## 1 Introduction
The filter aid must be carefully selected to allow for the highest flow while allowing a reasonable recovery of solid impurities. [PERSON] et al. [15] provide a useful illustration that shows how the filter aid works. [PERSON] and [PERSON] [8] previously compared candle and pressure-leaf filter technologies by describing tests with various grades of precoating and additions of body feed, and they recommended conditions for plant filtration of sulfur. In addition, [PERSON] [11] showed that the commonly used filter aids are diatomic and perlite. Diatomite, also known as diatomaceous earth, has been used as a filter aid for nearly a century [16; 17; 18; 19; 20; 21]. Sulfuric acid plants usually use diatomic earth as a filter aid, but it is suspected that the high silica content of the inorganic diatomaceous earth can lead, through abrasion, to an early degradation of the filter cloths, so that the sulfur impurities are not properly removed anymore or the units' filter times need to be reduced. In addition, the disposal of the used diatomaceous earth is an issue, as pointed out by [PERSON] et al. [22] in their study. To better understand the impurities present in the molten sulfur, solid samples were recovered from the furnace walls, boiler walls, and gas filters, and they were subsequently analyzed. The accumulation of the impurities at various spots in the sulfuric acid plant revealed high contents of sulfate, calcium, aluminum, and iron, which are shown in Table 2. The source of the calcium is the line used for neutralization. The other contaminants originate from the addition of the mineral diatomaceous filter aid, which is composed primarily of silica, along with impurities of alumina, iron oxide, and alkaline earth oxides [21]. \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Furnace Wall** & **Boiler Wall** & **Gas Filter** \\ \hline Fe & 8.32\% & 11.46\% & 10.76\% \\ Al & 21.90\% & 1.42\% & 1.16\% \\ Mg & 1.27\% & 1.16\% & 1.27\% \\ Na & 2.86\% & 2.34\% & 2.74\% \\ Ca & 25.03\% & 26.03\% & 15.62\% \\ SO\({}_{4}\) & 7.04\% & 41.41\% & 42.09\% \\ Si & 6.34\% & 5.95\% & 6.18\% \\ Cd & 4 ppm & 3 ppm & \textless{}1 ppm \\ Cl & 33 ppm & 27 ppm & 29 ppm \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Characterization of solid residues from different locations in the sulfuric acid plant. Figure 2: Typical horizontal tank-vertical pressure leaf filter used for molten sulfur filtration [12]. The aim of this work is to study a possible substitution of the silica-based diatomaceous earth filter aids with organic cellulose-based filter aids that do not contain silica and other impurities (namely, metals) that could be released into the molten sulfur during filtration. Two types of commercially available, inexpensive, cellulose-based filter aids, FILTER-900 and FILTER-1100, distinguished by the respective molecular weight cutoffs (MWCOs) characterized through the different Dalton numbers (900 and 1100 Da) as well as a diatomite precoat as a control, were used. The focus of the study was on the removal of the impurities present in the molten sulfur. The experiments were conducted with industrial units used in commercial fertilizer production in Morocco.