Ultra‐Fast Recyclable and Value‐Added Desulfation Method for Spent Lead Paste via Dual Intensification Processes

Abstract The new low‐cost clean pre‐desulfation technology is very important in pyrometallurgy and hydrometallurgy. However, traditional reactors have low space‐time yield and desulfation rate, resulting in high energy consumption and SO2 emissions in the industrial desulfation processes. Herein, dual rotating liquid film reactors (RLFRs) and lime are proposed to construct a recyclable, ultra‐fast, and value‐added desulfation method. Parameter optimization and kinetic calculations prove that the above reactions are controlled by internal diffusion, revealing that RLFR promotes the mass transfer and reaction rate. The new process greatly shortens the desulfation time of lead paste from 40 min to 10 s with a high desulfation rate of 99.7%, and the sulfation time of lime from 30 min to 30 s with a sulfation rate of 98.6% with a net profit of 55.99 ¥/ton by cost accounting. Moreover, ten batches of continuous scale‐up experiments demonstrate the stability of processes, the desulfation and sulfation rates are kept at 99.7% and 98.2%, which greatly reduces the emissions of waste desulfate liquor. This work provides a new universal strategy for a sustainable, low‐cost, and clean desulfation method of waste resources to achieve technical and economic feasibility.

Deionized water was obtained by water purification system in the laboratory.The lead paste was provided by Anhui Cilwee Power Group.Firstly, spent lead paste samples were washed with deionized water, then dried in a vacuum oven at 120 o C for 4 hours.Then it was crushed and sieved to obtain particles with a size of less than 100 µm.

Pretreatment of spent lead paste
The spent lead paste was obtained from the scrapped lead-acid batteries (LABs) provided by Anhui Chilwee Power group, China.Firstly, the scrapped LABs were automatically disassembled, crushed and sorted by an automatic splitter system to separate them into different components, such as lead paste, grids, sulfuric acid, separators, and plastics.Secondly, the sorted lead paste was washed several times with deionized water until the washing liquid was neutral, and then dried in an oven at 120 °C for 12 h.Finally, the lead paste was crushed with a crusher, and sieved with a 120-mesh sieve to remove the contained grids and fibers in the lead paste.

Material Characterization
The morphology and structure of the spent lead paste, the obtained highly pure PbCO3, and CaSO4 powder were analyzed with a scanning electron microscope (SEM, Japan Bruker Company, Model 8010) and a powder X-ray diffraction (PXRD, Japan Shimadzu Instruments Co., Ltd., XRD-6000).The samples were investigated with a thermogravimetric analyzer (TGA, Hitachi, STA7300) within 30-800 o C at a heating rate of 10 o C min -1 .Laser particle size analyzer (Malvern, MS2000) was used to test the size and distribution of particles.Moreover, an inductively coupled plasma atomic emission spectroscopy (ICP-AES, Thermo Fisher Technology China Co., Ltd., ICPA-6000) was used to determine the element concentrations of the samples.

The content measurement of PbO by the EDTA titration method
3 g lead paste was dissolved in a 5 wt% acetic acid solution (60 mL) for 30 min.The filtrate and filter residue were washed with 5 wt% acetic acid solution several times and separated by centrifugation.
Then 10 mL filtrate was placed in a conical flask, followed by ammonia (NH3•H2O) solution to adjust the pH of the filtrate to 5~6.Titration indicator was a mixture of 2 mL 20 wt% hexamethylenetetramine solution and 3 drops of 0.5 wt % xylenol orange.A 0.02 mol L -1 EDTA standard solution was employed for complexometric titration, and the result was calculated in the way: where ωPbO represents the mass fraction of the PbO component in the lead paste, cEDTA and vEDTA represent the concentration and volume of EDTA, MPbO is the molar mass of PbO (223.2 g mol -1 ), m stands for the total mass of the lead paste, respectively.

The content measurement of Pb the EDTA titration method
The above filter residue (step 1. Where ωPb is the mass fraction of Pb and MPb is the molar mass of Pb (207.2 g mol -1 ).

The content measurement of PbO2 the KMnO4 titration method
The filter residue (step 1.1.2) was dissolved in a mixture of 40 mL HNO3 and 5 mL H2O2 solution and stirred for 30 min.The filtrate and filter residue were washed with acetic acid solution several times and separated by centrifugation.
The newly obtained 10 mL sucked filtrate was placed in a conical flask and titrated with 0.1 mol L -1 calibrated KMnO4 solution, which turned light red without fade in 30 s, recorded the volume of KMnO4 solution (vKMnO4).According to the above method, the blank experiment was conducted under the same conditions and recorded the volume of KMnO4 solution (v0).
Where ωPbO2 represents the mass fraction of the PbO2 in the lead paste, cKMnO4 and vKMnO4 stand for the concentration and volume of KMnO4, v0 is the volume of KMnO4 solution required for the blank experiment, MPbO2 is the molar mass of PbO2 (239.2 g mol -1 ), respectively.

The content measurement of PbSO4 the EDTA titration method
The filter residue obtained from the previous step 1. 1.3 was dissolved in 20 wt% ammonium acetate solution, and boiled for 10 min at a heating mantle.After that, the reaction was naturally cooled down to room temperature and washed three times with deionized H2O by centrifugation.
The newly obtained filtrate was analyzed by referring to the titration analysis in step (1.1.1).
Where ωPbSO4 represents the mass fraction of the PbSO4 in the lead paste and MPbSO4 is the molar mass of PbSO4 (303.27g mol -1 ).

1 . 1 )
was dissolved in 40 mL HNO3 solution and stirred at 60 o C for 30 min.The filtrate and filter residue were washed with HNO3 solution several times and separated by centrifugation.The newly obtained filtrate was analyzed by referring to (1.1.1)titration analysis.

Table S1 .
The chemical composition of lead paste by chemical titration methods.

Table S2 .
The desulfation rates of spent lead paste within 10 s at different reaction temperatures.

Table S3 .
Estimated net costs of different desulfation technologies.