MX2007010901A - Producing calcium cyanide at a mine site using easily transportable starting materials. - Google Patents

Abstract

The present invention is a method by which the mine can obtain a supply of calcium cyanide for use in its leaching operations. Specifically, one preferred method within the scope of the invention involves producing hydrogen cyanide directly at a mine site using formamide as a starting material. Alternative methods of producing hydrogen cyanide at the mine site using easily transportable starting materials, such as methanol and urea, are disclosed. The hydrogen cyanide is neutralized at the mine site with slaked lime (Ca(OH)₂)to form the calcium cyanide.

Description

PRODUCTION OF CALCIUM CYANIDE IN A MINE SITE USING EASILY TRANSPORTABLE INITIAL MATERIALS FIELD OF THE INVENTION The present invention relates to the production of cyanide for mining applications. More specifically, the present invention relates to the production of cyanide for mining applications, and to the mining site using initial materials that are easily transportable.

BACKGROUND OF THE INVENTION Aqueous solutions of cyanide (or "cyanide") anions are widely used in the mining industry as a mechanism for extracting precious metals, such as gold and silver from the earth. In general, these aqueous cyanide solutions contain sodium cyanide, potassium cyanide or cyanide salts of other alkali metals. Although miners have tried to find other means of obtaining gold from minerals, cyanide extraction remains the only effective low-cost mining technique. Consequently, it is believed that cyanide will remain the material of choice for the mining of precious materials. Mining with cyanide is usually carried out either in a pipeline leaching operation or in tank leaching. In these leaching operations, the ore is REF. : 185781 put in contact with a basic dilute solution of cyanide. Cyanide solutions typically have a concentration of less than 1000 ppm. Under these basic conditions, the cyanide will react with the solid metal and oxygen in the air to form a soluble aqueous complex, which is generally in the form of [Ag (CN) 2] ~ or [Au (CN) 2] ~ Once the leaching process is completed, the aqueous complex of the precious metal is then collected and concentrated so that the precious metal can be extracted and reduced to its metallic state. The cyanide used in the leaching process is also recycled for subsequent leaching and / or disposal in accordance with government regulations. During the process, some of the cyanide is oxidized and some is lost for later use. Today, most of the precious metals mining industry receives its cyanide from these mining processes, either in the form of a liquid solution or in the form of dry sodium cyanide briquettes. Briquettes are generally manufactured in large, integrated chemical manufacturing sites. Several major producers of alkali metal cyanide in the world are: (1) E.l. du Pont de Nemours and Company (which has the cyanide facilities of Memphis, Tennessee and Texas City, Texas); (2) Degussa AG (a German company that has cyanide facilities in Weesling, Germany); and (3) Tae K ang Industrial Co. , Ltd. (Seoul, South Korea); (4) Tongsuh Petrochemical Corporation, Ltd.

(Seoul, South Korea); and (5) Australian Gold Reagents Pty, Ltd., (Kwinana, Western Australia). In general, the product of choice for precious metal mines that use the cyanide leaching process is sodium cyanide (NaCN). This product is usually manufactured from the neutralization of hydrogen cyanide (HCN) and caustic soda (NaOH). The HCN is either produced on purpose (mainly through the Andrussow process or the BMA process) or from the reaction to produce acrylonitrile (in which one of the byproducts of this reaction is HCN). There is some production of hydrogen cyanide from other processes, but it is limited and in general is not a source for NaCN. Once formed, the NaCN is then dried and made into briquettes to aid the shipment of products over larger distances. However, if the NaCN is produced near the mine site, the product of choice for the mine is usually a 30% solution of NaCN.

To produce HCN through the Andrussow process, the raw materials are natural gas (methane, CH4) and ammonia (NH3). These gases are reacted at elevated temperatures (1093 ° C, 2000 ° F) on a platinum-rhodium catalyst. The reaction is endothermic requiring the addition of heat. The reaction essentially involves the ammoxidation of methane, for example, partial oxidation affected by the addition of oxygen (air). HCN is produced in this reaction as one of the products. In some cases this HCN product is separated from the other gases in the product, primarily by the removal of ammonia (using either a sulfuric acid or phosphoric acid scavenger). The HCN is then absorbed into the water, which is distilled to obtain pure HCN. After the removal of unwanted impurities, such as ammonia, carbonates, formates, etc., HCN can be used in other downstream manufacturing processes including the production of NaCN that is dried. In other applications, the gases produced from HCN are contacted directly with the caustic soda to produce a NaCN solution. A NaCN solution is produced when the acquisition amine is within a suitable distance for the logical shipment by freight of the solution instead of a dry NaCN product, such as the gold fields of Australia, South Africa, Uzbekistan, China and / or the United States. When the acrylonitrile process is used to produce HCN, HCN is essentially a byproduct of acrylonitrile. However, in order to comply with current environmental and safety regulations, the manufacturing plant must treat and / or dispose of this HCN. In this way, these plants often choose to convert HCN into NaCN and sell this additional product to the mining industry. Despite the market for cyanide, there are growing interests in the mining industry that cyanide supplies are not available at a commercially viable price in the future. It is anticipated that the facilities and older technology currently used to manufacture hydrogen cyanide will become obsolete, thereby reducing the amount of HCN that is currently being produced and sold. In this way, it is expected that the total cost of cyanide will increase in the future. Cyanide is defined as an extremely dangerous chemical product. In fact, from the public point of view, the use of cyanide is considered a threat to the environment and a possible tool for the use of terrorists. In the United States, some railroad tracks do not transport cyanide. It is becoming increasingly difficult to ship cyanide internationally. The Exporting manufacturers must currently obtain export licenses from government agencies that define legitimate end users and the site of use. Many international mining operations are concerned that future restrictions / regulations may restrict the shipment of cyanide through the oceans, in the shipment of cyanide through ports in undeveloped regions of the world, and / or the transport of cyanide on roads with poor maintenance and / or less traffic. Such additional restrictions will make the (or even impossible) shipment of cyanide prohibitively expensive and may endanger the commercial viability of existing or future mines. Even with developed countries, there is a strong movement of lobbyists to decrease the use of cyanide in mines. Although it is expected that these lobbyists will send additional security guards to the mines, a complete ban on cyanide is not expected. However, there are certain states in the United States that have banned cyanide under certain conditions, namely Wisconsin and Montana. Other states restrict the use of cyanide for new mining operations. If additional states follow this course, mining with precious metal cyanide will become a commercial impossibility. Finally, there is a perceived probability in the mining industry that traditional large cyanide manufacturers can seek to exit the cyanide market. ICI and FMC, both previous manufacturers of cyanide, have already left the market. There are some possible reasons why cyanide manufacturers may wish to stop their cyanide production. First, the manufacture of cyanide may not be considered a major commodity element of some large manufacturers. Second, cyanide manufacturers are concerned about safety and environmental hazards regarding cyanide. Specifically, these producers are concerned that they can be held responsible if a disaster involving cyanide occurs. Also, many producers seem to believe that cyanides are becoming a "comfort" chemical. Typically, the chemicals are either classified as a "specialty" product or a "comfort" product. As a chemical moves from a specialty chemical to a comfort chemical, large, technologically innovative companies often exit the market for this product so that they can focus their efforts on new products with higher profit margins. Thus, it should not be surprising if these large producers start slowly emerging from the cyanide market.

Given the current climate with respect to cyanide, it would be advantageous for mines to find a new source of alkaline cyanide, which is commercially viable in the long term. It would also be advantageous to provide a method for the mines to obtain alkaline cyanide without having to face the difficulties associated with the transport of the alkaline cyanide, including the future risk of governmental prohibition of the transportation of cyanide. Such a method and process is described in the process.

BRIEF DESCRIPTION OF THE INVENTION This invention provides a method by which a supply of cyanide is produced directly in a precious metal mine. This cyanide supply can then be used by the mine in its leaching operations to extract the precious metal from the earth. The ability to produce cyanide at the mine site can provide the mine with long-term stability and / or commercial advantage. This can act as a type of "insurance" against the future disruption of the cyanide supply and thereby protect the substantial investment associated with the mining operation. Because mining operations typically have a short life span of approximately 8-10 years, a controlled supply of cyanide would be highly desirable.

In general, the mine will manufacture the cyanide supply using initial materials that are readily available and easily shipped. As used herein, the term readily available means that raw materials are commercially available and not subject to special and restrictive government control and management regulations. As used herein, the term "easily shipped" means that raw materials may be shipped via conventional and commercially available shipping means without special or restrictive government regulation regarding shipment. In a currently preferred embodiment, the starting material is formamide that can be broken down into HCN and water. Other raw materials can also be used. Once the HCN has been formed, the producer will neutralize the HCN with an inorganic base to produce a supply of cyanide anions. In some embodiments, the preferred inorganic base will be limestone (CaO) or slaked lime (Ca (OH) 2) because this much cheaper product compared to NaOH, readily available, and is often already used by the mine in other processes. In this way, when it reacts with the lime or slaked lime, the cyanide will be in the form of dissolved Ca (CN) 2.

BRIEF DESCRIPTION OF THE FIGURES In order that the manner in which the aforementioned and other characteristics and advantages of the invention are obtained is easily understood, a more particular description of the invention briefly described above will be made with reference to the modalities specific to it that are illustrated in the attached drawings. Understanding that these figures describe only specific embodiments of the invention and should therefore not be considered as limiting their scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying figures in which: 1 is a flow diagram of a method for producing a supply of calcium cyanide at the site of the mine, according to the present invention; Figure 2 is a flowchart of yet another method for producing a supply of calcium cyanide at the site of the mine, according to the present invention; Figure 3 is a flow chart of yet another method for producing a supply of calcium cyanide at the mine site, according to the present invention; Figure 4 is a flow diagram of another precious metal mining method that uses calcium cyanide that is produced at the mine site; Figure 5 is a schematic flow chart of a process for producing a supply of calcium cyanide; and Figure 6 is a schematic flow chart of a process for producing a supply of calcium cyanide, similar to the process of Figure 5, but with the recycle stream of the calcium cyanide produced to increase its concentration.

DETAILED DESCRIPTION OF THE INVENTION The presently preferred embodiments of the present invention will be better understood with reference to the figures, wherein like parts are designated by similar numbers throughout. It will be readily understood that the components of the present invention as described and illustrated generally in the figures herein, could be accommodated and designed in a wide variety of different configurations. Thus, the following more detailed description of the embodiments of the method of the present invention, as represented in Figures 1 to 6, is not intended to limit the scope of the invention, as claimed but is merely representative of the currently preferred embodiments of the invention. With reference now to Figure 1, a method 120 of the present invention is illustrated in a flow diagram. Method 120 represents a method that allows a precious metal producer to make cyanide directly at the site of the mine. Such on-site cyanide production can provide the mine with long-term stability, commercial viability and / or other advantages. For example, in some modalities, this cyanide production at the site can provide the mine with long-term stability since the supply of cyanide to the mine no longer depends on manufacturers that can exit the market. Likewise, such cyanide production can give the mine more stability since its cyanide supply is not influenced by the cyanide shipping difficulties that can be developed around the world. Finally, such cyanide production at the site may also reduce the operating costs of the mine since the mine will no longer be forced to pay shipping costs, taxes, duties and other charges that may be associated with the transportation of the cyanide. This includes eliminating the hazards associated with shipping the cyanide to a site in the mine. As a result, this method of cyanide production at the site can provide significant advantages. Method 120 includes the step of obtaining 122 from a supply of formamide (HC0NH2). Formamide is a relatively expensive chemical that is commercially available in liquid form from a variety of sources including the company BASF Aktiengesellschaft located in Ludwigshafen, Germany, the Kemira Oyj Industrial Chemicals company located in Helsinki, Finland, and other small producers. Formamide is much less dangerous and is subject to much less restrictions than cyanide. Currently formamide is not considered a terrorist threat and can be shipped to the whole world without much difficulty. Once the formamide supply has been obtained, the supply of formamide can be decomposed 124 at the site of the mine to produce an amount of HCN. As used herein, the term "at the site of the mine" refers to any site or location that is associated within a nominal distance to a precious metal mine. Thus, in some currently preferred embodiments, this phrase means that the decomposition and neutralization steps of method 120 are accomplished either on the same property and / or on a property that is next to or adjacent to the lot from which the goods are being extracted. precious metals. In some cases, there may be multiple mining operations within a few kilometers. In such cases, there may be only one facility to manufacture the cyanide that supports the multiple mining operations.

In general, the heterogeneous catalytic reaction step that converts formamide to HCN is relatively straightforward.

HCONH2 (g)? H20 (g) + HCN (g) The reaction takes place in the vapor phase at elevated temperature and low and essentially quantitative pressures. The exact methods and processes by which the reaction can occur are described in U.S. Patent No. 4, 693,877 entitled "Formamide excision to give hydrocyanic acid and water". This patent, which is now expired, is expressly incorporated by reference herein. Additional information and descriptions of the conversion of formamide to HCN are described in the following patents and book summaries: U.S. Patent No. 2,042,451, U.S. Patent No. 2,529,546, U.S. Patent No. 2,534,000, U.S. Patent No. 2,604,380, U.S. Patent No. 3,702,887, U.S. Patent No. 4,745,207, U.S. Patent No. 4,693,877, • U.S. Patent No. DE 476,662; German Patent No. DE 477,437; German Patent No. DE 498,733: German Patent No. DE 561,816; German Patent No. DE 944,547; German Patent No. DE 1,000,796; German Patent No. DE 1,211,612; German Patent No. DE 2,445,168; and James A Kent, ed., RIEGEL'S HANDBOOK OF INDUSTRIAL CHEMISTRY, 8th edition, Van Norstrand Reinhold Company, Inc., New York, 1983, pp. 203-209. Each of the patents defined above, as well as the aforementioned treaties, are expressly incorporated by reference herein. While some of the aforementioned documents or patents describe the decomposition of formamide to a microscale, these reactions can easily be scaled up for large industrial applications. Once the formamide has been decomposed to form the amount of HCN, the HCN can then be neutralized at the site of the mine to produce an aqueous solution of calcium cyanide, step 126. The aqueous solution of calcium cyanide can then be diluted concentrate for use in mining operations. In general, this processing step may involve one or more of the following steps: contact the HCN gas with a solution or suspension of slaked lime (Ca (OH) 2) to produce the aqueous solution of calcium cyanide; dilute the cyanide solution formed to concentrations that are appropriate for the mine leaching processes (such as 1000 ppm, etc.). Figure 5 contains a general schematic representation of a process at 150 to produce calcium cyanide from the decomposition of formamide. As shown in Figure 5, a supply of formamide is provided 152 to an evaporator 152 that evaporates the formamide. The formamide stream 156 passes through a heat exchanger 158 to heat the formamide to a temperature suitable for the catalytic decomposition reaction occurring in a 160 catalytic contactor. The formamide is decomposed into hydrogen cyanide gas and water vapor.

These gases 162 are introduced in a contactor 164 together with the calcium hydroxide 166, the slaked lime, to neutralize the HCN and form Ca (CN) 2. The neutralized product stream 168 is introduced into a separator 170 to which a vacuum pump 172 is connected for the removal of the gases 174. The separator 170 includes a liquid barrier 176 which is permeable to gases, but not to liquids. . The aqueous solution of the stream of product 178 of Ca (CN) 2 is then recovered. In some applications, it may be desirable to concentrate the Ca (CN) 2 formed in this way. Figure 6 contains a general schematic representation of a process 180 similar to process 150 except that a portion of the aqueous solution of calcium cyanide is recycled back to the additional contactor with the HCN gas. The aqueous solution of the product stream of Ca (CN) 2 178 is passed through a heat exchanger 182 to capture the heat from the product stream 178. The concentrated aqueous solution of Ca (CN) 2 184 is removed for use at the mine site. Additional Ca (0H) 2 186 may be added to the recycle stream 188, as necessary, to react with the HCN. The sterile stream (which has no mineral) 190 can be added to the recycle stream 188. The sterile stream 190 usually contains a small amount of aqueous cyanide. The recycling of the sterile stream in this way can allow the aqueous cyanide to be efficiently used. Some fresh water may be added to the recycle stream or the sterile stream 190 may be replaced by a stream of fresh water. However, fresh water usually contains dissolved carbonate, which can react with calcium to form calcium carbonate, an incrustation problem. The sterile current 190 would have carbonate less dissolved than fresh water, reducing any problem of calcium carbonate scale. As noted above, lime or slaked lime is the inorganic base for converting HCN into a solution or suspension of calcium cyanide Ca (CN). Lime or limestone can be converted to slaked lime by contact with water in a damper. As is known in the art, lime mainly comprises calcium oxide, but may contain measurable amounts of other compounds including silica, alumina and / or other metal cations including magnesium, iron, etc. Thus, when lime or slaked lime is used as the inorganic base, the main product will be calcium cyanide; however, given the other ingredients in the lime, there may be measurable amounts of other impurities that contain magnesium, iron, silica, alumina, etc. The presence of such impurities is not harmful to the applications proposed here. In fact, the ability to use an organic base, such as lime, of lower purity and cost is a distinct advantage according to the invention. Lime is a currently preferred inorganic base because it is substantially less expensive than caustic soda (NaOH) or other inorganic bases. In general, caustic soda that is frequently not produced in areas near the mine, and in this way, the caustic soda can be shipped for long distances as a 50% aqueous solution or in some cases as caustic soda in flakes. Such boarding can be prohibitively expensive. On the other hand, lime is generally produced locally (for example, in areas near the mine) and is substantially cheaper to ship and buy. In fact, many mines will already have a supply of lime or slaked lime on hand, because lime is often added to the leach solution to ensure that the leach solution maintains a pH range of about 9 to about 10.5. . Lime is also often used to neutralize effluent from autoclave or calcination processes in the mine. In this way, the lime needed to neutralize the HCN may already be available at the mine site. It should also be noted that the reaction equipment needed to produce the calcium cyanide directly at the mine site could also be designed as a pre-fabricated product. In other words, the reaction equipment used to produce the cyanide can be manufactured and assembled using readily available parts and accessories, and then shipped (via freight, slides, etc.) to the site of the mine. Once this pre-fabricated reaction equipment arrives, it can be quickly constituted and used by the producer to manufacture calcium cyanide. In addition, when the producer finishes his cyanide production, the reaction equipment could be sold or shipped to another mine, etc. Due to the simplicity of the decomposition process of the formamide described above, the reaction processing equipment can be easily scaled up to meet the desired production requirements. In addition, the reaction processing equipment is relatively simple to operate. This will be particularly advantageous when operating in remote locations where expert work is expensive and not widely available. Referring now to Figure 2, a flow diagram of a second method 220 of the present invention is illustrated. Method 220 again represents a method that allows cyanide to be produced directly at the site of the mine. As noted above, this cyanide production at the mine site can provide the mine with significant advantages. Contrary to method 120 described above, the first step in method 220 involves obtaining 222 of the initial materials for the Andrussow process to produce HCN. As described above, these initial materials include gas natural and ammonia. However, because these initial materials may not be readily available at the mine site or may be expensive and / or difficult to ship, methanol (CH3OH) and urea (CO (NH2) 2) may be used as the materials initials. Method 240, shown in Figure 3, is substantially the same as method 220, except that step 242 replaces step 222. Step 242 includes obtaining a supply of methanol and urea at the mine site. , as initial materials to prepare HCN. Other embodiments may also be performed, in which the raw materials are a combination of some of the aforementioned reagents, such as natural gas and urea or methanol and ammonia. Urea is available in large quantities and is used as a fertilizer throughout the world. This is shipped and used as a non-hazardous material. Methanol is also a comfort chemical in the world and can be easily shipped. It is known that neither urea nor methanol are a terrorist threat. It is known that urea decomposes into ammonia. For example, U.S. Patent Nos. 5,252,308, 2,797,148 and 3,718,731 describe the ways in which urea can be decomposed into ammonia. These patents are incorporated by reference herein and show the methods / reaction conditions by which these reactions can take place. Again, once adequate starting materials have been obtained, the next step in method 220 is to react 224 the initial materials at the site of the mine to form an amount of HCN. Such reactions are known in the art. For example, information regarding the reaction of ammonia and methanol to produce HCN is also known. Methanol together with propylene and ammonia are currently used to produce HCN on catalysts at lower temperatures as part of the process to form acrylonitrile (which forms HCN as a byproduct). In these reactions, the excess ammonia is reacted with methanol to produce a greater amount of HCN. As is known in the art, such reactions may involve fixed and fluidized beds operating at lower temperatures. In fact, the following patents, which are all incorporated by reference herein, teach specific information that may be useful in performing the reaction of ammonia / urea with methanol / methane to produce HCN: United States Patent No 5,158,787, U.S. Patent No. 3,911,089, • U.S. Patent No. 4,485,079, U.S. Patent No. 5,288,473, U.S. Patent No. 3,716,496; U.S. Patent No. 3,988,359, U.S. Patent No. 4,511,548 U.S. Patent No. 718,112; British Patent No. 913836; Japanese Patent No. JP 74-87,474; Japanese Patent No. JP 79-08655; Japanese Patent No. JP 78-35232; and German Patent No. DE 1,143,497 (without oxygen present) Once the HCN supply has been formed, the next step is to neutralize HCN 226 at the mine site to produce the calcium cyanide. The processes for step 226 are similar and / or identical to the methods and processes associated with the processing step 126 described above in conjunction with Figure 1. For purposes of brevity, however, this discussion will not be repeated; rather, the reader can simply examine the previous description for more information. It should also be noted that lime is not widely used as the inorganic base of choice for the Andrussow process, due to the possibility that some of the calcium (or other metal cations) cations will react with carbon dioxide and form a precipitate. carbonate of calcium (or some other type of precipitate). In addition, Ca (CN) 2 has not been a desirable alkaline cyanide, because it can not be dried and formed into briquettes, in a manner contrary to NaCN. In addition, Ca (CN) 2 can only be concentrated to approximately 15-17% by weight, which is substantially more dilute than concentrated NaCN solutions. In this way, boarding costs for the Ca (CN) are significantly higher than the boarding costs for the NaCN. Another reason why Ca (CN) 2 is not a desirable alkaline cyanide is because it decomposes at higher temperatures and at higher concentrations than NaCN. As a result, cyanide manufacturers have generally avoided making Ca (CN) 2. However, according to the present invention, the ability to manufacture an aqueous solution of Ca (CN) 2 directly at a mine site, as necessary for direct use, will eliminate the above advantages. Referring now to Figure 4, a flow diagram of a different method 320 of the present invention is illustrated. Method 320 is a method for mining precious metals that use calcium cyanide. The first step of this method involves the production of 322 calcium cyanide at the mine site, using at least one initial material. This production step can be achieved using any of the methods 120, 220, 240 previously described. Consequently, the initial material (s) can be formamide, methane and ammonia, methanol and urea, etc. The next step in the 320 mining process is to leach 324 the ore with the cyanide solution. The manner in which such leaching can be performed is known in the art. Once this leaching 324 has been carried out, the mother solution is then collected 326. The mother solution is the solution that is obtained after the leaching has been carried out. This stock solution will contain a quantity of precious metal ions bonded to the cyanide anions as an aqueous complex. Again, the precise methods / processes for obtaining this stock solution are well known in the art and are currently being practiced in all precious metal mines that use cyanide. Finally, the last step 328 in method 320 is to recover the precious metal from the stock solution. The exact method for recovery 328 of the precious metal will depend on a variety of factors including cost, availability of reagents, etc. Of course, all of these specific methods / steps associated with the recovery of the precious metal are well known in the mining industry and will fall within the scope of the present invention. The present invention can be exemplified in other specific forms without departing from their structures, methods or other essential characteristics as they are widely described here and claimed later. The modalities described are going to be considered in all aspects only as illustrative and not as restrictive. The scope of the invention is, therefore, indicated by the appended claims, rather than by the foregoing description. All changes that fall within the meaning and range of equivalency of the claims have to be encompassed within their scope.

It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention is that which is clear from the present description of the invention.

Claims (3)

CLAIMS Having described the invention as above, the content of the following claims is claimed as property:

1. A method for producing an aqueous solution of calcium cyanide directly at a mine site, characterized in that it comprises: obtaining a supply of formamide at the site of the mine; the reaction of formamide at the site of the mine to form a quantity of hydrogen cyanide; and contacting the amount of hydrogen cyanide at the mine site with slaked lime to produce a supply of calcium cyanide.

2. The method according to claim 1, characterized in that the slaked lime comprises mainly calcium hydroxide.

3. The method of compliance with the claim 1, characterized in that it also comprises the step of diluting the calcium cyanide to concentrations that can be used at the mine site in at least one mining operation. . A method to produce an aqueous solution of calcium cyanide directly at a mine site, characterized in that it comprises: obtaining a supply of ammonia at the site of the mine; Obtaining a methanol site at the mine site; the reaction of ammonia and methanol at the site of the mine to produce a quantity of hydrogen cyanide; and contacting the amount of hydrogen cyanide at the mine site with slaked lime to produce a supply of calcium cyanide. 5. The method according to claim 4, characterized in that the ammonia is obtained by the decomposition of the urea. 6. The method of compliance with the claim 4, characterized in that the slaked lime comprises mainly calcium hydroxide. The method according to claim 4, characterized in that it further comprises the step of diluting the calcium cyanide to concentrations that can be used at the site of the mine in at least one mining operation. 8. A method of mining precious metals using a calcium cyanide solution, characterized in that it comprises: producing the calcium cyanide solution in a site of a mine from at least one initial material; leach an area of interest at the mine site with the cyanide solution; collect a stock solution containing a quantity of precious metal ions, linked to the cyanide anions as an aqueous complex; and recover the precious metal ions from the mother solution. The mining method according to claim 8, characterized in that the production step comprises the decomposition of the formamide to form hydrogen cyanide and neutralizing the hydrogen cyanide with slaked lime to form the calcium cyanide solution. The mining method according to claim 8, characterized in that the production step comprises the creation of the cyanide solution using methanol and urea as the starting materials. The mining method according to claim 8, characterized in that the production step comprises the production of a quantity of hydrogen cyanide which is then converted to a solution of calcium cyanide using lime or slaked lime. 12. A method for producing calcium cyanide directly at a site of an amine, characterized in that it comprises: obtaining a supply of the initial materials at the site of the mine for use in a process of Andrussow to produce hydrogenated cyanide reaction of the initial materials at the site of the mine to produce a quantity of hydrogen cyanide; and process the amount of hydrogen cyanide at the mine site to produce a supply of calcium cyanide. 13. The method according to the claim 12, characterized in that the processing step is accomplished by contacting the hydrogen cyanide with slaked lime to produce calcium cyanide.