Abstract:
A method and apparatus for bioprocessing particles wherein particles are entrapped a porous material and have biologically active microorganisms on their surfaces. A liquid (carrying oxygen and/or nutrients for said microorganisms) is passed through the entrapped particles and microorganisms and microorganisms are active to breakdown said particles. After a suitable period of time the particles are removed from the matrix.

Description:
FIELD OF INVENTION 
     The present invention relates to bioprocessing of particulate material, more particularly to bioprocessing of sulphidic minerals or other particulate solids with active microorganisms and to an improved apparatus for such processes. 
     BACKGROUND OF THE INVENTION 
     Virtually all of the bioreactors presently used for the industrial scale oxidation of particulate solids such as gold-bearing sulfide minerals, copper, zinc, nickel sulphides are slurry reactors, i.e. aerated vessels containing ore slurry which are equipped with mechanical mixer. While considered more efficient than chemical processes for the treatment of sulphidic ore concentrates, current day slurry bioreactors have four major disadvantages: 
     1. In order to keep the particles (e.g. pyrite) in suspension, it is necessary to apply large amount of mechanical energy for mixing mainly because of the high density of pyrite. 
     2. The interparticle friction is very strong because of the high solids concentration and the intensive mixing. Since microorganisms grow mainly on the surface of the particles (e.g. sulfide crystals), the particle friction in slurry bioreactors causes detachment of microorganisms which significantly limits the reaction rate. This results in long reaction times (e.g. 3 to 5 days for pyrite). 
     3. Low oxygen transfer rate due to the presents of suspended solids. 
     4. Liquid and solid retention times are equal. 
     The above discussion indicates that both substantial increase in the process efficiency and decrease in energy consumption can be achieved if the bioreaction is carried out under low shear stress conditions and without the need to suspend the particles in liquid. 
     A recently proposed concept of soil immobilization (see Karamanev et al. “Hydrodynamics of Soil Immobilization in the Immobilized soil Bioreactor” AIChE Journal Vol. 43. No.5 May 1997 pages 1163 to 1170; and Karamanev et al.; “Soil Immobilization: New Concept for Biotreatment of Soil Contaminants” Biothechnology and Bioengineering, Vol 57 No. 4 Feb. 20, 1998 pages 471-476) and the teaching of these publications are incorporated herein by reference. This type of reactor is hereinafter referred to throughout the Application as an immobilized solid particle bioreactor or immobilized bed bioreactor the operation of which is based on the entrapment of solid particles into the pores of a highly porous inert matrix, such as a non-woven textile. 
     Generally the matrix of an immobilized bed bioreactor will have a wide pore size distribution of between several microns and 2 mm. so that as a slurry containing solids within the same size range as that of the pores of the matrix is circulated repeatedly through the matrix, solid particles get entrapped inside the pores. When soil was used as a solid phase, the resulting structure was named immobilized soil. Immobilized soil particles contain surface-immobilized microorganisms (biofilm). In order to supply the microorganisms with substrate, inorganic salts and oxygen, aqueous solution of these compounds is circulated through the immobilized soil structure. 
     These immobilized bed bioreactors when used for treating for treatment of solid particles require a system of removing the treated solid particles for further processing, The systems for treating liquids as in the immobilized soil reactors of the prior art or do not provide or teach any systems for removal of solids from the bed. 
     Due to the depletion of rich ores, mining companies are paying more attention to low-grade ores. The present disclosure will be discussed below with specific reference to gold and the gold industry, but it is to be understood that it is believed the invention has wider application outside of the gold industry (ex other base metals such as copper, zinc and nickel). 
     The gold in the lower grades of ore (e.g. refractory gold-bearing ores) is encapsulated as fine particles (sometimes on molecular level) in the crystal structure of sulfide (typically pyrite with or without arsenopyrite) ore. This makes it impossible to extract refractory gold by cyanidation since cyanide solution cannot penetrate the pyrite/arsenopyrite crystals and dissolve gold particles, even after fine grinding. To effectively extract gold from these ores, an oxidative pretreatment is necessary to break down the sulphide ore. The most popular methods of pretreatment include nitric acid oxidation, roasting, pressure oxidation and biological oxidation by microorganisms. 
     Roasting is highly energy consuming and produces off-gases containing sulfur dioxide and arsenic trioxide, which require costly treatment. Both pressure oxidation and oxidation by nitric acid require high pressure, high temperature and/or corrosion-resistant materials. 
     The biological pretreatment of refractory gold ores is based on the ability of some microorganisms such as  Thiobacillus ferrooxidans  to oxidize and dissolve pyrite and arsenopyrite, thus liberating the entrapped gold particles. Either whole ore or concentrate can be used for biotreatment. Whole ore is usually treated in heaps while concentrate is treated in bioreactors. 
     The process of microbial oxidation of concentrate is usually carried out in slurry bioreactors with a unit volume of 200 to 1000 m 3  at temperatures between 20 and 55° C. and under atmospheric pressure. This process is considered to be less expensive and more environmentally friendly than other methods. However, there are some disadvantages of this method. The bioprocess rate is low: it takes usually between 3 and 5 days to treat the sulfide particles, compared to several hours in the case of chemical processes. The energy consumption is relatively high. This is especially important, taking into account that the power cost is usually between 40 and 60% of the total operating cost of the slurry biooxidation. Therefore, the increase in the bioreaction rate and the reduction of energy consumption are important. 
     BRIEF DESCRIPTION OF THE PRESENT INVENTION 
     It is an object of the invention to provide a process and apparatus for bioprocessing particles. 
     It is a further object of the present invention to provide a system for removal of immobilized material from an immobilized bed bioreactor. 
     It is a further object to provide a bioprocess for liberating gold from pyrite in an immobilized soil type reactor. 
     Broadly the present invention relates to a method and apparatus for bioprocessing particles comprising providing an entrapping matrix for said particles, entrapping said particles in said matrix, providing biologically active microorganisms on said particles in said matrix, passing liquid carrying nutrients and oxygen for said microorganisms through said matrix containing said entrapped particles and said microorganisms, permitting said microorganisms to breakdown said particles and form processed particles and processed liquid and recovering said processed particles from said matrix and said processed liquid from the bioreactor. 
     Preferably, said bioprocessing is carried out in an immobilized bed bioreactor wherein said matrix is substantially vertical and said liquid is made to flow upward on one side of said matrix by air injection and downward on an opposites side of said matrix and a portion of said liquid flows through said matrix from said opposite side to said one side. 
     Preferably said microorganism cause dissolving of parts of said particles by biochemical reaction of said parts. 
     Preferably said processed particles are recovered from said matrix by washing said processed particles from said matrix. 
     Preferably said particles are pyrite and/or arsenopyrite containing gold particles. 
     Preferably liquid is bled from said vessel as a liquid bleed. 
     Preferably said particles are sulfidic minerals containing metals including copper, zinc, nickel cobalt and/or manganese and said liquid bleed is further treated to recover said copper, zinc nickel cobalt and/or manganese. 
     The apparatus for bioprocessing particles of the present invention comprises a vessel, an entrapping matrix for said particles in said vessel, a first passage on one side of said matrix and a second passage for liquid on the side of said matrix opposite said one side, means for passing said liquid in a first direction through said one passage and permitting said liquid to flow in an opposite direction to said first direction in said second passage, a washing system for directing washing liquid through said matrix from one of said first and said opposite sides to remove particles immobilized in said matrix from said matrix to provide removed particles and means for collecting said removed particles and removing them from said vessel. 
     Preferably, said matrix is substantially vertical and said means for passing liquid comprises means injecting air into said liquid adjacent to a bottom of said first passage. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S) 
     Further features, objects and advantages will be evident from the following detailed description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings in which; 
     FIG. 1 is a schematic illustration of an immobilized bed bioreactor. 
     FIG. 2 is a schematic illustration of a portion of the immobilized bed bioreactor of FIG. 1 within the ellipse  10  shown in FIG.  1 . 
     FIG. 3 is a schematic illustration of a particle (in the illustration a pyrite particle) trapped within the immobilized bed bioreactor of FIG.  1  and contained within the ellipse  12  of FIG.  2 . 
     FIG. 4 is a schematic illustration of a multistage immobilized bed bioreactor in reaction position with the recovery (washing) mechanism retracted. 
     FIG. 5 is a schematic illustration in plan view of a cylindrical immobilized bed bioreactor that may be operated on a continuous or batch basis. 
     FIG. 6 is a schematic illustration in along the line  6 — 6  of FIG.  5 . 
     FIG. 7 is a schematic illustration in along the line  7 — 7  of FIG.  6 . 
     FIG. 8 is a graph showing the effect of the air flow rate on the maximum amount of immobilized pyrite. 
     FIG. 9 is a graph showing the relationship between the number of textile layers in the matrix and maximum amount of immobilized pyrite. 
     FIG. 10 is a graph showing the change of iron dissolved from pyrite in time. 
     FIG. 11 is a graph showing a comparison between the volumetric bioreaction rates of pyrite oxidation in the immobilized bed and slurry bioreactors. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following description will deal mainly with recovery of gold, but it is to be understood the particles being processed may be other material combined with other suitable microorganisms and conditions. 
     The system for practicing the present invention uses an immobilized bed bioreactor  14  is similar to the an immobilized bed bioreactor system described in the Karamanev et al. references referred to above and incorporated herein by reference. Referring to FIG. 1 the system used with the present invention includes a vessel  20  having a immobilized bed  22  contained therein and positioned vertically spaced from the walls of the vessel  20  to provide a pair of passages  24  and  26  with opposite sides  22 A and  22 B of the bed  22  forming one side of each of the passages  24  and  26 . The bottom  32  of bed  22  is also spaced from the bottom  28  of the vessel  20  to provide a passage  30  interconnecting the passages  24  and  26 . Preferably a cover (not shown) is provided to eliminate the effect of atmospheric elements such as sunlight and rain. 
     The vessel  20  contains a liquid  34  introduced as indicated at  31  (usually water that may contain added nutrients for the microorganisms being used) up to a level above the top  36  of the bed  22  so that when filled with liquid  34  the passages  24 ,  26   30  and the liquid above the top  36  form a continuous path of liquid  34  surrounding the bed  22  on 4 sides i.e. top  36 , bottom  32  and opposite sides  22 A and  22 B. 
     In most applications of this system liquid will be bled from the system as indicated by line  33 . In some processes valuable material to be recovered leaves the vessel  20  in the bleed through line  33 . 
     A suitable gas such as air is introduced at the bottom of one of the passages  24  and  26 —in the illustration passage  24  via line  38  from a pump  40 . The air acts to lift the fluid in the passage  24  and leaves the fluid at the top of the fluid in the vessel  20  i.e. at the fluid air interface adjacent to the top of the vessel  20 . The air also participates in the oxidation reaction when pyrite is being processed. 
     The amount and a rate of air pumping is set to cause circulation of the liquid up though passage  24  across the top  36  of the bed  22  and to flow downward in the passage  24  and through passage  30  back into the bottom end of passage  24  as indicated by the arrow  44 . Some of the liquid or fluid  34  as indicated by the arrows  46  also flows through the bed  22  from the passage  26  back into the passage  24  due to the difference in hydrostatic pressure on opposite sides of the bed  22 . 
     At least at the startup of the system particles to be processed are introduced as schematically represented by the line  48  and shut off  50  into the vessel  20  preferably into the up-flow passage  24  and are carried in the flow to the passage  26  from which they enter the bed  22  from the side  22 B due to the flow through the bed  22  from the side  22 B to the side  22 A. These particles are trapped in and form part of the immobilized bed  22 . 
     It will be apparent that the size of the particles, interstices of the bed  22 , velocity of fluid  34  etc. must be coordinated to obtain the required movement and deposition of the particles in the bed  22 . 
     If desired nutrients for the microorganisms may be introduced to the fluid  34  in the vessel  20  as schematically indicated by the line  52  and valve  54  and/or may be introduced in the incoming liquid introduced via line  31 . 
     When the treatment of the particles is complete the particles are recovered from the bed  22  for example by creating a sufficiently higher fluid pressure say on one side (e.g. side  22 B) and driving the particles out through other side (side  22 A) or vice versa. One such system to extract processed particles from the vessel  20  is in the form of washers schematically illustrated in elevated position in FIG.  1 . In the illustrated arrangement the washer  70  (in a system with multiple beds  22  obviously there will be one for each bed  22 ) direct jets  72  of fluid against the adjacent face of the bed  22  to force the particles within the bed to move to and out of the bed at the opposite face of the bed. When multiple adjacent beds are used a single washer  70  modified to direct liquid jets in opposite directions may be used to clean two adjacent beds  22 . 
     When the washers  70  are lowered to operative position as indicated by the arrow  74  fluid ejected from the jets  72  drives the processed particles from the adjacent bed  22  and into the passage on the side of the bed remote from the side of the bed being impinged by the washing fluid. The removed particles are carried from the vessel  20  through the line  56  provided with a suitable shut-off system schematically represented at  58  to a location for further treatment or processing as required. 
     FIG. 2 is a magnified illustration of section (within the ellipse  10 ) of the bed  22 . As illustrated the bed is form by a one or more textile layers  60  of the required mess size for the particles being processed. The textile layers are contained between a pair of reinforcing grids  62  on each side  22 A and  22 B of the bed  22 . 
     The particles trapped or immobilized within to form part of the bed  22  are schematically illustrated at  64  are as shown of a variety of different sizes each having the required microorganisms  66  positioned thereon as indicated in FIG.  3 . In FIG. 3 the particle  64  has been designated a pyrite crystal and the microorganisms  66  as  T ferrooxidans  cells. 
     Generally the microorganisms will be added at start-up (e.g. by inoculation) and will continue to multiply and the nutrients will be added preferably continuously to the system in the liquid added via line  31  or through the line  52 . However, in some cases microorganisms can be present on the surface of solid particles before the latter are introduced to the bioreactor. 
     FIG. 4 schematically illustrates a multistage system wherein a plurality of beds are arranged in side by side relationship with tops  36  of the beds  22  below the fluid level to permit flow thereover and are spaced from the bottom of the tank  20 B so that liquid can flow completely around each of the beds  22 . 
     Similar to the system shown in FIG. 1 each of the up passages  24 B is provided with an air inlet fed from air line  38 . If desired each passage  24 B may also be provided with a particle inlet and/or a nutrient inlet (not shown) as schematically illustrated at  48  and  52  respectively in FIG.  1 . 
     The bottom  25  of the vessel  20 B illustrated in FIG. 4 is formed with a plurality of V-shaped troughs  27  with their apexes  29  positioned directly below and preferably centered on their respective up-flow passage  24 B. The air inlets for the up-flow passages  24 B are positioned at the apexes  29  to direct air up into the center of their respective up-flow passages. The V-shape troughs held in entraining particles that are intended to be deposited in the bed  22  to form the immobilized particles in the bed  22 . In this case the solid particles are removed as indicated by the line  74  having a suitable shut off  76 . Where a tapered bottom is provided for each bed  22  when the system of FIG. 1 (as opposed to that of FIGS. 5,  6  and  7 ) is used the level of liquid in the reactor is lowered and the liquid and solids withdrawn via line  74 . 
     FIGS. 5,  6  and  7  schematically illustrate a system that may be operated on a continuous or batch basis. In this system a cylindrical retaining vessel  220  contains series of annular immobilization beds  222  concentric with the vessel  220 . Each of the beds  222  is constructed in the same manner as and equivalent to bed  22  but are in the form of a cylinder. In this arrangement an annular inlet manifold  238  is provided to introduce air (as above described) into the annular up passage  224  formed between a pair of beds  222 . Liquids containing nutrients as required are introduces as schematically indicated at  31  and liquid is bled from the system as indicated at  33 . 
     In order to remove the processed or treated particles from the system of FIGS. 5,  6  and  7  a substantially radial arm  300  is mounted above the vessel  220  for example on brackets (not shown) for rotation by a motor or the like schematically represented by the arrow  304 . At appropriate locations along arm  300  there is provided in alternating relationship a first hollow member  306  and a second having one surface  308  (see FIG. 8) adjacent to (and substantially parallel to (i.e. vertical)) the surface  222 A. A plurality of perforations  310  are provided through the surface  308  to direct fluid jets  309  from the member  306  against the adjacent surface  222 A of the bed  222  for washing particles (equivalent to particles  64 ) from the bed  222 . 
     A second hollow member (a collector member)  312  is provided with a perforated surface  314  adjacent to (and substantially parallel to (i.e. vertical)) the surface  222 B on the side of the bed  222  radially opposite to the member  306  in a position so that fluid containing particle driven from the bed  222  are received in the member  312  for removal from the system. The members  306  and  312  preferably extend the full axial length (height) of the bed  222  so that particles may be collected over the full length of the bed  222 . 
     In the illustrated arrangement the incoming fluid is indicated by the arrows  316  and the outgoing fluid carrying the particles by the arrows  318 . 
     A suitable branch arm  350  is connected to and moves with (trails) the arm  300  in the direction of rotation of these arms as indicated by the arrow  304 . The branch arm  350  is provided with a plurality of particle dispensing lines  354  one for delivering particles into each of the passages  224  (and/or  226 ). Particles to be processed are provided through the lines  354  to replace the particles extracted by the washing action of the arm  300  locally and shortly after each local area of the bed  222  has been cleansed of particles. 
     The arm  300  and thus the members  306  and  312  are rotated as represented by the arrow  304  at a rate commensurate with the time the particles are to be subjected to treatment in the system, for example if the particles are intended to remain in the bed  222  for about 2 days the rate of rotation of the arm  300  would be once every 2 days. 
     It will be apparent that the basic principle of operation of the different embodiments of the invention described above are all the same as described above with respect to FIGS. 1 to  3 . It will also be apparent that a washing system similar to that described above and illustrated in FIGS. 5,  6  and  7  with appropriate modification could be applied to the FIG.  1  and/or FIG. 4 embodiments. 
     It is believed the process and apparatus of the present invention has wide application, however, one of the more significant applications of the present invention is in the treatment of sulphidic minerals or ores. 
     Generally the mechanisms for the oxidation of the sulfides is a s follows. 
     Direct Oxidation of Sulfides                           
     Typical sulphidic minerals or ores that may effectively be processed using the present invention include metal sulfides containing gold, copper, zinc or nickel (and/or other metals such as cobalt, manganese it is believed may be processed using the present invention). Gold is the most unique in that processing of gold requires recovery of the treated particles from the bed and further treating the recovered particles (in known manner) to obtain the gold. The processing of copper, zinc or nickel or other metals requires further treating the bleed in line  33  in known manner to obtain the desired mineral. In either case the solid particles must be removed from the bed and be replenished with new ones to carry out the process as it is the particles that are being processed and provide the source of the material to be recovered. 
     EXAMPLES 
     Materials and Methods 
     The immobilized bed bioreactor was a vertical cylindrical glass vessel with a diameter of 4.7 cm and a height of 37 cm. Its working volume was 500 mL. The cylinder was divided vertically into two semicylindrical sections by means of a porous matrix (bed  22 ) composed of one or more layers of non-woven polyethylene textile (FIG.  2 ). The total thickness of the matrix was 30 mm. It was kept in place by two sheets of stainless steel mesh (62) with 1 mm openings. The mesh and the textile formed a sandwich-like structure (bed  22 ). A membrane air pump (Hagen Inc.) was used to deliver air to the bioreactor. The air flowrate, measured by a calibrated rotameter, was in the range between 10 and 100 L/h. Prior to entering the bioreactor; air was saturated with water in a washing bottle in order to avoid evaporation of liquid in the reactor. The bioreactor was operated at room temperature (22° C.). It was wrapped in aluminum foil in order to avoid the effect of direct sunlight on  Thiobacillus ferrooxidans . The bioreactor was filled with a 9 K mineral nutrient media of Silverman and Lundgren (Journal of Bacteriology, Vol. 77, p. 642, 1959), containing ammonium sulfate 3 g/L, potassium hydrogen phosphate 0.5 g/L, magnesium sulfate 0.5 g/L, potassium chloride 0.1 g/L, calcium nitrate 0.01 g/L and sulfuric acid for pH correction to pH of 2.2. The microbial culture used was  Thiobacillus ferrooxidans . It has been isolated from acid mine drainage. The solid phase used was ground pyrite with different size fractions. During the hydrodynamic experiments, the bioreactor was equipped with a magnetic stirrer in order to avoid settling of pyrite. After the immobilization of pyrite, the stirrer was stopped. 
     The slurry bioreactor was a cylindrical vessel with an ID of 10 cm and a height of 13 cm. Its working volume was 500 mL. It was equipped with an 8 cm long magnetic, Teflon-covered, metallic rod. It was rotated by a magnetic stirrer which was placed below the reactor. The reactor was aerated using the same equipment as the immobilized bed bioreactor described in detail in the Karamanev et al references referred to above. 
     The mineral salts used were analytical grade. The pyrite used was purchased from Ward Natural Science (St. Catharines, Ontario). It was ground mechanically and only the fraction below 250 μm was used. 
     The concentrations of ferrous and ferric ions in the bioreactor were measured using a new spectrophotometric method using sulfosalicilic acid as an indicator. The method allowed to measure both ferric and ferrous ions concentrations in a single sample. The measurement was performed at wavelengths of 425 and 500 nm for each form of iron ions, respectively. The spectrophotometer used was Philips PU-8625. pH was measured by a Fisher-119 pH meter with a precision of 0.01. An optical phase-contrast microscope Microstar (American Optical) was used to observe visually the free suspended microorganisms (magnification 1000×). 
     The concentration of solid particles in liquid was measured in a sample of 1 mL, withdrawn from the reactor and replaced by 1 mL of distilled water. The sample was placed for 15 min in a test tube to separate the solid particles from liquid by settling. The sediment was dried at 100° C. for 5 hours and weighed after cooling down. The total amount of immobilized pyrite particles was calculated from the difference between the amount of pyrite particles added to the reactor and the amount of particles suspended in liquid. 
     Hydrodynamic Study of the Process of Pyrite Entrapment 
     The immobilized bed bioreactor was first studied from the hydrodynamic point of view. The dynamics of pyrite immobilization into the pores of the porous textile was studied. It was found previously that nonwoven textile is very appropriate as a porous matrix. The textile used in this work was described in details elsewhere (Karamanev et al., 1998). Its porosity was 0.995 and the mean fibre diameter was 50 μm. The rate and total amount of immobilized bed particles was measured as a function of the number of layers of textile, the air flow rate and the particle size. When the number of textile layers was changed, the total thickness of the textile matrix was kept constant (3 cm). In order to occupy the same volume, multiple layers of textile were squeezed, and therefore, the increase in the number of layers led to a decrease in the porosity of the textile matrix. 
     The experiments show that if more than 31 g of ground pyrite was added to the specific reactor used at once, they could not be kept in suspension by air bubbles because of the high density, and thus, high settling velocity of pyrite particles. Therefore, pyrite was added to the reactor step-wise. Initially 30 g of pyrite were added, followed by the addition of 3 g every minute during the first 5 minutes. After that, 3 g of pyrite was added each 5 minutes. In the first experiment, the matrix contained a single layer of non-woven textile. During the repeated addition of pyrite, it was visually observed that the process of pyrite immobilization was very fast: liquid became transparent in 10 to 15 seconds after the addition of pyrite. The concentration of pyrite in suspension, measured experimentally 1 minute after the addition of pyrite, was found to be practically zero. However, after the total addition of 43 g of pyrite, its concentration in slurry increased to 10 g/L. The addition of a new 3 g resulted in the formation of sediment at the reactor bottom. At this point it was assumed that the textile matrix was saturated with pyrite and could not accept more solid particles. The maximum amount of pyrite immobilized was studied as a function of the air flow rate. It can be seen (FIG. 9) that with the increase in the air flow rate, the amount of immobilized pyrite initially decreases, and after 40 L/h it levels off. 
     Similar experiments were performed for the case of 2, 3, 4 and 5 layers of textile in the matrix. The air flow rate was 30 L/h. The relationship between the number of textile layers and the maximum amount of pyrite immobilized is shown in FIG.  10 . It can be seen that the increase in the number of textile layers up to 4 led to the proportional increase in the amount of pyrite at saturation and after that the slope slightly decreases. The effect of the number of textile layers and the matrix porosity on the maximal amount of pyrite immobilized (at saturation) is also shown in FIG.  10 . The maximum quantity of immobilized pyrite was 192 g which was obtained when 5 layers of textile were used. Since the liquid volume in the reactor was 500 mL, the fraction of solids in the reactor is equal to (192/500)×100=38.4% w/v. This solids fraction is two times larger than that used in industrial slurry bioreactors which is between 15% and 20% v/w. 
     Macrokinetic Study of Pyrite Biooxidation 
     The number of textile layers (4) and the quantity of pyrite (100 g) in this study were determined according to the results of the hydrodynamics of pyrite immobilization given in the previous section. The microbial culture was activated by aerobic cultivation with ferrous sulfate as an energy source. Once activated, the culture medium was used as an inoculate for the immobilized bed bioreactor. The bioreactor was filled with 450 mL of 9 K nutrient salts solution plus 50 ml of inoculate. After the aeration started (30 L air/h), 100 g of pyrite were added to the bioreactor and immobilized in the textile matrix according to the procedure described in the above section. The concentrations of ferrous and total iron as well as pH in liquid were measured in time. In order to compare the rate of pyrite biooxidation in immobilized bed bioreactor with those in a slurry bioreactor, a second, slurry bioreactor was operated in parallel. The conditions in the slurry bioreactor were the same as those in the immobilized soil bioreactor, including the liquid volume, amount of pyrite, air flow rate, liquid composition, inoculate volume, temperature, initial pH. The only difference was in the amount of energy consumption since slurry bioreactor required intensive mechanical mixing, and therefore, higher energy input. Unfortunately, the exact amount of energy input by the mechanical mixer can not be measured in the case of magnetic stirring. 
     The amount of iron dissolved from pyrite in both bioreactors as a function of time is shown in FIG.  11 . Almost all the dissolved iron in both bioreactors was in the form of ferric ions, which shows that the microbial oxidation of ferrous iron was faster than the chemical reduction of ferric iron by pyrite. The time of biooxidation in both reactors was longer than that reported in industrial processes. However, it was similar to that reported in most laboratory-scale experiments. This difference can be explained by the use of adapted microbial cultures in most industrial operations and non-adapted ones in laboratory experiments, as the one reported in this work. The most important conclusion from FIG. 11 however is that iron concentration in the immobilized soil bioreactor increased much faster than that in the slurry bioreactor. The difference was 2.5 times at the end of the run. 
     The rate of pyrite dissolution, calculated from the change in the iron concentration in liquid, is shown in FIG.  12 . This figure also shows clearly that the immobilized bed bioreactor has much higher volumetric efficiency of pyrite dissolution than a slurry bioreactor; the difference increases with time. At the same time, it requires significantly less energy. 
     It will be apparent that the advantages of the present invention include 
     elimination of the intraparticle friction and 
     elimination of the power input required to keep solid particles in suspension. 
     This allowed to significantly increase the volumetric efficiency of the process and to decrease the power requirements. The performance of the bioreactor was tested by the oxidation of pyrite by the bacterium  Thiobacillus ferrooxidans . Our experimental results showed that: 
     the volumetric efficiency of biooxidation in the new bioreactor was approx. 2.5 times higher than in a slurry reactor. All operational parameters (including the volumetric pyrite fraction) were kept equal in both bioreactors, except for the power input which was much lower in the new reactor 
     The maximum amount of pyrite that can be treated per unit volume of the new reactor is twice as high as that in a slurry bioreactor. 
     This novel bioreactor process can be used in different biohydrometallurgical processes, and especially for biologically-assisted gold processing. 
     Having described the invention, modifications will be evident to those skilled in the art without departing from the scope of the invention as defined in the appended claims.