Reduction of Lead Sulfate Using Fine-Grained Carbon-Bearing Materials ^†

1. Introduction

The volume of lead production in Poland ranks third in the group of non-ferrous metals, after copper and zinc. Lead is used primarily for the production of lead-acid batteries. These types of batteries are used in internal combustion vehicles, such as cars, motorcycles and mopeds, tractors, and agricultural machines. Lead is an example of a metal whose production from secondary raw materials has, for many years, exceeded production from primary sources, i.e., metal ores [1]. This situation occurs both in Poland and around the world. Lead from secondary raw materials is produced mainly by processing used lead-acid batteries. Pyrometallurgical methods definitely dominate in these processes.

By mass, a used lead-acid battery contains on average 51% lead-containing battery paste, 26% lead-containing metallic fractions, 14% electrolytes, 6% polypropylene and 3% PVC. One of the main ingredients of battery paste is lead sulfate. During pyrometallurgical processing of battery scrap in the presence of carbon-containing fuel/reducer, the lead sulfate contained in the battery paste may transform into the form of PbS, PbO × PbSO₄ oraz PbO [2,3,4,5,6,7,8,9,10,11,12,13,14]. The fuel/reducer traditionally used in pyrometallurgical technologies for processing battery scrap is coke or coke breeze. Due to the high prices of these raw materials, attempts are being made to replace them with alternative, cheaper or more efficient fuels/reducers [15,16,17,18,19,20,21,22,23]. As part of the present work, research was carried out on the lead sulfate reduction process using fine-grained carbon-bearing materials in the form of anthracite dust and coal flotation concentrate. For comparison purposes, coke breeze was also used for the reduction process.

2. Research Methodology

The thermogravimetric analysis was carried out using an STA 449 F3 Jupiter analyzer, Netzch (Selb, Germany). The F3 Jupiter analyzer is a device which enables a series of measurements with the use of thermal analysis methods. It contains a graphite furnace which works in a protective atmosphere (argon and helium) and facilitates measurements at up to 2000 °C. Before the experiment, a sample of a particular weight (approx. 200 mg) was placed inside a small DTA/TG Al₂O₃ crucible which was then attached to the measuring head in the working chamber of the analyzer. The measurements were carried out in an air atmosphere. The adopted sample heating program consisted of three main stages:

• Heating the sample to temperatures of 900, 925, 950, 9750 and 1000 °C at a rate of 20 °C/min;
• Isothermal heating of the sample at the chosen temperature for 30 min;
• Cooling the sample to a temperature of 700 °C.

3. Results and Discussion

The example results of thermogravimetric tests of the lead sulfate reduction process using three carbon-bearing materials (loose samples) are presented in Figure 1 and Figure 2 and in

Table 1. Changes in the mass of samples during the PbSO₄ reaction using anthracite dust (loose form).

Substrates	Temperature, °C	Total Mass Change, %
PbSO4 + anthracite dust	900	−30.74
	925	−31.77
	950	−32.71
	975	−32.11
	1000	−37.17

. By analyzing the reduction processes carried out for individual PbSO₄ samples, it is possible to notice the similar nature of the thermogravimetric curves obtained for all the reducers used. Figure 1 shows a summary of the TG curves obtained for the lead sulfate reduction reaction using anthracite dust as a reducer. As a result of the analysis of thermogravimetric curves, two areas of different mass loss of the tested samples were distinguished (Figure 2). The first stage for all tested samples was characterized by a rapid mass loss associated with the reduction of PbSO₄ (formation of PbS), the reduction reaction of the PbO and the Boudouard reaction (formation of CO). In the second range, characterized by slower mass loss, in addition to the above-mentioned reactions, there could also be a reaction between the formed lead sulfide and lead oxide to form lead [24].

Analyzing the results of the tests carried out in this work (

Table 2. Changes in sample mass during PbSO4 reduction (loose and agglomerate samples).

Substrates	Sample Form	Temperature, °C	Total Mass Change, %
PbSO4 + anthracite dust	loose	900	−30.74
	agglomerate		−29.69
	loose	1000	−37.17
	agglomerate		−18.88
PbSO4 + flotoconcentrate	loose	900	−41.30
	agglomerate		−38.34
	loose	1000	−41.66
	agglomerate		−43.22
PbSO4 + coke breeze	loose	900	−39.51
	agglomerate		−33.59
	loose	1000	−39.67
	agglomerate		−26.43

), it can be concluded that the increase in process temperature does not affect the nature of the thermogravimetric curves of lead sulfate reduction. Similarly to the reduction of lead oxide with anthracite dust and coke breeze, the reduction of lead sulfate took place in two successive stages. Reduction using flotoconcentrate was of a slightly different nature, similar to that of lead oxide [23]. The introduction of agglomerate material into the process, for mixtures containing anthracite dust and coke breeze, results in a reduction in the recorded weight loss of the samples. The greatest weight losses of samples were obtained in mixtures in which coke breeze and flotoconcentrate were used as carbon-bearing materials.

4. Summary

In the PbSO₄ reduction processes carried out, the nature of the thermogravimetric curves for all used reducers (anthracite dust, flotoconcentrate, coke breeze) does not change with the increase in temperature. In all cases, two ranges can be distinguished, clearly differing in the rate of mass loss.

In the analyzed temperature range (900–1000 °C), in the case of a bulk charge using anthracite dust, an increase in temperature increases the efficiency of the process (the sample mass loss increases). The maximum mass loss was recorded at a temperature of 1000 °C and amounted to approximately 37%. When using coke breeze, the highest weight loss was observed at a temperature of 950 °C and it amounted to approximately 42%. When using flotoconcentrate, the highest weight loss was observed at a temperature of 925 °C (approximately 43%). Agglomeration of input materials does not improve process efficiency.

Based on the analysis of the research results, it can be concluded that coal flotate concentrate may be a better alternative to the traditionally used fuel/reducer (coke), due to its similar reduction capabilities and significantly lower price.