Online analysis of sulfuric acid and zinc sulfate in the viscose process

Viscose, often called rayon, is the original man-made fiber. Created from regenerated cellulose materials like wood pulp and cotton linters, its market size is expected to reach 40.26 billion USD by 2032 [1,2].

Viscose is gaining popularity through a growing demand for sustainable fashion. Its soft, breathable, and absorbent qualities make it a comfortable and eco-friendly alternative to cotton and polyester.

In the first production step, the wood pulp is immersed in sodium hydroxide (NaOH) to convert it to alkaline cellulose (Figure 1). After pressing and shredding, the alkaline cellulose is aged to depolymerize. A solution of carbon disulfide (CS₂) is added to form cellulose xanthate. The resulting crumbs are dissolved in NaOH to obtain a viscous solution called viscose.

After ripening, filtering, and degassing, the viscose solution is pumped under pressure through metal spinnerets submerged in a spin bath. The spin bath contains sulfuric acid (H₂SO₄) to acidify the cellulose xanthate, sodium sulfate (Na₂SO₄) for rapid coagulation, and zinc sulfate (ZnSO₄) to cross-link the cellulose molecules.

Many types of viscose fibers can be made by changing various process conditions and adding chemicals. The final steps are drawing, washing, and bleaching.

To optimize the wet-spinning process (Figure 1, purple star), it is crucial to measure the acid and zinc concentration 24/7. Traditionally, total sulfur and byproduct sulfur have been quantified through a laborious gravimetric process involving sulfate precipitation [3]. However, the technical expertise, time, and space required for this method have limited its practical application. Thus, the implementation of rapid and reliable analytical techniques is essential for effective process control.

Metrohm Process Analytics offers several options to measure the critical chemical components in the viscose spin bath. The 2060 TI Process Analyzer (Figure 2) is integrated to measure the sulfuric acid and zinc concentration simultaneously. This analyzer plays a vital role in closed-loop control. This increases product throughput and yield while minimizing chemical consumption.

Additional analytical techniques can be employed to optimize the wet-spinning process. For instance, X-ray fluorescence (XRF) can provide real-time monitoring of trace elements like zinc in the spinning solution. Accurate zinc concentration measurement is crucial, as it can fluctuate due to factors including measurement duration, background interference, detector sensitivity, and sample preparation. The 2060 XRF Process Analyzer from Metrohm Process Analytics is fully capable of this online analysis.

 AN-PAN-1004 ABC Titration: Analysis of alkali, carbonate, hydroxide, and sulfide in pulping liquors 

AN-PAN-1011 Determination of permanganate absorption number (PAN) 

AN-PAN-1035 Automated online analysis of indigo, hydrosulfite, and other parameters in textile dye baths

• Safer working environment for employees (nuclear reactor).
• Fully automated diagnostics – automatic alarms for when samples are out of specification parameters.
• Guarantee compliance with environmental standards.
• High accuracy for lower detection limits of boron.