SUMMARY OF THE WORK DONE IN DECEMBER 2014 AND JANUARY 2015
With reference to the information contained in the report of November 2014, there are two important changes: the first one refers to an alternative to the expected pyrolysis process (action B4) and the second one to the methodology applied for electrolysis (action B5).
Action B4 – Why the alternative to the expected pyrolysis process
In the control phase of the emissions of gases from the pyrolysis of the sandwich it has been noted the presence of traces of fluoride. Although the first analysis of the backsheet had not shown these values, more accurate tests have established that about 50% of the panels include a microlayer of fluorinated plastic. Given that the Sasil’s pyrolysis equipment is not authorized for the treatment of fluorinated plastics, we have considered an alternative process to incinerate the sandwich, as the treatment of the fumes generated by the combustion is much simpler and does not imply unknown technological processes. It is a widespread process and normally it is applied downstream of the incineration plant. Moreover,a lower consumption of nitric acid in the process of leaching related to to the incineration combustion was found. Therefore it was decided to investigate this incineration process considering the possibility to treat all the sandwiches, regardless of whether or not they contain fluorinated plastic.
Even though this choice can represent a disadvantage if considered in terms of energy efficiency only, still it must be taken into account that the incineration of combustion, unlike the pyrolysis, allows to treat 100% of the panels, and not only those without fluorinated plastics, thus avoiding to send to landfill 50% of previously treated sandwich. Furthermore, the incineration from combustion allows a considerable saving of HNO3 (25%). Also we have considered the option of treating the sandwiches of fluorinated plastic with a technology patented by a company of Novara, but we have estimated that the industrialization of such a system is very complex and it also has to do with solvents which, however, have a certain environmental impact.
In conclusion, we are researching an industrial incinerator capable of ensuring this intermediate step, between detachment of sandwich (Action B3) and the successive steps of leaching and electrolysis (action B5), and thus replacing the pyrolysis (action B4). In terms of environmental impact, the disadvantage of having to transfer on the outward (by the SASIL establishment to the incinerator) about 770 t / year of sandwich, and in return (by the incinerator to the SASIL establishment) about 300 t / year of ashes, it is compensated by the lower use of nitric acid and the consequent lower consumption of calcium hydroxide, with relative lower production of calcium nitrate in solution. This alternative to pyrolysis involves a modification of the development program of the FRELP project that, at this stage, will not include any longer the adaptation and use of the existing Sasil pyrolysis plant when performing B4 action. But it will have to rely on an external incinerator to reduce to ashes the sandwich by heat treatment.
Optimization process of chemical recovery: characterization of the ashes before leaching.
After the thermal cracking, the ashes are separated by sieving with a mesh sieve (ASTM 3 mm). Subsequently, the small pieces of metal, probably coming from the disintegration of the metal grid during thermal cracking, are carefully removed by hand. This represents about 30% of weight of ash, with respect to the initial weight of the plastic sheet, after the detachment of the glass. The obtained material is shown below:
Action B5 – METHODOLOGY OF ELECTROLYSIS
In December 2014 and January 2015 the experiments carried out by Darsa with samples of ash resulting from the pyrolysis in laboratory have allowed to get closer to the completion of the last stage of the recovery process for the electrolysis of metals, particularly silver and copper. From these experiences it emerges that the process has a high recovery yield, close to 95%, but requires constant power conditions in order to calibrate the control parameters of the electrolysis. In particular it is necessary, in the acid solution, to minimize the presence of free nitric acid, and to act with additions of silver, copper, aluminum and various metals fairly constant. We will have to intensify such tests on significant quantities of ashes (few kg), in order to mediate the presence of metals and thus obtaining a calibration of electrolysis aimed at constant recoveries and efficiencies that will lead to an enhancement of the functional selectivity. In order to minimize the use of nitric acid for leaching, we have also considered the use of a concentrator to fractional distillation before the electrolysis phase. The solution of this problem with electrolysis in function of leaching, for the recovery of copper and silver, will be the subject of further investigation in the next three months, jointly by Sasil and SSV for what refers to leaching, and by Darsa for electrolysis.
Description of the pilot test in Darsa
The goal of the pilot test was to find the best condition for the collection of silver from the solution of nitric acid for galvanic deposition, performed with the Electro-winning DEW M 100 that works with a constant value of Ampere and with various concentrations and pH of the solution.
It is expected to conclude the last tests of electrolysis at Darsa by April. Meanwhile, in order to obtain a precise comparison between the various options that allow a final decision on the most suitable strategy, several LCA about technically feasible scenarios will be prepared. Subsequently we should proceed to the submission of a project amendment (in May), so that it can be completed by the presentation of the mid-term report expected by the end of July.