Reduction of Metallurgical Slag with High Cu and Pb Content Using a Bioreducer in the Form of Rapeseed Cake ^†

1. Introduction

The 21st-century economy, and industry in particular, is witnessing a growing focus on environmental issues, driven by heightened public awareness and the implementation of EU directives, notably the Green Deal. There are numerous methods by which the economy can be transformed to become more environmentally friendly. Nevertheless, one of the most significant strategies is to halt or diminish the utilisation of non-renewable raw materials (frequently fossil materials) and to enhance the deployment of renewable raw materials. One of the most significant developments in the industry is non-ferrous metallurgy, which constitutes one of the foundations of the industry. In this industry, the production of metals necessitates the utilisation of production additives with reducing properties (in the chemical sense). Currently, the most prevalent additive is coke, which is derived from the gasification of hard coal, a non-renewable raw material. In recent times, extensive research has been conducted with the objective of partially or even completely eliminating coke and replacing it with another raw material with suitable reducing properties. Hydrogen is frequently considered a potential alternative. However, hydrogen-based extractive metallurgy technologies are not without their drawbacks. These include the explosive properties of this gas as well as issues related to storage and transport. Nevertheless, there are other raw materials that meet the renewability criterion, which could be used instead of coke in metallurgical processes. These are biomass, which is organic material that is most often a by-product of agriculture or the timber industry. Due to their relatively high carbon content, very fast renewability, and the fact that they are a neutral source of CO₂ emissions in EU countries, it is worth considering them alongside hydrogen as a reducing agent in non-ferrous metallurgy [1,2,3,4].

One such biomass studied is rapeseed cake, a by-product of rapeseed oil production. This material is chosen for its high availability in local markets, especially in Poland and Europe, which are leading exporters of rapeseed oil [5].

2. Materials and Research Methods

In this research, experiments were conducted to reduce metallurgical slag with high Cu and Pb content (composition listed in Table 1) using rapeseed cake as an alternative reductant, with coke used for reference purposes. All experiments were conducted at a maximum temperature of 1400 °C and held for 2 h. The variables in the experiments were the type of reductant (rapeseed cake, coke), their addition (2, 5, 10, 12, 15% to the slag weight for rapeseed cake and 5, 10 and 15% for coke) The feedstock for the experiments was reductant, covered with slag (200 g). In each series, two re-melts were conducted.

Table 1. Composition of the slag.

Component	Concentration, %
O	25.08
Cu	17.25
Pb	14.10
Si	13.15
Fe	11.70
Ca	5.57
C	3.36
Al	2.43
K	1.55
Mg	1.22
As	1.16
Zn	0.92
Co	0.55
Ni	0.29
Ti	0.18
Mn	0.15
Mo	0.12

3. Results of Experiments

Table 2 presents the averaged results of the experiments conducted for each series. The compositions of the experimental inputs (slag mass and reductant mass) are displayed, along with the resulting post-reduction remelting products (alloy and post-reduction slag).

Table 2. Results of the experiments.

Reducing Agent	Average Mass of Slag, g	Mass of Reducing Agent, g	Reducing Agent to Mass of Slag, wt. %	Average Mass of Alloy, g	Average Mass of Post-Reduction Slag, g	Average Mass of Slag, g
Rapeseed cake	200.02	4	2	19.27	177.59	200.02
	200.02	10	5	35.61	158.21	200.02
	200.01	20	10	52.09	139.30	200.01
	200.02	24	12	56.19	132.88	200.02
	200.03	30	15	58.50	129.98	200.03
Coke	200.03	10.00	5	29.83	153.37	200.03
	200.02	20.04	10	37.11	158.77	200.02
	200.04	30.06	15	48.22	143.24	200.04

In both series, the addition of reductant in the form of coke and rapeseed cake resulted in a higher mass of alloy being obtained. A comparison of the reduction experiments with coke and rapeseed cake revealed that, for the same measuring points, the average mass of the alloys obtained was higher when biomass was used. For a 5% reductant addition to the charge weight, the average mass of the alloy obtained was approximately 16% higher, for 10% it was approximately 29% higher, and for 15% it was approximately 18% higher.

Subsequently, chemical analyses were conducted on the alloys and post-production slags. The results of these analyses, along with the resulting converted mass proportions of Cu, Pb, and Fe in both products, are presented in Table 3.

Table 3. Concentrations of Cu, Pb, and Fe and their mass in the alloys and slags.

Reducing Agent	Reducing Agent to Mass of Slag [%]	Alloy						Post-Reduction Slag
		Concentration, %			Mass, g			Concentration, %			Mass, g
		Cu	Pb	Fe	Cu	Pb	Fe	Cu	Pb	Fe	Cu	Pb	Fe
Rapeseed cake	2	94.48	3.29	0.14	18.22	0.63	0.03	5.47	12.79	11.58	9.71	22.71	20.56
	5	77.90	19.56	0.21	27.7	6.96	0.07	1.81	10.65	13.88	2.86	16.85	21.96
	10	60.55	35.32	0.73	31.54	18.4	0.38	0.94	3.2	16.4	1.31	4.46	22.85
	12	58.67	34.58	2.16	33.09	19.43	1.22	0.84	1.84	16.8	1.12	2.44	22.32
	15	56.94	30.97	6.38	33.61	18.13	3.73	0.54	1.29	16.85	0.70	1.67	21.9
Coke	5	86.24	11.59	0.06	25.73	3.46	0.02	2.06	10.01	14.09	3.21	15.11	21.26
	10	73.55	23.32	0.14	27.29	8.65	0.05	1.08	6.75	15.15	1.68	10.48	23.54
	15	64.94	32.01	1.31	31.31	15.44	0.63	0.59	2.18	15.69	0.84	3.12	22.47

With regard to alterations in the chemical constitution of the products. The concentration of copper (Cu) in the alloys was observed to decline with the incorporation of bioreducer and coke. This phenomenon can be attributed to the increasing proportion of lead (Pb) and, subsequently, iron (Fe) in the alloy. The lowest concentration of Cu in the slag was recorded with the 15% addition of rapeseed cake, reaching a satisfactory level of 0.54% for Cu and 1.29% for Pb. In contrast, the addition of 15% coke resulted in Cu concentrations in the slag below 1%. with a similar Cu concentration to rapeseed cake (0.59%) and a significantly lower Pb concentration.

4. Conclusions

• The reduction efficiency with rapeseed cake under experimental conditions is higher than with metallurgical coke. The weights of the alloys after reduction with rapeseed cake relative to coke for their same additives in relation to the feedstock slag (5, 10, 15%) were 16. 29 and 18% higher.
• The post-reduction slags for the rapeseed cake series contained 5.5 to 0.5% Cu and 12.8 to 1.3% Pb. The lowest concentrations were obtained at the highest bioreducer addition (15%).
• The rapeseed cake represents an interesting material in terms of its reducing properties. The results obtained for the given experimental parameters demonstrated the superiority of the biomass reductant over the conventional reductant.