Decoppering of Metallurgical Slag Using Printed Circuit Boards as a Reducer ^†

1. Introduction

Coking coal was included in the EU’s list of critical raw materials in 2014, 2017, and 2020 [1]. Like other raw materials on the list, it is of strategic importance for the functioning and economic development of the European Union, and its deficit could have serious economic consequences for the entire European economy. The European Union currently imports about 75% of the coking coal it consumes from countries such as Australia, the United States, Canada, Russia, and Mozambique [2]. Only about 25% of the annual demand for coking coal comes from EU countries, namely Poland and the Czech Republic, which are the only producers of this critical raw material. The European Commission has also recognized this raw material as a material that plays a key role in the process of transforming the steel industry into a climate-neutral economy [3].

Coking coal is a fundamental raw material for coke production, which serves as both a fuel and a reducing agent in metallurgical processes for iron and steelmaking, as well as non-ferrous metal production.

One material that could offer an alternative to the reductant currently used in metallurgical processes, i.e., metallurgical coke, could be printed circuit boards (PCBs).

The current state of knowledge on the course of reduction reactions in metallurgical processes in the liquid–gas phase system indicates that the mass transport of reactants in both the gas and liquid phases is the rate-limiting step in the process. The reduced metal, in the case of a difference in the densities of the resulting phases, may sediment. In order to intensify the mass transport of the components of the system, it is necessary to set it in motion by, among other things, stirring the liquid phase. The movement of the liquid phase also promotes coagulation and sedimentation of the separated metal. Intensification of mixing can be achieved, among other things, by a material that will generate a significant amount of gas during the thermal process. Such a role can be played by plastics contained in PCB scrap, of which hydrocarbons are an important component.

From the available literature [4], it can be concluded that it is possible to use this type of material for the reduction process of metallurgical slag. Positive results have already been achieved at 1300 °C. In this publication, the results of a metallurgical slag decopperization process using PCB scrap as a reductant are presented, thus extending the temperature range of the process to 1450 °C. LOW GRADE” type PCB scrap from Tesla Recycling Sp. z o.o. (Grodzisk Mazowiecki, Poland) was used as the reducing material. For comparative purposes, tests were also carried out on the reduction of metallurgical slag using metallurgical coke—the currently used reductant in metallurgical processes.

2. Materials and Research Methods

Investigation of the slag -de-copperization process was carried out in a laboratory, high-temperature, resistance chamber furnace with chamber dimensions of 220 × 300 × 350 mm. Metallurgical slag containing 9.58% wt. Cu was used for the reduction tests. The reductant was “LOW GRAGE” PCB scrap with a fraction size of ≤2 mm and an average copper content of 10.0% wt. and metallurgical coke.

Variable addition of PCB scrap as a reductant, ranging from 10 to 30% of the slag mass, and 5% addition of coke as a reference measure, were used in the reduction process. The mass of slag used in the tests was 200 g. The conducted tests involved four measurement series, differing in the type and mass of reductant used:

• Series I—Addition of “LOW GRADE” PCB scrap at 10% wt.,
• Series II—Addition of “LOW GRADE” PCB scrap at 20% wt.,
• Series III—Addition of “LOW GRADE” PCB scrap at 30% wt.,
• Series IV—Addition of coke at 5% wt.

A total of 24 tests were performed. The tests within each measurement series were carried out at a temperature of 1450 °C for 2, 3 and 4 h. Each measurement was performed twice. After cooling the furnace, the sample was removed from the crucible in order to separate the process products, i.e., metal and slag. The slags obtained in the tests were prepared appropriately and then passed on for chemical analysis.

The copper content in the slag samples was determined using the X-ray fluorescence method.

3. Research Results

The results of the average of two product mass melts and average chemical analyses obtained during the research are presented in

Table 1. Chemical analyses of slags after reduction and average amounts of products obtained during the process.

	Series Name	Process Time, h	Average Amount of Slag Produced, g	Average Amount of Alloy Formed, g	Average Cu Content in Slag after Reduction, wt. %
SERIES I	PCB LGwt. 10%	2	163.03	78.21	0.26
		3	168.54	71.04	0.42
		4	163.54	78.4	0.24
SERIES II	PCB LGwt. 20%	2	152.28	105.68	0.16
		3	159.21	101.51	0.29
		4	156.37	105.65	0.2
SERIES III	PCB LGwt. 30%	2	152.52	121.74	0.27
		3	150.17	129.92	0.23
		4	152.29	129.56	0.27
SERIES IV	COKEwt. 10%	2	142.94	79.8	0.15
		3	139.26	85.81	0.21
		4	141.82	81.78	0.28

.

On their basis, the value of the degree of de-copperization of metallurgical slag was calculated using the obtained reducers. This value was determined from the relationship:

S_Cusr. = (%Cu_Z0 − %Cu_Zk)/%Cu_Z0 × 100 [%]

(1)

where:

• %Cu_Z0—initial copper content in metallurgical slag,
• %Cu_Zk—copper content in slag after reduction.

The results of these calculations are presented in

Table 2. The copper content in the obtained metallic alloy calculated on the basis of slag analysis and the average degree of decopperization.

	Series Name	Process Time, h	Mass of Copper Introduced into the Process, g	Cu Content of the Alloy, g	Theoretical Cu Concentration in the Alloy, wt. %	Cu Yield to Alloy, %	Average Degree of Slag Decoppering, %
SERIES I	PCB LGwt. 10%	2	42.33	41.46	53	97.96	97.23
		3		40.91	57.6	96.66	95.66
		4		41.54	53	98.15	97.44
SERIES II	PCB LGwt. 20%	2	46.32	45.84	43.4	98.95	98.33
		3		45.4	44.7	98.01	97.02
		4		45.7	43.3	98.65	97.96
SERIES III	PCB LGwt. 30%	2	50.33	49.51	40.7	98.36	97.23
		3		49.64	38.2	98.63	97.65
		4		49.51	38.2	98.37	97.23
SERIES IV	COKEwt. 10%	2	38.32	37.89	47.5	98.88	98.43
		3		37.74	44	98.47	97.81
		4		37.53	45.9	97.93	97.08

.

The metallic alloy formed during the reduction process is characterized by a tendency to segregate its components. This is due to the fact that the immiscibility of the components in the Cu–Fe–Pb system occurs in both the solid and liquid states [5]. The segregation area in the liquid phase turns into a copper-rich liquid, and iron-rich liquid increases with decreasing temperature [6,7]. This makes it difficult to obtain a representative sample for analysis.

Therefore, using the data in Table 1, the theoretical copper content of the resulting alloys and the yield of this metal to the resulting alloy were calculated on the basis of the slag analysis. The results obtained are also shown in Table 2.

4. Conclusions

The conducted research confirmed the possibility of using PCB scrap as a metallurgical slag reducer. With the assumed experimental parameters, the degree of decopperization of metallurgical slag using PCB scrap as a reductant was within a narrow range of 95.6 to 98.4%. The results obtained as part of the research indicate that at a temperature of 1450 °C, the process time and the amount of PCB scrap introduced had no significant impact on the level of final copper content in the slag after reduction and the degree of decopperization of the slag. Comparative tests carried out using metallurgical coke as a reducer—a reducer currently used in the steel industry—achieved almost identical results to the case of the addition of PCB scrap (from 97 to 98.5%)