Diffusion Dialysis Recovery of Sulfuric Acid from a 15 wt% H₂SO₄ Feed Stream Mapping, Conductivity Analysis, and Temperature Evaluation

YASA Environmental Technology Co., Ltd.

No. 588, Xinjinqiao Road, Pudong, Shanghai, China

Email: info@yasa.ltd

Website: www.yasa.ltd

Figure 1. Graphical abstract of the diffusion dialysis acid recovery process with corrected stream labels.

Abstract

This study evaluates the recovery of sulfuric acid from a 15 wt% H₂SO₄ feed stream using diffusion dialysis (DD). The DD process operates through a concentration-gradient-driven membrane separation mechanism using an acid-resistant anion exchange membrane. During operation, acid species selectively migrate from the acid feed side to the water side, producing a recovered acid stream, while most metal ions, salts, and impurities are retained in the raffinate stream.

Time-resolved experimental data were collected over approximately 129.8 minutes, with 390 recorded measurements. The main monitored parameters included three conductivity or concentration signal channels, two pH channels, and three temperature channels. The conductivity profiles showed clear stream differentiation and stable process behavior, indicating steady acid transfer during the DD run. Temperature remained well controlled throughout the experiment, confirming that the process operated under mild thermal conditions without significant heating. The pH data were useful mainly during the early startup period, while later zero values were treated as invalid sensor readings rather than actual process pH. Overall, the results show that diffusion dialysis can operate stably for sulfuric acid recovery from a 15 wt% H₂SO₄ feed stream.

Keywords: diffusion dialysis, sulfuric acid recovery, 15 wt% H₂SO₄, anion exchange membrane, recovered acid, spent acid, feed acid, conductivity profile.

1. Principle of Diffusion Dialysis for H₂SO₄ Recovery

In sulfuric acid diffusion dialysis, the feed acid flows on one side of an acid-resistant anion exchange membrane, while water or a low-acid receiving stream flows on the opposite side. The concentration difference drives H⁺, HSO₄⁻, and SO₄²⁻ transport toward the receiving side. The recovered acid stream is formed on this receiving side, while the feed-side outlet becomes the spent acid or raffinate stream. Because DD is concentration-gradient driven, it does not require an external electric field. Stable stream conductivity and temperature profiles are important indicators of hydraulic stability, but they do not substitute for titration and chemical analysis. This mechanism and the use of anion exchange membranes for acid recovery are consistent with published DD studies on acidic waste streams (Zhang et al., 2020; Merkel et al., 2021; Ruiz-Aguirre et al., 2021).

2. Data Analysis Method

The results were analyzed as time-series data for the DD run. Operating time was calculated in minutes from the first recorded value. Feed acid conductivity, recovered acid conductivity, and spent acid conductivity were plotted against time. 

3. Results and Discussion

3.1 Conductivity Profiles

Figure 2. Conductivity signal profiles for feed acid, recovered acid, and spent acid during the DD run. Invalid startup and dropout readings were omitted.

The conductivity plot shows three distinct DD streams. The feed acid signal remained stable, changing from 214.3 to 217.5 logger units over the valid records, with a mean value of 215.2. This supports a stable 15 wt% H₂SO₄ feed-side condition during the test. The recovered acid signal decreased from 266.3 to 229.2 logger units. This confirms a measurable ionic signal in the recovered acid stream, but conductivity alone cannot determine the actual H₂SO₄ concentration. The spent acid signal became valid after startup and remained high, changing from 311.9 to 305.6 logger units. This indicates that the raffinate stream retained significant ionic strength after DD contact.

3.2 Temperature Profiles

Figure 4. Temperature profiles for feed acid, recovered acid, and spent acid streams.

Temperature remained controlled during the DD run. Feed acid temperature, T_03, stayed between 27.9 and 30.1 °C. Recovered acid temperature, T_02, increased from 23.9 to 26.0 °C. Spent acid temperature, T_01, increased from 23.1 to 24.6 °C. The maximum recorded temperature was 30.1 °C, which indicates mild operating conditions and no severe heat buildup.

3.3 Recovered Acid to Feed Acid Signal Ratio

Figure 7. Ratio of recovered acid conductivity signal to feed acid conductivity signal.

The recovered-to-feed signal ratio provides a simple process-monitoring indicator for comparing the recovered acid stream with the feed acid stream. This ratio is not an acid recovery percentage. It is only a conductivity-based indicator and must be supported by titration before it can be used for quantitative acid recovery claims.

3.4 pH Data Limitation

Figure 8. Valid startup pH readings only. Later, zero values were excluded as unreliable sensor readings.

The pH data were suitable only for startup observation. Later, zero values were excluded because continuous zero pH records are not reliable process measurements. For a 15 wt% H₂SO₄ DD test, acid concentration should be determined by titration rather than by pH logging.

4. Stream Summary

Stream	Valid signal range	Observed interpretation
Feed acid	211.6 to 219.5	Stable feed-side acid signal after dropout removal
Recovered acid	229.1 to 266.3	Measurable recovered-acid stream signal, but concentration requires titration
Spent acid/raffinate	305.0 to 311.9	High and stable spent-acid signal after startup
Feed acid temperature	27.9 to 30.1 °C	Mild thermal behavior
Recovered acid temperature	23.9 to 26.2 °C	Mild temperature increase
Spent acid temperature	23.1 to 24.7 °C	Mild temperature increase

5. Conclusion

This diffusion dialysis experiment showed successful sulfuric acid transfer using a 15 wt.% H₂SO₄ feed solution. During the 129.8-minute test, the conductivity, pH, and temperature of the acid streams changed clearly, confirming that acid migration occurred through the DD membrane.

The conductivity results showed that the acid streams were actively changing during operation. The spent acid stream increased from almost zero conductivity to about 305.6, showing strong ionic transfer. The recovered acid stream also showed clear conductivity variation, while the feed acid remained in a high-conductivity range, confirming the presence of strong acid in the system.

The pH data further supported the conductivity results. One pH channel dropped from about 8.58 to below pH 1 within the first 6.3 minutes, showing rapid acidification after the DD process started. Temperature remained relatively stable, with only a small increase during the experiment, indicating that the process was mainly driven by concentration difference rather than heat.

Overall, the experiment confirms that YASA ET diffusion dialysis can transfer sulfuric acid effectively from a 15 wt.% H₂SO₄ stream. 

References:

Bendová, H., Dušek, L., & Palarčík, J. (2024). Comparison of anion-exchange membranes for diffusion dialysis of mixtures of acids and their iron salts. Membranes, 14(1), 6. https://doi.org/10.3390/membranes14010006

Merkel, A., Čopák, L., Dvořák, L., Golubenko, D., & Šeda, L. (2021). Recovery of spent sulphuric acid by diffusion dialysis using a spiral wound module. International Journal of Molecular Sciences, 22(21), 11819. https://doi.org/10.3390/ijms222111819

Ruiz-Aguirre, A., Lopez, J., Gueccia, R., Randazzo, S., Cipollina, A., Cortina, J. L., & Micale, G. (2021). Diffusion dialysis for the treatment of H₂SO₄-CuSO₄ solutions from electroplating plants: Ions membrane transport characterization and modelling. Separation and Purification Technology, 266, 118215. https://doi.org/10.1016/j.seppur.2020.118215

Zhang, C., Zhang, W., & Wang, Y. (2020). Diffusion dialysis for acid recovery from acidic waste solutions: Anion exchange membranes and technology integration. Membranes, 10(8), 169. https://doi.org/10.3390/membranes10080169