Manual Metallurgical Applications of Bacterial Leaching and Related Microbiological Phenomena

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1 Introduction

Using these conditions, the reaction is immediate and does not require drying. Quick bio-oxidation by adding dehydrating agents to decrease the water activity may also be applied for other ores, thereby decreasing or eliminating the drying requirements. Nevertheless, in each case, the economy of the process must be considered in connection with the cost and quantity of the employed dehydrating agent, as well as the economic value of the ore in question.

Dehydrating agents may be used for bio-oxidation of tails, but it could be costly, but they are very attractive for use with concentrates.

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With respect to mineral, between 5 and 15 kilograms of CaCl 2 per ton of a copper mineral notably reduce the requirements in drying, giving better extraction yields, when compared with the same assays without the use of a dehydrating agent. The previous concept relating to the use of dehydrating agents cannot be entirely separated from reducing the volume of the acidic solution. They are different ways to achieve the same objective: to establish the lowest thermodynamically available water or water activity in the system.

As noted above, the quantity of acid for conditioning the ore is determined before-hand as the most convenient, in order to neutralize the ore to prevent compaction and to provide the proper acidity for the microorganisms. The quantity of acid used is preferably contained in the smallest possible volume of a solution, in such a way as to acidify homogeneously the ore introducing into the system the smallest quantity of water.

It should also be noted that, rather than providing a microbial inoculum, one may enrich the mineral ore's own microbial flora, as noted previously. This is accomplished by homogeneous acidification of the ore with a sulphuric acid solution. The most convenient volume and concentration must be determined before-hand as was indicated previously.

The best conditions of acidification depend on the physical and chemical characteristics of the ore in question.

As noted above, the system must be dried. Natural or induced drying is allowed until the thermodynamically available water is sufficiently low enough so that the microorganisms are not suspended in a liquid film. Thus, one allows for the spontaneous or induced water excess that may be present in the system by evaporation or by drying with flowing air until the thermodynamically available water is sufficiently low enough for obtaining the bio-oxidation products in solid state.

At this point, each microbial cell attached to a sulphide compound can bio-oxidate the compound giving a solid soluble product looking like a crystal and containing in the solid product thousands or millions of cells coming at least theoretically from one cell.

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In this manner, the ore is numerically enriched by its own microbial microflora. The bio-oxidation products constitute microbial colonies. In a second cycle, the ore must be homogeneously humidified another time with the same criteria applied in the first cycle. In this way, the soluble solid products produced in the previous cycle are, at least partially solubilized, liberating the contained microbial cells, which can come into contact with other insoluble compounds not bio-oxidized in the first cycle. By subsequent natural or induced drying, they will be bio-oxidized.

Microbial Leaching

The prior processes may be repeated until one reaches a commercial yield of bio-oxidation for subsequent extraction. When a microbial inoculum is used, the microorganisms may be added during or subsequent to the addition of the conditioning acidic solution.

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The drying of the system is one way for getting the microorganisms not to be suspended in a water film, and for allowing for a stable attachment of the microorganisms to the insoluble sulphide or oxide compounds which are their substrates. A high bacterial multiplication velocity associated with a high bio-oxidation velocity is obtained.

The air contact is very important for the bio-oxidation process, taking into account that the bacterial growth requires carbon dioxide, nitrogen and oxygen from the air. The concentrates are generally in the form of thin powders. One way for getting a high specific surface area of a conditioned and inoculated concentrate in contact with air is by pelletizing said conditioned and inoculated concentrate with a coarse inert material such as: triturated rocks, gravel, glass balls, plastic balls, rubber balls, etc.

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A layer of the conditioned concentrate surrounds the inert pelletization material allowing for a high specific surface area in contact with air. Good results have been obtained also by distributing the conditioned and inoculated concentrate as layers on both sides of an inert material sheet. The prior criteria may be applied for other ores when they are of sugar grain-sized or smaller. In this manner, it is possible to get a higher contact between the substrates and air.

Generally, the metal sulphides or oxides or sulphur are found in nature as particles disseminated in rocks, the association of both being considered as an ore. For reaching a high conversion, by bio-oxidation of those particles giving soluble compounds, it is necessary that at least one microbial cell comes into contact with each particle, getting a stable attachment and a high multiplication velocity associated with a high bio-oxidation velocity of those particles which have got at least the attachment of one cell after drying. The insoluble compounds which, at the point of adequate dehydration do not have at least one cell attached, will not be oxidized.

So, it is possible even using the addition of an inoculum, not to get a commercial bio-oxidation yield. It depends on the number of cells in the inoculum and the dissemination degree of the insoluble substrates. In this case, equivalent procedures as those indicated for the ore's own microflora enrichment may be carried out. The water content may be increased and decreased alternatively through a number of cycles. The water content increase is allowed according to step a to release the microorganisms contained in the bio-oxidized solid products obtained through the step of drying in the first cycle.

After that, the water content is decreased by drying in the same form as it was in the first cycle in order to obtain the bio-oxidation of the particles not bio-oxidized in the previous cycle. A number of cycles, including humidification and drying, may be carried out in order to get a commercial bio-oxidation yield.

Different microbial species have different ranges of temperatures for optimal growth and high activity, as is shown in Table I Process for microbial species important in Biometallurgy. Below, in Table II, by way of example, strains corresponding to different species are indicated and have been employed in the present invention. The strains have been characterized, among other features, according to the optimum temperature range. Microorganisms having different temperature range of growth, may be employed in the present invention. For the best results, the ore, at least during the bio-oxidation steps, must be at a temperature within the optimum temperature range of the microbial species employed.

As noted above, in all the known biohydrometallurgical systems, bio-oxidation is carried out with the microorganisms suspended in a liquid phase. Thus, the known systems have two phases: a solid phase, constituted by the ore, and a liquid phase constituted by the leaching solution, where the microorganisms are suspended. The bio-oxidized products are obtained as solutions, that is, they are produced solubilized in the liquid phase. Thus, during the bio-oxidation step and when the bio-oxidation finishes, the system has two phases, a liquid and a solid phase, which may be separated.

Even in known systems, where words such as dry, drying period, dry cycles or irrigation and dry are used, meaning that there is no irrigation being applied during such periods, heaps or dumps, or the parts of a mine subjected to leaching continue producing drainages. This is to say, the operations are over-supersaturated moisture and the system during the bio-oxidation step has two phases: a liquid and a solid phase. Both phases may be separated giving the bio-oxidized products solubilized in the leaching solution, that is in the liquid phase.

By the present invention, the bio-oxidation is carried out when the system has a moisture below the supersaturated state. The supersaturated state occurs when additional water cannot find room without generating a pore pressure that once again forces the particles apart, causing them gradually to lose contact with one another.

By the process of the invention, during the bio-oxidation and when the bio-oxidation finishes, it is not possible to separate a liquid and a solid phase. It is also not possible to have drainage during the bio-oxidation step or when bio-oxidation finishes. The system has only one phase, that is, a solid phase and the soluble bio-oxidized products are in solid state, being microbial colonies. By the process of the invention, the soluble bio-oxidized compounds are obtained in solid state during the bio-oxidation step. For separating them from the remainder of the material, the bio-oxidation step must be followed by a separation step.

The separation step may be carried out by suitable methods such as: washing with water, washing with aqueous solution, screening, gravity separation, or flotation. When separating by washing, the soluble bio-oxidized compounds in solid state, obtained during the bio-oxidation step, are solubilized in the washing solution and must be separated in this step by separating the washing solution from the remaining solid material. When the invention is applied to metal extraction, the soluble bio-oxidized compounds are the products of interest in the process.

They are separated in the separation step, and the remaining material constitutes the residue or tail. When the invention is applied to purification processes, such as coal desulphurization or precious metal purification, the soluble bio-oxidized compounds separated by the separation step constitute the residue while the remaining material constitutes the product of interest. The first stage of interaction of bioleaching microorganisms with a solid inorganic substrate consists in their attachment to the surface, whereupon the substrate being oxidized is attacked biochemically.

Attachment is specific to the mineral compounds which offer a source of energy, but such attachment is not frequent and does not always occur in the systems tried so far.

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The conditions which allow or facilitate a stable and efficient attachment, enabling the bacteria to transform the substrate and multiply quickly, had not heretofore been explained. From the physiological characterization and development conditions hereinafter described, it is clear that these microorganisms have a definite hydrophobic character. In other words, the water, or at least the water levels in conventional systems, make the stable attachment of cells to the substrates difficult. The phenomena that take place when the cells and the surface of mineral compounds interact or the mechanism of destruction of the sulphide mineral lattice are not clear.

Although there are different theories, it is generally believed that enzymatic mechanisms are involved in this interaction. In such case, the intervening enzymes must not be diluted or washed out from the reacting surface.

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One important aspect of the present invention is the micro-environment in which the microorganisms are confined to develop. If a finely ground ore is agglomerated with an acid solution, it is possible to obtain quick growth and to obtain formation of products in solid state when the liquid to solid ratio is and sometimes higher, depending on the specific area and the chemical composition of the ore.

The higher the ore specific area, the thinner the water film for a fixed liquid to solid ratio. The water film must be thin enough to ensure the proper microbial attachment to the substrate surfaces and to allow the microorganisms to multiply quickly giving the soluble bio-oxidized products in solid state. A second consideration is the pattern of the solution layer which is the boundary between the solid surface and the solution. The thickness of that layer, which performs as a solid, is a function of the chemistry of the solution. There are ions which permit a thick layer of water to perform as a solid.

A third consideration is the water activity which is a measurement of the available free water next to the solid surfaces, and depends on the water present and on the electrolytes present which are different for different ores. Furthermore, the water activity may be artificially reduced, as shown in the present invention, by the addition of chemical compounds known as dehydrating agents as polyethyleneglycols, inorganic salts or any chemical compound with the ability to decrease the water activity.

From the above considerations, it is clear that the range of moisture contents present at the time of developing the microorganisms in a faster way, yielding bio-oxidized products in solid state, is unique for each ore and for each chemical composition of the system being assayed. The culture and bio-oxidation of mineral compounds in the conditions described below were carried out by using the strains indicated in Table II, by way of example and not limitation.