Patent Description:
The proposed process is low-cost, eco-friendly, easily manageable and innovative in that it is based on the "zero-liquid discharge" (ZLD) philosophy, with recycling of most of the water used.

The generation of waste of electric and electronic equipment (WEEE) has been continuously increasing during the last few decades.

In each of these devices at least one electronic board is present. This type of waste is somewhat difficult to treat owing to the presence of more than <NUM> different elements in the components. As regards the materials, about <NUM>% by weight consists of plastics, <NUM>% by weight of ceramic materials and <NUM>% by weight of base, precious and special chemical elements. Consequently, various physical/chemical technologies have been proposed for the treatment of waste electronic boards. The physical methods, although more sustainable in environmental terms and very efficient with regard to the recovery of the base metals (<CIT>), have the drawback that they use a large amount of energy, but in particular high percentages of precious metals are lost in the various fractions which are separated.

Chemical processes based on high-temperature treatments at present constitute the technologies which are most widely used for such waste. However, these processes have a high energy consumption. Moreover, the formation of toxic fumes, loss of plastics and high capital investment represent other important factors which must be taken into account.

Hydrometallurgical processes, combined with pre-treatments, are regarded as being the most suitable technologies. There exist already published patents which use these technologies (<CIT>; <CIT>; <CIT>). In general, they use highly toxic chemical reagents (for example aqua regia) which could generate toxic gases and also require special equipment resistant to these chemical compounds. Moreover, large quantities of liquid and solid waste are also generated.

It is therefore required to develop a technology able to solve the problems described above. The hydrometallurgical method described in this invention offers the main advantages of easy control and management of the various operations, minimization of the water consumption, good levels of recovery of copper, tin, gold and silver, as well as no discharge of waste solutions. Studies relating to this type of treatment were conducted during the preparation and drafting of the previous works published by the inventors (Birloaga et al, <NUM>, <NUM>, <NUM>). These studies formed the initial basis for the subject of the present invention.

The present invention relates to a hydrometallurgical process which differs mainly from the previously published data (Birloaga et al. , <NUM>, <NUM>, <NUM>) in that a further metal (tin) is recovered, the process waste solutions are treated and the water recovered via the treatment is used again.

The flocculation process based on EPI-DMA used for the treatment of waste water is mentioned by the patent <CIT>, but the specific application of this chemical compound for the selective recovery of tin from the solution for leaching electronic boards using sulfuric acid and hydrogen peroxide is not mentioned in any patent and in scientific literature in general.

Many of the methods used for the recovery of metastannic acid are based on neutralization with alkaline reagents or on centrifuging.

Moreover, in the present invention, the use of EPI-DMA for the flocculation of colloidal tin does not require any neutralization step, since the reagent is added directly into the solution as such obtained from leaching.

The treatment of the waste solutions resulting from the process for recovery of gold and silver from the electronic boards (leaching using thiourea, ferric sulfate and sulfuric acid, neutralization using sodium hydroxide and cementation using metallic zinc powder) by means of Fenton's reagents and calcium hydroxide (Ca(OH)<NUM> is not cited in any document of the known scientific and patent literature.

Despite the fact that the Fenton process is widely used for degradation of organic compounds, it has never been used for the treatment of waste solutions of this type.

A further innovative feature of the invention consists in the integration of this treatment in the mixture of heterogeneous waste waters and solutions resulting from different stages of the recycling process.

The innovative aspect of the entire hydrometallurgical treatment also consists in the application of the "zero liquid discharge" philosophy to the recovery of Au, Ag, Cu, Sn from boards, the entire recovery process comprising all the various treatments and not each procedure separately from the other ones.

The hydrometallurgical treatment of waste electronic boards (PCBs) is claimed in the accompanying claims.

Further objects will become clear from the detailed description of the invention which follows.

<FIG> shows a flow chart of the invention.

The treatment method according to the invention comprises the following main stages:.

Since the reagents used in this process are not based on the use of cyanides and high recovery levels are achieved, this method is considered to be economical and more sustainable from an environmental point of view compared to the currently used processes.

The flow chart illustrating the process is shown in <FIG>.

With reference to <FIG>, the preferred, but non-limiting version of the invention is described in detail below.

Stages (i) and (ii) are carried out using methods and procedures known per se.

The electronic boards which have already been decontaminated and stripped of the undesirable electronic components and already ground are leached by means of a first counter-current leaching stage. This first leaching operation is preferably carried out in two steps, during each of which the ground material obtained from the pre-treatments is treated and separated beforehand into two fractions. In the first step a freshly prepared acid solution is used, while in the second step the same solution is used again after filtering (i.e. after separating it from the solid fraction). In this way, a solution with a greater concentration of base metals, in particular copper and tin, is obtained: each of the two steps has a solid concentration of <NUM>-<NUM>% wt/vol in a solution of sulfuric acid with concentration of <NUM>-<NUM> mol/l, and oxygenated water, typically in an amount of <NUM>% vol. The necessary conditions for achieving a successful outcome of the process are mechanical stirring in a reactor, at a speed of at least <NUM>-<NUM> rpm, and ambient temperature and pressure conditions for a duration of <NUM>-<NUM> for each of the two steps. In the test conditions indicated, practically all of the copper is extracted as sulfate and use of the reagents is optimized since the mass of metal extracted per unit of acid used is maximum. At the end of each step, the solution, in which the copper is solubilized as sulfate and the tin is present as metastannic acid, is separated from the solid by means of filtration and the solid, consisting of the ground boards in which gold and silver to be recovered are still present, is washed with a sufficient quantity of water, after counter-current leaching. This washing water is recovered and may be used for the preparation of a new solution for a further copper leaching procedure as described above. This is due to its highly acid nature, since it removes nearly all the acids and base metal complexes retained in the solid after its separation by solution.

At the end of the first leaching process the liquid is separated by means of filtration and transferred to the second stage (ii), and likewise the ground solid is separated and transferred to the following stage (v).

Flocculation is carried out on the waters obtained from stage (iii) and containing copper mainly as sulfate and tin as metastannic acid, which were obtained after separation of the solid by means of filtering.

During the flocculation stage the tin is recovered by means of precipitation with flocculating agents. Flocculation is preferably carried out in two steps. The metastannic acid is very difficult to recover from the solution obtained in the previous stage (iii), using a simple filtration procedure, so that the use of cationic polyelectrolytes is considered appropriate. This type of polyelectrolyte, which belongs to the group of polyamine flocculants, known per se, such as the epichlorohydrin-dimethylamine polyamines [EPI-DMA], is able to capture and aggregate the colloidal particles in large floccules which may be more easily separated by means of filtration from the solution. Then a solution of <NUM>% wt polyamine is added to the solution which is rich in (non-precious) base metals obtained during the previous step (iii). This process is carried out with constant stirring at <NUM>-<NUM> rpm for about <NUM>-<NUM>. The solid consisting of metastannic acid is separated by means of filtration and washed with water. The final recovery of tin, in the form of metastannic acid aggregated in particles of a certain size, is more than <NUM>% of the initial quantity of tin contained in the electronic boards.

Cementation is performed using zinc powder in the solution obtained from stage (iv A) without the flocculent precipitate, which has been removed. The washing water obtained from stage (iii) may be advantageously added to this solution in order to extract the copper content. Cementation is performed by adding metallic zinc powder in a greater quantity, generally <NUM>-<NUM>%, than the stoichiometric quantity required by the chemical reaction. Cementation is carried out under ambient temperature and pressure conditions while stirring at <NUM>-<NUM> rpm for <NUM>-<NUM>. At the end, the metallic precipitate is separated from the solution by means of filtration and then washed with a suitable volume of water. The degree of total recovery (leaching and cementation) of the copper is greater than <NUM>%.

The resultant solution and the washing water may be sold to companies which produce fertilizers for intensive agriculture methods.

The second leaching is performed in co-current conditions on the remaining solid obtained from stage (iii); preferably the solid is divided up into two or more fractions. In order to carry out the second leaching operation a solution containing the following is used: thiourea as reagent in a concentration of <NUM>-<NUM>/l, ferric sulfate as oxidizer in a concentration of <NUM>-<NUM>/l, and <NUM>-<NUM> mol/l of sulfuric acid so as to ensure an acid reaction environment, with pH of between <NUM> and <NUM>, and the solid is placed in the solution thus prepared. The reaction is carried out with continuous stirring at <NUM>-<NUM> rpm for <NUM>-<NUM> in ambient temperature and pressure conditions and with a concentration of solid resulting from filtration during the counter-current process (stage iii) equal to <NUM>-<NUM>% wt/vol. Then, after filtration and subsequent washing of the remaining solid, <NUM>-<NUM>/l f of thiourea are added to the filtered solution and it is used again to leach further gold and silver from other remaining solid obtained again from filtration during the process of stage (iii). After separation of the liquid (solution containing gold and silver) from the solid, washing of this solid is performed using the same water as that used in the previous washing operation.

In this way, by reusing both the leaching solution and the water for washing the solid, preferably the two or more fractions of solids, it is possible to increase approximately twofold the concentration of gold and silver in the two or more liquid streams.

Moreover, cost savings are achieved since the use of chemical reagents and water is reduced.

The mixture of solution and washing water obtained in stage (v) is neutralized with a basic compound, preferably with a <NUM>% wt/vol solution of sodium hydroxide, until a pH of <NUM>-<NUM> is reached. This procedure is carried out in ambient temperature and pressure conditions with stirring at <NUM>-<NUM> rpm for <NUM>-<NUM>.

Then metallic zinc powder is added in a suitable amount, i.e. equal to five to twenty times the concentration of the gold in the solution. Once the reaction time has ended, the particles of metallic silver and gold which have formed are separated from the solution by means of filtration and washed with a certain volume of water. The total amount of precious metals recovered is more than <NUM>%.

The resultant solution and the washing water from the previous stage (vi) are mixed together and more than <NUM>% of the total volume is recycled in stage (v) for preparation of a new leaching solution. The remaining mixture fraction is treated using two different methods:.

Owing to the chemical reaction between thiourea and ferric sulfate, dangerous organic complexes are formed, such that a suitable treatment of the waste solution is required. The Fenton reagent is widely used because of its powerful oxidizing properties. This reagent consists of a mixture of oxygenated water and ferrous sulfate which produce hydroxyl ions which cause degradation of the organic substances. In connection with the present invention, this process is carried out in ambient conditions for <NUM> while stirring at <NUM> rpm. Once this procedure has been completed, the waste is neutralized, for example with a <NUM>% wt/vol solution of Ca(OH)<NUM> which causes precipitation of the inorganic elements at a pH of <NUM>-<NUM>. The treatment lasts <NUM>-<NUM> and is carried out in ambient conditions while stirring at <NUM>-<NUM> rpm.

The water thus treated is recovered by means of filtration and recycled in stage (iii) as water for the preparation of the new leaching solution. The remaining solid may be further treated for recovery of the palladium (<NPL>) in an external plant, or removed for disposal in a tip or in a waste-to-energy plant.

The present invention is not limited to the form and sequence of operations described above, but the details thereof may be varied.

The following examples are to be regarded as illustrating the present invention and are not to be regarded as limiting in any way the scope thereof.

Claim 1:
Process for the treatment of waste electronic boards, which comprises the following main stages:
(i) removal of the electronic components containing dangerous substances from the surface of the boards; followed by
(ii) crushing and grinding the boards to a particle size of less than <NUM>;
(iii) bringing the ground boards into contact with a first leaching solution, in which the first leaching is performed in counter-current conditions and carried out in two steps in each of which the ground material obtained from the treatments (i) and (ii) is treated;
wherein in a first step a freshly prepared acid solution is used, said solution comprising sulfuric acid in a concentration of <NUM>-<NUM> mol/l and hydrogen peroxide, and in a second step the same solution used in the first step is used again after separating it from a first solid fraction; and wherein, in order to recover tin and copper, at the end of stage (iii) a solution containing copper solubilized as sulfate and tin as metastannic acid is separated from a second solid fraction by means of filtration and the solid fractions, consisting of the ground boards in which gold and silver to be recovered are still present, is washed with water;
(iv) selective recovery of the tin and copper from the solution of stage (iii) by means of flocculation and cementation and precipitation techniques, wherein the tin is recovered by means of a flocculation stage with flocculating agents in order to precipitate a solid flocculent precipitate consisting of metastannic acid which is separated by means of filtration from the solution of stage (iii), and copper is recovered by means of a cementation stage carried out by adding zinc powder to the solution obtained from the flocculation stage, without the flocculent precipitate which has been removed, a metallic precipitate being separated from the solution by means of filtration at the end of cementation;
(v) bringing the solid fractions obtained from the first leaching treatment of stage (iii) into contact with a second leaching solution including thiourea, ferric sulfate and sulfuric acid, in which the second leaching is performed in order to extract gold and silver;
(vi) recovery of gold and silver from the solution obtained from stage (v) by means of neutralization and cementation.