A Mobile Pilot Plant for the Recovery of Precious and Critical Raw Materials

Abstract

In order to furtherly proceed with the recycling of raw materials from e-wastes, PCBs must be treated in a hydrometallurgical process able to extract useful materials from them. This chapter presents some details of the hydrometallurgical pilot plant developed in FENIX.

5.1 Introduction

Precious metals (PMs) are crucial in the global economy as they are key constituents of a vast number of industrial products and processes. Large amounts of wastes with various contents of precious metals are generated every year. The wastes of electrical and electronic equipment know the worldwide largest increment. With the current growing trends there is estimated that this amount will arrive to 120 metric tons/year by 2050 and the consumption of the raw materials with be two-fold [1]. The waste printed circuit boards represent an important secondary resource of precious metals (Au, Ag, Pd) but also of base metals (Cu, Zn, Ni, Sn, Pb, Fe, Al). As was expressed in the paper of Wand and Gaustad [2] the main economic drivers in the recycling of such waste, considering their concentrations and market price, are in the following order: Au, Pd, Cu, Ag, Pt, Sn and Ni. According to the study of Golev et al. [3], the waste printed circuit boards represent about 40% of the metal recovery value from the entirely equipment.

The application of hydrometallurgical methods for recycling is preferred to pyrometallurgical methods, as the latter usually require high temperatures, produce harmful gases (such as SO₂) and dust, and require high capital costs.

The first step of each hydrometallurgical recycling technique is represented by leaching. In order to achieve a high leaching efficiency of precious metals (PMs), the aqua regia, cyanide, thiol groups (thiourea, thiosulfate and thiocyanate), halides (chloride, iodide and bromide) with the presence of different oxidants (oxygen, ferric complexes, hydrogen peroxide, chlorine, bromide, iodine) are employed. The generations of highly polluted NOx and HCN gases, as well as various harmful elements in wastewater, have restricted the use of aqua regia and cyanide leaching systems. Table 5.1 presents a patent list of the hydrometallurgical processes with their brief overview that are used for base and precious metals recovery from WPCBs.

Table 5.1 Patents on waste printed circuit board treatment for recovery of both base and precious metals by hydrometallurgical procedures

The hydrometallurgical processes have gained the largest interest of application for waste printed circuit boards treatment. In addition the circular economy principle stared to be of high interest for researchers that activate in the field of WPCBs treatment. However, till present, for recovery of all elements that are present within the structure of WPCBs was not possible to be obtained using hydrometallurgical processes.

For FENIX Project, the authors of this chapter have developed and tested the efficiency of two hydrometallurgical technologies for e-waste recycling at both laboratory and pilot levels. For both processes, commercially named GOLD REC 1 [9] and GOLD REC 2 [10], patent applications have been deposited at both Italian and European levels. The main core was to recover both precious and base metals content from electronic waste and to use them as material for the production of metallic powders (USE CASE 1), 3D printing filaments (USE CASE 3) and jewelries (USE CASE 2). Within this chapter the processes description and a summary of the activities undertaken for this core achievement are presented.

5.2 Pilot Plant Design and Description by Process Performing

The pilot plant was designed considering its installation in a real industrial environment logistically useful to carry out experimental tests for the researchers involved in this project. The advantages to operate in this industrial site are:

• Working in a real environmental context;

• Availability of the e-waste necessary for the pilot plant;

• Availability of several services (compressed air, grinding section, electricity, working men for dismantling, grinding and other technical operations)

In this way the following action were carried out:

• a. Design and construction of the pilot plant;

• b. Testing activities (comparing in parallel the same results with pilot lab-scale tests)

• c. Production of some suitable amount of materials for their characterization and to be used for the other partners of the FENIX project.

The pilot plant was constructed within a container that was devised within three sections, namely: one section for operator and control panel, a second section that has a chemical reactor (R101) with an useful volume of 200 L and a working temperature of 70 °C; this reactor was and it is used for leaching, precipitation and cementation; a filter press (FP 101) for filtration of the solutions that have more than 1 g/L of solid content (e.g. solid residue of leaching or base metals precipitates) and a candle filter for solutions with a solid content lower than 1 g/L; two electrochemical cells (EC1—for base metals and EC2—for precious metals); one scrubber; the third section with 12 storing tanks (TK 101–106 for reagents and TK 107–112 for solutions and wastewater) (See Figs. 5.1, 5.2 and 5.3).

Fig. 5.1

FENIX hydrometallurgical plant—3D view

Fig. 5.2

P&I of the FENIX’s hydrometallurgical plant

Fig. 5.3

FENIX’s hydrometallurgical plant—real view

5.2.1 GOLD REC 1 Process Description

The hydrometallurgical process has started with the HydroWEEE EU Project and fully developed within the HydroWEEE DEMO EU Project. The current hydrometallurgical procedure, as is shown in Fig. 5.3, consists in the following operations:

• The waste printed circuit boards are firstly subjected to a physical mechanical procedure where the Al- and Fe-based components are removed from PCBs surface. Then, the depopulated PCBs are shredded and milled to suitable particles sizes;

• The milled PCBs are then leached with water, sulfuric acid and hydrogen peroxide for the extraction of base metals by the precious metals (Eqs. 5.1 and 5.2).

  $${\text{Cu}} + {\text{H}}_{ 2} {\text{SO}}_{ 4} + {\text{H}}_{ 2} {\text{O}}_{ 2} = {\text{CuSO}}_{ 4} + 2 {\text{H}}_{ 2} {\text{O}}$$

  (5.1)
  $${\text{Sn }} + {\text{ H}}_{ 2} {\text{SO}}_{ 4} + {\text{H}}_{ 2} {\text{O}}_{ 2} = {\text{ SnSO}}_{ 4} + 2 {\text{H}}_{ 2} {\text{O}}$$

  (5.2)

• The solid separation by the leach liquor is carried out by filtration process followed by washing with water. The resulted solution is subjected to a precipitation process for Sn precipitation. Then, also this solid precipitate is separated from solution by filtration and further washed with water. The solution achieved after Sn recovery is sent to an electrowinning cell for Cu recovery (Eq. 5.3).

  $${\text{CuSO}}_{ 4} + {\text{H}}_{ 2} {\text{O}} + 2 {\text{e}}^{ - } = {\text{ Cu }} + {\text{ H}}_{ 2} {\text{SO}}_{ 4}$$

  (5.3)

• Then, the resulted solution is recycled in the first leaching process for the leaching of another PCB material.

• The solid residue of base metals leaching process is involved into another leaching process with thiourea as reagent, ferric sulfate as oxidant in diluted sulfuric acid for Au and Ag dissolution (Reactions 5.4 and 5.5).

  $${\text{Au }} + {\text{ 2CS}}\left( {{\text{NH}}_{ 2} } \right)_{ 2} + {\text{ Fe}}^{ 3+ } \to {\text{Au}}\left[ {{\text{CS}}\left( {{\text{NH}}_{ 2} } \right)_{ 2} } \right]_{ 2} + + {\text{ Fe}}^{ 2+ }$$

  (5.4)
  $${\text{Ag }} + {\text{ 3CS}}\left( {{\text{NH}}_{ 2} } \right)_{ 2} + {\text{ Fe}}^{ 3+ } \to {\text{Au}}\left[ {{\text{CS}}\left( {{\text{NH}}_{ 2} } \right)_{ 2} } \right]_{ 3}^{ + } + {\text{ Fe}}^{ 2+ }$$

  (5.5)

• Then, after removal of solid suspension form solution by filtration, the electrowinnig process is also applied on this solution for Au and Ag recovery (Eqs. 5.6 and 5.7). Once the process is finished, the solution is also recycled for leaching of precious metals from other solid residue of base metals leaching process.

  $${\text{Au}}\left( {{\text{CSN}}_{ 2} {\text{H}}_{ 4} } \right)_{ 2}^{ + } + {\text{ e}}^{ - } = {\text{Au}}^{0} + 2 {\text{CSN}}_{ 2}$$

  (5.6)
  $${\text{Ag}}\left( {{\text{CSN}}_{ 2} {\text{H}}_{ 4} } \right)_{ 3}^{ + } + {\text{ e}}^{ - } = {\text{Ag}}^{0} + 3 {\text{CSN}}_{ 2} {\text{H}}_{ 4}$$

  (5.7)

• It is important to specify that solutions recycling after electrowinnig process will not be total. A part of these solutions are treated by proper technologies of waste water treatment. The treatment of the wastewater coming from the base metals recovery step consist in precipitation of the impurities with calcium hydroxide. The residual solution of precious metals recovery step is treated firstly with hydrogen peroxide and ferrous sulfate for degradation of organic complexes and then with calcium hydroxide for impurities precipitation. At the end of wastewater treatment process, the filtration is performed for solid removal from the treated water (Fig. 5.4).

  Fig. 5.4

  

  Flow diagram of GOLD-REC 1 hydrometallurgical process

Various tests of metals recovery with this hydrometallurgical technology have been applied on a milled sample of WPCBs of personal computers. Under the optimal conditions of the leaching process (solid concentration of 15%, under continuous agitation for 2 h for each step and a reagents concentration of 1.8 M of sulfuric acid and 20% vol/vol of hydrogen peroxide), which take place using the two-step counter current method, over 95% of Cu recovery and 60% for Sn have been achieved. The resulting solution was subjected to a coagulation process with polyamine solution in a concentration of 10% wt./vol. and 90% of tin content from solution was recovered. The obtained product had a tin concentration of about 50%. Furthermore, the electrolysis process was performed using graphite as cathode and zirconium-titanium electrode as anode.

At the end of the process, the purity of Cu product was 89% and the determined power consumption was 2.39 kWh/kg of Cu. The obtained copper deposit (Fig. 5.5) has been used for the additive manufacturing process (USE CASE 1).

Fig. 5.5

SEM image and photography of copper deposit

5.2.2 GOLD REC 2 Process Description

The original process patented is presented in Fig. 5.6.

Fig. 5.6

Flow diagram of GOLD REC 2 hydrometallurgical process

This hydrometallurgical process could be synthetically described as indicated in the follow:

• The chemical process can be applied on the e-waste without grinding (with whole WPCB as an example) avoiding important loss of precious metals also described in the literature;

• The process uses san unique step of metals dissolution with a chemical leaching using HCl, H₂O₂, acetic acid in water solution at room temperature (21 °C ± 3 °C) with a solid/liquid ratio of 10–20% (Eq. 5.8–5.15). The chloroacetic acid is produced by in situ chemical process within two steps: firstly, hydrochloric acid reacts with hydrogen peroxide and acetic acid to produce peracetic acid, water and chlorine (Eq. 5.8); in the second step chloroacetic acid and hydrochloric acid are produced by the chlorination of the unreacted acetic acid (Eq. 5.9). The global reaction of this process is represented by Eq. 5.10.

$$2 {\text{ HCl }} + {\text{ 2 H}}_{ 2} {\text{O}}_{ 2} + {\text{ C}}_{ 2} {\text{H}}_{ 4} {\text{O}}_{ 2} = {\text{ C}}_{ 2} {\text{H}}_{ 4} {\text{O}}_{ 3} + {\text{ 3 H}}_{ 2} {\text{O }} + {\text{ Cl}}_{ 2}$$

(5.8)

$${\text{C}}_{ 2} {\text{H}}_{ 4} {\text{O}}_{ 4} + {\text{ Cl}}_{ 2} = {\text{ C}}_{ 2} {\text{H}}_{ 3} {\text{ClO}}_{ 2} + {\text{ HCl}}$$

(5.9)

$${\text{HCl }} + {\text{ H}}_{ 2} {\text{O}}_{ 2} + {\text{ C}}_{ 2} {\text{H}}_{ 4} {\text{O}}_{ 2} = {\text{ C}}_{ 2} {\text{H}}_{ 3} {\text{ClO}}_{ 2} + {\text{ 2 H}}_{ 2} {\text{O}}$$

(5.10)

$$1. 5 {\text{ C}}_{ 2} {\text{H}}_{ 3} {\text{ClO}}_{ 2} + { 1}. 5 {\text{ HCl }} + {\text{ Au }} = {\text{ AuCl}}_{ 3} + { 1}. 5 {\text{ C}}_{ 2} {\text{H}}_{ 4} {\text{O}}_{ 2}$$

(5.11)

$${\text{C}}_{ 2} {\text{H}}_{ 3} {\text{ClO}}_{ 2} + {\text{ HCl }} + {\text{ 2 Ag }} = {\text{ 2 AgCl }} + {\text{ C}}_{ 2} {\text{H}}_{ 4} {\text{O}}_{ 2}$$

(5.12)

$${\text{C}}_{ 2} {\text{H}}_{ 3} {\text{ClO}}_{ 2} + {\text{ HCl }} + {\text{ Cu }} = {\text{ CuCl}}_{ 2} + {\text{ C}}_{ 2} {\text{H}}_{ 4} {\text{O}}_{ 2}$$

(5.13)

$${\text{C}}_{ 2} {\text{H}}_{ 3} {\text{ClO}}_{ 2} + {\text{ HCl }} + {\text{ Sn }} = {\text{ SnCl}}_{ 2} + {\text{ C}}_{ 2} {\text{H}}_{ 4} {\text{O}}_{ 2}$$

(5.14)

$${\text{C}}_{ 2} {\text{H}}_{ 3} {\text{ClO}}_{ 2} + {\text{ HCl }} + {\text{ Ni }} = {\text{ NiCl}}_{ 2} + {\text{ C}}_{ 2} {\text{H}}_{ 4} {\text{O}}_{ 2}$$

(5.15)

$${\text{C}}_{ 2} {\text{H}}_{ 3} {\text{ClO}}_{ 2} + {\text{ HCl }} + {\text{ Pb }} = {\text{ PbCl}}_{ 2} + {\text{ C}}_{ 2} {\text{H}}_{ 4} {\text{O}}_{ 2}$$

(5.16)

$${\text{C}}_{ 2} {\text{H}}_{ 3} {\text{ClO}}_{ 2} + {\text{ HCl }} + {\text{ Zn }} = {\text{ ZnCl}}_{ 2} + {\text{ C}}_{ 2} {\text{H}}_{ 4} {\text{O}}_{ 2}$$

(5.17)

• Precious (Au and Ag) and base metals (Cu, Sn, Zn, Ni, Pb) are dissolved leaving the WPCB with mainly epoxy resins and fiberglass structure intact (with some residues of metals);

• The liquid solution is easily separated from the S/L system and selective reduction-precipitations steps are considered in the process to recover the dissolved metals. These steps are synthetically described in the follow:

• a. Reduction and precipitation of Au chloride to its metallic form by ascorbic acid;

$${\text{AuCl}}_{ 3} + { 1}. 5 {\text{ C}}_{ 6} {\text{H}}_{ 8} {\text{O}}_{ 6} = {\text{ Au }} + {\text{ 3HCl }} + { 1}. 5 {\text{C}}_{ 6} {\text{H}}_{ 6} {\text{O}}_{ 6}$$

(5.18)

• b. Cooling the solution to less than 15 °C for precipitation of AgCl;

• c. Selective reduction and precipitation of Cu by metallic Sn or co-reduction of both copper and tin ions with iron metal;

$${\text{CuCl}}_{ 2} + {\text{ Sn }} = {\text{ SnCl}}_{ 2} + {\text{ Cu}}$$

(5.19)

$${\text{CuCl}}_{ 2} + {\text{ Fe }} = {\text{ FeCl}}_{ 2} + {\text{ Cu}}$$

(5.20)

$${\text{SnCl}}_{ 2} + {\text{ Fe }} = {\text{ FeCl}}_{ 2} + {\text{ Sn}}$$

(5.21)

• d. Reduction and precipitation of SnCl₂ by metallic Zn;

$${\text{SnCl}}_{ 2} + {\text{ Zn }} = {\text{ ZnCl}}_{ 2} + {\text{ Sn}}$$

(5.22)

• e. Exploitation of the residual solution for its recycling within the process or by adding iron in order to produce a FeCl₂-FeCl₃ solution useful for coagulation processes in the treatment of wastewaters;

• 2. The main products are: Au (after melting process in an inductive electrical oven adding some slug compound), AgCl, Cu and Sn in powder forms (mainly in the range of 10–90 µm) and a residual chloride solution that can be regenerated by make-up with proper reagents concentration or treated with iron metal to achieve a high concentrated iron solution (extensively and usually utilized in the coagulation processes in wastewater treatments);

Various tests have been conducted at both laboratory and pilot levels. These were carried out on various streams, namely: RAM modules, PCBs of mobile phones and CPU. The runs were performed using the following conditions: 3.5 M of HCl, 10% wt./vol. of C₂H₄O₂, 5% wt./vol. of H₂O₂, 15–20% of solid concentration, room temperature, 3 h. Under these conditions, recovery yields between 60 and 95% were achieved for Au, Ag, Cu and Sn content of the three kind of waste. Not complete dissolution is achieved since the process is performed in whole material. The waste materials (RAM modules and PCB of mobile phone) have entrapped within their layers and components these elements. Therefore, not complete exposition of elements to the leaching media is realized. The results carried out within the hydrometallurgical plant revealed a recovery of about 50–75% for these four elements. The process of reduction with ascorbic acid had an efficiency of over 95% for Au recovery at both pilot and laboratory levels. Figure 5.7 presents the photographic aspects of one of the Au precipitates recovered at pilot level and final achieved product after thermal refining.

Fig. 5.7

Photographic aspect of Au product after precipitation and thermal refining

The further step of AgCl precipitation from solution by cooling revealed over 75% of recovery at laboratory scale after 3 h of reaction. At pilot level, Ag was coprecipitated during the copper cementation process. This was mainly since the plant does not have a cooling system. The copper recovery was performed either with Sn metal or Fe metal powders at laboratory scale level. The runs have been carried out at different stoichiometric amounts of both base metals and the optimal results in terms of recovery from solution and purity of products was achieved with tin metal at a stoichiometric amount of 0.8 (82% of recovery and 97% of purity). This is mainly since tin metal has close value to copper within the reactivity series of metals. The copper recovery with Fe revealed over 99% of recovery of copper at a stoichimetric excess of 45% and a purity of 84%. At pilot level, the produced copper powder (Fig. 5.8) had a copper content of about 50% with Fe and Sn as main impurities,

Fig. 5.8

Copper metal powder composition

The achieved gold and copper products have been used for jewelry (USE CASE 2) and filaments (USE CASE 3) production.

5.3 Conclusion

In order to achieve a circular economy for metals, FENIX Project, has as one of the main cores to perform the recovery of base and precious metals from e-wastes and to reuses them for manufacturing of new products. For this reason, two hydrometallurgical processes have been tested at both laboratory and small industrial levels. There have been performed various experiments and according to the results achieved at pilot level, both technologies must be further improved to achieve better recovery degrees and properties of final products.