WO2005085483A1

WO2005085483A1 - Metal leaching

Info

Publication number: WO2005085483A1
Application number: PCT/SE2005/000338
Authority: WO
Inventors: Raymond F. Swanson
Original assignee: Swanson Raymond F
Priority date: 2004-03-10
Filing date: 2005-03-09
Publication date: 2005-09-15
Also published as: SE0400589D0; SE527454C2; SE0400589L

Abstract

A method for leaching or dissolving metals from a metal containing material is described. According to this method the metal containing material is brought into contact with a mixture comprising three different chemicals i.e. a solution of a metal leachant, a pH-adjuster, in an amount sufficient to lower the pH of the solution of the metal leachant to a value below 3.5, and an oxidizer, in an amount sufficient to provide an oxidation/reduction potential (ORP) of the solution that will lead to oxidation of the metals.

Description

METAL LEACHING

Technical field of the invention The present invention relates to metal leaching, and more specifically to a method for removing metals from metal containing materials, such as soil and crushed rock.

Background of the invention There are many different materials, occurring naturally or being introduced into the nature by humans, that contain metals. These materials may in some cases constitute environmental hazards in that metal ions may leak into ground-water. Examples of such materials are soil on shooting-ranges which often comprises large amounts of lead and other metals, or crushed rock that has been discarded from mining operations. It is highly desirable to remove these contaminating metals from the soil and other materials.. Several methods for recovering metals, especially precious metals, from metal bearing materials have earlier been described. For example US 4,778,519 describes a method for recovering gold and silver form precious metal bearing materials such as ores and jewelry scrap. According to this method the gold and silver are extracted from the precious metal bearing material with a thiourea solution to form a thiourea leach which then is contacted with carbon to adsorb the gold and the silver. US 4,816,235 describes a method for obtaining silver and manganese metal from a silver-manganese ore, including the step of leaching the ore with acidified thiourea. WO 91/13177 describes extraction and recovery of gold. The process according to WO 91/13177 comprises treating gold-bearing material with an acidic lixiviant solution containing thiourea, ferric ions and a comple- xing agent for ferric ions, such as di- and tri- carboxylic acids, phosphoric acid and phosphate salts, thiocyanates, fluorides, fluosilicic acid and fluosili- cate salts, EDTA and EDTA salts. The complexing agent for ferric ions is a very important part of the invention described therein. Derwent's Abstract of RU 2070588 describes process for metal leaching. However, it only describes separation of precious metals from ores and concentrates by sulfuric acid dissolution of admixtures, followed by leaching of precious metals with acidic solution of thiourea in the presence of iron sulfate. All the methods described in the above mentioned patent documents include the use of thiourea. However, several reports indicate thiourea as being carcinogenous, and use thereof should therefor be avoided. The above mentioned methods also have other disadvantages. Summary of the invention The object of the invention is to provide a method for safe and efficient removal of metals from materials of the above mentioned type. During the work leading to the present invention it was found that it is possible to leach or dissolve all types of metals, including precious metals, from different metal containing materials, such as soil or crushed rock, by exposing the material to a combination of three different chemicals. The advantages with the method according to the invention is that the method can be performed at low temperatures which means that no heating is necessary, that no exothermic heat is generated during the dissolution process, that the method can be performed at atmospheric pressure, that only environmentally safe chemicals are used, and that very simple and inexpensive equipment can be used to perform the method. Thus, the present invention relates to a method for removing metals from a metal containing material, wherein the metal containing material, in a finely divided form, is brought into contact with three different chemicals: I) a solution of a metal leachant, which metal leachant preferably comprises urea, II) a pH-adjuster, preferably hydrochloric acid, sulfuric acid or sodium sulfate, in an amount sufficient to lower the pH of the solution of the metal leachant to a value below 3.5, and III) an oxidizer, preferably ferric chloride (Fe₂Cl₆) , in an amount sufficient to provide a suitable oxidation/reduction potential (ORP) of the solution comprising the metal leachant and the pH-adjuster. The characterizing features of the invention will be evident from the following description and the appended claims . Detailed description of the invention Thus, the present invention relates to a method for dissolving metals from different metal containing materials. Metal containing materials suitable for the treatment according to the invention are for example metal contaminated soil, such as soil on shooting-range, debris, such as crushed rock, from mining refining opera- tions or mine dump materials from the processing of ores. The metal containing materials treated according to the invention should occur in or be transformed into a finely divided, or crushed form. The purpose for dissolving the metals from the metal containing materials may be the cleaning of contaminated materials for environmental reasons, or the extraction of metals for marketing reasons . The term "metal" used herein and in the appended claims encompass all metals, including precious metals. The three different chemicals used according to the invention all have some metal dissolving effect when they are used alone, but the effect of the combination is highly synergistic and good results are obtained through use of the combination compared to use of the different chemicals separately. In the method according to the invention three different chemicals are thus used to obtain the desired effect. The first chemical, Chemical I, is called herein a metal leachant. As the name suggests, this metal leaching performs the main leaching of the metals. The choice of the metal leachant is based on the application and the physical environment in which the process is performed. Preferably the metal leachant used according to the invention is or comprises urea (H₂N-CO-NH₂) . The second chemical, Chemical II, used in the method according to the invention is a pH-adjuster. When added to the solution of the metal leachant this pH-adjuster lowers the pH to a value below 3.5, preferably to a value below 3, and most preferably to a value between 1.7 and 2.8. There is no lower limit. The pH-adjusters preferred according to the present invention are hydrochloric acid (HCl) , sodium sulfate (Na₂S0) , and sulfuric acid (H₂S0) . Hydrochloric acid is the most preferred pH-adjuster. The selection of the pH-adjuster is based on the total composition of the media used in the process. The third chemical, Chemical III, used in the method according to the invention is an oxidizer. This chemical is used to control the oxidation/reduction potential (below called ORP) of the media used in the process. This oxidizer is preferably a metal chloride, and most preferably ferric chloride (Fe₂Cle) . The method comprises use of an appropriate solution of Chemical I. When urea is used an appropriate concentration may be 2-5%, e.g. 2% or 5%. Chemical II is added to the solution of Chemical I. The amount of Chemical II used is the amount that provides the desired pH, i.e. a pH lower than 3.5, preferably lower than 3, and most preferably between 1.7 and 2.8. The metal containing material is added followed by addition of Chemical III until a desired ORP level of the solution comprising Chemical I, Chemical II, Chemical III and the metal containing material- below this solution is referred to by the term "fluid" - is obtained. The term ORP level used herein relates to the potential necessary for accomplishing oxidation. The ORP, or redox, value of a solution is a measure of the electron activity in the solution. The ORP level is measured in a known way by use of an electrode. The suitable ORP level varies depended on the metals to be oxidized, but as an example it can be mentioned that to dissolve all metals from a metal containing material an ORP level in the order of 200-700 mV is needed, depending on the fluid and the metals to be dissolved. When the metals from the metal containing material are dissolved the metals are suspended in ionic form in the fluid. As the concentration of metal ions increases in the fluid the ORP level decreases. When the ORP decreases to below a specific value the dissolution of the metals from the metal containing material will stop. It is then optional to either extract the metal ions, as described below, or to add fresh Chemical III, which will lead to an increase of the ORP level and further dissolution of metals. It is thus possible to supervise the oxidation process by controlling the ORP level. It is preferable to maintain an essentially constant ORP level until all metals are dissolved, either by constantly adding Chemical III to the fluid or by adding Chemical III at suitable intervals. In order to boost the ORP level and obtain even better results it is also possible, and desirable, to add hydrogen peroxide, H₂0₂, to the fluid. It is preferable to use a concentration of hydrogen peroxide of about 5%. It is also possible to use air, oxygen or ozone instead of hydrogen peroxide. Preferably hydrogen peroxide or dissolved atmospheric air is used. The gas is then injected into and dissolved in the fluid. In order to obtain a better blending of the different components it may be advantageous, and in some cases necessary, to agitate the fluid, either constantly or with intervals. The metals may be recovered from the fluid by any appropriate known recovery technique, such as precipitation, cementation, solvent extraction, electroplating, ion exchange or carbon column reduction. When the metal ions have been removed, the fluid may be reactivated and reused for additional metal leaching. According to one embodiment of the present invention it is possible to perform a selective leaching or dissolution of the metals. Then, only one or some of the metals in the metal containing material is dissolved. This is accomplished by choosing an ORP level that is high enough to enable dissolution of some metals but not of all metals. It is also possible to selectively dissolve one or a few metals at a time. The process is then initiated at a relatively low ORP level, which is just high enough to dissolve one or a few of the metals according to their activity. This metal or these metals are removed from the fluid and the ORP level is raised to a value at which some or all of the remaining metals are dissolved. By gradually increasing the ORP- level it is thus possible to remove one metal at a time from the metal containing material . In order to make the process even more efficient, it is possible to use a higher temperature and a higher pressure. However, it is preferable to operate at surrounding temperature and atmospheric pressure. Since low temperatures, atmospheric pressure and non-aggressive solutions and chemicals can be used in the method according to the invention, the method is a very safe way to dissolve metals. Practically, when the method according to the invention is used to clean contaminated soil, it is desirable to perform the method at the contamination site. It is possible to use conventional leaching techniques, such as heap leaching or dump leaching. The soil or crushed rock is placed in containment, such as a processing vessel or a leaching pad. The containment wall should be non- metallic and should not have any hydrocarbon, carbon or chloride materials in contact with the contaminated soil or rock. A suitable material is polyethylene. The three chemicals are then mixed as described above, and the leaching process begins. Fluid comprising the metal ions are constantly removed from the containment through a filtering system that collects the metals from the fluid. The fluid is (after reconditioning to adjust the ORP level and optionally also the pH) then recirculated into the container. When no metals remain in the leached materials, they can be replaced to the site they were removed from. The equipment, which mainly consists of a leach pad, chemical holding tanks, pumps and filters, which are portable and easy to handle, are then transported to a new contaminated site. The stripped fluid can either be cleaned and disposed or transported to the next site to be reused. The metals are collected from the filters as a metallic sludge and are then further processed and marketed. The invention will now be further explained in the following examples. These examples are only intended to illustrate the invention and should in no way be considered to limit the scope of the invention. Brief description of the drawings In the example below reference is made to the appended drawings on which figure 1 and figure 2 shows graphs illustrating the result of dissolution of metals from soil at a shooting-range. Example 1 The method according to the invention was used to clean soil at Kviberg shooting-range in Gδteborg, Sweden. 155.3 kg of soil was collected from the shooting- range. The dry weight of this soil was 143 kg. This soil was treated with the method according to the invention as follows. 600 1 of a 2% solution of urea (Chemical I) was provided and the pH of the solution was adjusted to 1.8 by addition of 30% HCl (Chemical II) . The soil was added and the ORP of the fluid was adjusted to approximately 550 mV by addition of Fe₂Cl₆ (Chemical III) . Samples off 50 ml were taken from the fluid day 2, day 3, day 5, day β, day 7 and day 9, and the contents of metals in the samples were analyzed at a commercial laboratory, Gδteborgs Kemanalys AB, Gδteborg, Sweden, by use of inductive coupled plasma spectrometry and by VBB Viak, Malmδ, Sweden, by X-ray fluorescence measurements. The results are shown in table 1 below, and in appended figure 1. The graph in figure 1 illustrates the increase of the amount of metals in the fluid (i.e. the total dry weight of metals in the fluid) .

Table 1

Exemple 2 The method according to the invention was used to clean soil at Eskilstuna shooting-range in Eskilstuna, Sweden. 10 g (dry weight) of soil was collected from the shooting-range. This soil was treated with the method according to the invention as follows. 42 ml of a 5% solution of urea (Chemical I) was provided and the pH of the solution was adjusted to 1.3 by addition of 30% HCl (Chemical II) . The soil was added and the ORP of the fluid was adjusted to approximately 685 mV by addition of Fe₂Cl₆ (Chemical III) and H₂0₂. Samples of 5 ml were taken from the fluid day 1, day 2 and day 3, and the content of lead in the samples were analyzed at Sydkraft SAKAB, Kumla, Sweden, by use of X- ray fluorescence measurements. The results are shown in table 2 below, and in appended figure 2. The graph in figure 2 illustrates the recovered amount of lead in the fluid.

Exemple 3 Exemple 3 was done as example 2 except that the solution of urea was replaced by water.

Samples, 5 ml, were taken from the fluid day 1, day 2 and day 3, and the content of lead in the samples were analyzed at Sydkraft SAKAB, Kumla, Sweden, by use of X- ray fluorescence measurements. The results are shown in table 2 below, and in appended figure 2. The graph in figure 2 illustrates the recovered amount of lead in the fluid. Table 2

Claims

1. A method for removing metals from a metal containing material, characterized in that the metal containing material is brought into contact with a solution of a metal leachant comprising urea, a pH- adjuster in an amount sufficient to lower the pH of the solution of the metal leachant to a value below 3.5, and an oxidizer in an amount sufficient to provide an oxidation/reduction potential (ORP) of the solution comprising the metal leachant and the pH-adjuster that will lead to oxidation of the metals.

2. A method according to claim 1, wherein the ORP obtained by addition of the oxidizer is 200-700 mV.

3. A method according to claim 1 or 2, wherein the concentration of urea is approximately 2%.

4. A method according to claim 1 or 2, wherein the concentration of urea is approximately 5%.

5. A method according to any one of the claims 1-4, wherein the pH-adjuster is hydrochloric acid, sulfuric acid or sodium sulfate.

6. A method according to any one of the claims 1-5, wherein the oxidizer is ferric chloride (Fe₂Cl₆) .

7. A method according to any one of the claims 1-6, wherein the pH is adjusted with the pH-adjuster to a value below 3.

8. A method according to any one of the claims 1-7, wherein hydrogen peroxide is added.

9. A method according to any one of the claims 1-8, wherein dissolved atmospheric air is added.

10. A method according to any one of the claims 1-9, wherein the metal containing material is used in a finely divided form.

11. A method according to any one of the claims 1-9, wherein the metal containing material is used in an encapsulated or agglomerated form.

12. A method according to any one of the claims 1-11, wherein fresh oxidizer is added at suitable intervals to maintain the original ORP.

13. A method according to any one of the claims 1-11, wherein the ORP first is set to a value that will lead to the oxidation of one or several low oxidizing metals, and wherein the ORP is then, optionally gradually, increased to a value where higher oxidizing metals will oxidize, thus resulting in selective leaching of the metals.