, it is necessary to capture the hydrophobic particles on a gas-liquid interface, and the greater the interfacial area which is presented to a given volume of liquid, the greater will be the capacity of the interface and hence the flotation cell to remove the flotable material.
It is one of the purposes of the present invention to be able to operate at superficial gas velocities well in excess of 5 cm/s, and therefore to be able to provide much greater interfacial areas for capture and retention of particles than are provided in existing flotation apparatus, relative to the volume of liquid in the equipment at any one time.
Another feature of the invention relates to the ratio of the volume of gas to the volume of liquid with which it is in contact at any given time.
In conventional mechanical and column flotation cells, the contact between bubbles and hydrophobic particles takes place in the slurry phase. Because of the hydrodynamic phenomena associated with two-phase gas-liquid mixtures, the voidage of gas in each form of flotation device is limited to 10 to 20 percent of the total slurry volume. Accordingly, the distances between bubbles is large and the probability of capture of the particles, which are customarily very small in comparison to the bubble size, is correspondingly low. It would be very advantageous to be able to capture the particles in a medium with a much higher void fraction, such as a froth, where the liquid films between the bubbles are very thin, and a particle does not have to travel very far before finding a gas-liquid interface to which it can adhere.
It is therefore a feature of this invention to provide an environment for the capture of flotable particles substantially in a froth phase.
In some flotation applications, for example the removal of oil droplets from waste water, the purity of the product is not important. In such cases, flotation is used as a water cleaning process, and the quantities of material which are floated are only a small proportion of the total feed to the flotation equipment. In mineral processing however, a different problem arises. Often, the valuable mineral is present in the ore as mined in very small amounts, sometimes less than 1 percent by weight. With exsiting technology it is impossible in such cases to separate the values into a high grade product in a single stage, because of the difficulty of reducing the amount of entrained gangue down to the required levels.
Accordingly, the overall flotation operation is carried out in a number of separate steps, known respectively as "roughing", "cleaning" and "scavenging". The finely-crushed ore suspended in water is first subjected to rougher flotation, in which the aim is to remove the values into a low-grade rougher concentrate which is then subjected to one or more further stages of purification by flotation or cleaning, at each stage producing a concentrate of higher and higher grade. Sometimes only one cleaning stage is required, but two or three cleaning stages are common place, and in a few exceptional cases many more than three stages are required. Scavenging is the term used for a further flotation operation applied to the tailings from the rougher stage. The concentration of values in these tailings is usually very low, but to maximise the overall recovery or yield of valuable material, the tails are subjected to one more flotation operation in the scavengers in order to capture any final traces. The scavenger concentrate is usually returned to the feed to the roughers, although it may also pass direct to the cleaners.
It would obviously be advantageous to be able to carry out all of these flotation operations in the one piece of equipment, but with existing technology this has not been possible because of the long residence times required to capture the values and the slow drainage of entrained gangue from the froth.
It is therefore an aim of one embodiment of the present invention to be able to carry out the operations of roughing, cleaning and scavenging in the one flotation apparatus.
The froth from conventional flotation machines whether of the mechanical or the column variety, discharges in the form of a voluminous aerated stream, often containing less than 10 percent by volume of liquid, the remainder being the flotation gas. It is necessary before further processing, to collapse the froth into a slurry of particles in water, and this froth-breaking step may require a considerable residence time. It would be advantageous to be able to produce a concentrate directly in the form of a liquid rather than as a froth, and it is an aim of one embodiment of the invention to break the froth within the machine and produce a concentrate in liquid form.
In many cases, it is desired to make a product from which the water has been substantially removed. It is the aim of one realisation of the present invention, to be able to make a product in a semi-dry form.