INT1202/MAJR/jes BIOLEACHING PROCESS CONTROL BACKGROUND OF THE INVENTION [0001] This invention relates generally to a bioleaching process and more particularly is concerned with controlling this type of process, when implemented in a stirred tank 5 reactor, to reduce energy consumption. [0002] Bioleaching in stirred tank reactors is used to oxidise refractory sulphidic gold concentrates and copper sulphide concentrates and Is also applicable to nickel and zinc sulphides. In these operations the electrical power consumed in the compression of air or oxygen, delivered to a reactor, and the energy requirement for dispersing the gas in 10 the reactor, represent a substantial operating cost. [0003] In a stirred tank bioleaching operation slurry, consisting of water, nutrients and a sulphide concentrate, Is fed to a reactor In which appropriate microorganisms oxidise ferrous iron and sulphide. This oxidation process requires oxygen. [0004] The oxygen is delivered to the reactor in gaseous form as air, oxygen enriched 15 air or oxygen. This gas Is sparged Into the reactor below a high solidity downward pumping agitator or a radial type impeller. This device shears the slurry and the incoming gas stream is broken into small bubbles. This greatly Increases the surface area of the gas stream and the rate at which oxygen is transferred into the slurry is increased. 20 [0005] The rate of transfer of oxygen is proportional to: INT1202/MAJR/jes 2 a) the amount of gas added, unless the gas rate exceeds the impeller's ability to disperse it; b) the partial pressure of oxygen within the gas; and c) the amount of power transferred to the slurry by the agitator. 5 [0006] The rate of transfer of oxygen is however inversely proportional to the amount of dissolved oxygen within the slurry. [0007] The rate of oxygen consumed by reaction (oxygen demand) is dependent on the rate at which sulphide Is fed to the reactor and this, in turn, can be affected by the throughput or by the grade of the concentrate. 10 [0008] A typical bioleaching process usually functions within a number of boundary conditions such as minimum and maximum dissolved oxygen concentrations, an agitator's limitation on gas dispersion, and so on. Within these boundary conditions there are optimum conditions at which to operate the process depending on the plant's location, its size and its oxygen demand. It is normal practice to design a plant for what 15 are estimated to be feed conditions over a set portion of the plant's lifetime. (0009] In practice a plant is often operated far from Its designed concentrate grade or feed rate. This is due to a variety of factors such as changes in mineralogy as mining progresses, deviation between design and actual sulphide flotation and treatment of other ores, a variation in the concentrate tonnage because of operational reasons, 20 mining rates and the like, INT1202/MAJR/jes 3 [0010] If oxygen demand in a reactor drops then equipment such as a compressor or blower, used to deliver the gas, is operated at a lower level. Figure 1 illustrates energy consumption per kilogram of sulphide treated as a function of agitator input power for a thermophile plant that includes primary and secondary reactors, A lower curve gives 5 the aforementioned relationship for full sulphide tonnage delivered to the plant while an upper curve gives the relationship when the sulphide feed tonnage Is reduced by 50% and the gas delivery power is reduced to compensate. It is evident that despite reducing the rate at which gas is supplied to the plant, to compensate for a reduced sulphIde feed rate, the energy consumed per unit sulphide treated increases by from 10 19% to 39% depending on the amount of agitator power supplied. [0011] Figure 2 Includes similar graphs for an air-supplied mesophile plant of the same duty. A lower curve is for full sulphide tonnage, and an upper curve is for half sulphide tonnage. The energy cost per unit sulphide treated, arising from a reduction in sulphide tonnage, Increases by from 26% to 60%. 15 [0012] The invention is concerned with a control technique which aims to reduce the energy consumption per unit sulphide treated under the aforementioned conditions. SUMMARY OF THE INVENTION [0013] The invention provides a method of conducting a bioleaching process which includes the steps of feeding a sulphide mineral slurry to a reactor, sparging the slurry in 20 the reactor with a gas, agitating the slurry in the reactor with a motor-driven agitator, causing blooxidation of the sulphide mineral to take place, and controlling the rate of supply of the sparging gas to the reactor and of the energy supplied to the motor In -4 response to at least one of the following: measured oxygen demand; and inferred oxygen demand. The invention also provides a method of improving the energy efficiency of a bioleaching process in which a sulphide mineral can vary in terms of throughput rate or 5 grade comprising: (a) feeding the sulphide mineral in a slurry to a reactor; (b) sparging the slurry in the reactor with a gas from a gas source; (c) agitating the slurry in the reactor with a motor-driven agitator; (d) causing biooxidation of the sulphide mineral to take place, which gives rise to io an oxygen demand; (e) determining the oxygen demand either by calculating the oxygen demand from data dependent on a rate of supply of the gas to the reactor and a composition of an off-gas from the reactor or by estimating the oxygen demand from data dependent on power drawn by the agitator, a rate supply of the gas to the reactor and dissolved 15 oxygen level in the slurry; and (f) in response to the determined oxygen demand, controlling the energy input to the gas source to control the rate of supply of the sparging gas to the reactor and controlling the energy supplied to the motor, wherein an aggregate of the energy inputted to the process is optimised. 20 BRIEF DESCRIPTION OF THE DRAWINGS [0014] The invention is further described by way of example with reference to the accompanying drawings in which: Figures 1 and 2 relate to existing situations and have already been described; 25 Figure 3 reflects energy consumption per unit sulphide for the compressors and agitators of a thermophile plant at normal sulphide grade, half sulphide grade, and half sulphide grade with reduced agitation power, respectively; Figure 4 reflects energy consumption per unit sulphide for the compressors and agitators of a thermophile plant at full tonnage, two-thirds tonnage, and two-thirds 30 tonnage with reduced agitator power, respectively and; Figure 5 is a block diagram representation of the manner in which the method of the invention is implemented. 24157441 (GHMatters) 21/09/10 - 4a DESCRIPTION OF PREFERRED EMBODIMENT [0015] Figure 5 of the accompanying drawings is a block diagram representation of a plant 10 in which a stirred tank bioleaching sulphide oxidation process is carried out in s accordance with the principles of the invention. [0016] The plant includes a series of stirred tank reactors although only one tank, designated 12, is shown in Figure 5. The tank includes an agitator or impeller 14 (these terms are used interchangeably in this specification) which is driven by means 10 of an 24157441 (GHMatters) 21109110 INT1202/MAJR/les 5 electrical motor 16, using techniques which are known in the art. Electrical energy is supplied from a source 18 to the motor. [0017] A gas source 20 Is used to introduce sparging gas Into diffusers or similar emission devices 22 at a lower end of the tank 12, The gas which Is introduced may be 5 air, oxygen enriched air or substantially pure oxygen. The source 20 may include one or more compressors, air pumps or the like. Conventional devices may be used in this respect. The rate at which gas Is supplied by the source to the tank is monitored by a sensor 24 which relays this information to a control computer 26. The controller receives data Input from other sources and Is capable of exerting a control function so 10 as to vary the rate at which the source supplies gas to the reactor, [0018] A slurry 28 is introduced into the tank in a controlled manner. The slurry comprises water, nutrients and a mineral sulphide concentrate. The use of the principles of the Invention Is not restricted to a particular mineral type and typically these principles may be used in processes for the recovery of gold, copper, nickel and zinc, 15 [0019] The tank in question is not necessarily a primary reactor, but may be a secondary reactor receiving the product from a primary reactor for further oxidation. [0020] The tank 12 contains a self-sustaining population of microorganisms which act as catalysts for the oxidation of ferrous Iron and sulphide. These reactions require oxygen and, as Indicated, this is applied from the gas source 20, 20 [0021] The level of dissolved oxygen within the slurry, in the tank 12, Is monitored by means of a suitable sensor 30. The off-gas composition Is measured by means of a sensor 32. The oxygen demand is either calculated from data derived from the outputs INT1202/MAJR/Jes 6 of the gas supply 20 as detected by the sensor 24, and the sensor 32, or is estimated by using a suitable algorithm and the outputs of the agitator power 18, the gas supply 20 and the sensor 30, or the oxygen uptake rate is manually measured by an operator. [0022] The estimated or measured oxygen uptake rate, as the case may be, is used in 5 a suitable algorithm to establish the most efficient level at which energy should be supplied to the motor 16, taking into account the overall power used to supply gas to all the tanks in the plant and any limitation of gas supply, The algorithm can be implemented automatically via the control computer 26 or manually by means of signals input by a trained operator. The total power consumption for all tank agitators and gas 10 supply compressors is measured and recorded to track usage. [0023] An objective of the invention is to increase the efficiency of the oxidation process by controlling power which Is Input to the agitator and the quantity of gas delivered to the slurry. [0024] Figure 3 shows three points of operation, namely a normal operating point 40, a 15 point 42 which depicts the effect if the sulphide grade drops by half and the agitator power Is not lowered, and a point 44 which depicts the effect if the sulphide grade drops by half and the agitator power is reduced by an appropriate amount. [0026] Figure 3 shows that at half sulphide grade operation the energy cost, which is the cost of operating the motor 16 and the gas source 20, is about 35% more if the 20 energy supply to the agitator Is not reduced. However if this energy level is reduced the energy consumption per unit sulphide treated Is about 21% more.
INT12O2IMAJRJes 7 [0026] If the sulphide grade is reduced then the efficiency of the oxidation process is decreased in the manner which has been described in connection with Figures 1 to 3. It Is to be borne In mind that In a biological plant a decrease in grade does not necessarily allow for a decrease in process retention if the same percentage valuable mineral 5 recovery is to be achieved. [0027] Often It is a decrease In concentrate tonnage which causes a reduction throughput In a plant. Figure 4 shows the plant efficiency for normal operation (point 48) and operation at a level of two-thirds design capacity with (point 50) and without (point 52) a reduction in the agitator energy consumption. The reduction in concentrate 10 feed rate results in a 24% Increase in energy per unit sulphide treated if only the power of the gas source 20 Is reduced. However if optimising control is exerted via the processor 30 on the motor 16 and on the source 20 to control agitation energy and the aeration energy then the increase in energy consumption per unit sulphlde treated is of the order of 14%. 15 [0028] The invention seeks to optimlse the operation of the bioleaching section of a plant. Aspects relating to energy control can be Implemented by making use of variable speed drives which, typically are electronically based. It is also possible to use measured or inferred oxygen utilisation rates together with mathematical correlation and models to optimise the whole process or Individual reactor requirements, In the latter 20 respect it should be borne in mind that the Invention has been described with reference to a single reactor or tank but, in practice, a series of tanks are used. [0029] The invention thus allows for an Improvement In overall efficiency with a corresponding reduction In energy consumption.
8 [0030] A reduction in energy consumption can be achieved by an interplay between the energy input to the motor and the energy input to the gas source. The aggregate of the energy which is input is then optimised, as opposed to optimising the energy consumption of the motor. [0031] The information which is generated by the process can be utilised automatically or can be made available to control personnel who can then take appropriate manually implemented control steps. [0032] In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word "comprise" or variations such as "comprises" or "comprising" is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.