## 1 Introduction  
Sulfuric acid has a crucial role in the production of phosphate fertilizers that are of significant importance to Morocco's economy [1]. Mineral fertilizers are usually produced from wet phosphoric acid (WPA), which is again produced by reacting phosphate ore with sulfuric acid. The attack of one metric tonne (t) of phosphate ore is usually done with 0.6 t concentrated sulfuric acid and generates 0.4 t WPA [2]. In 2021, Morocco produced nearly 10 million t phosphoric acid [3], resulting in a tremendous sulfuric acid consumption. Globally, more than 265 million t sulfuric acid are produced annually. Liquid sulfuric acid is usually produced from solid sulfur. The manufacturing process involves three main steps: the first consists of the combustion of sulfur, followed by the oxidation of sulfur dioxide in the presence of a catalyst, and, finally, the absorption of the resulting sulfurtrioxide into sulfuric acid to produce ileum, which is then diluted with water to obtain the desired acid concentration (see Equations (1)-(3)) [4]. All of the steps are exothermic, and modern phosphate fertilizer plants can harvest the resulting energy effectively [5]. \[\text{Combustion}:\;\text{S}+\text{O}_{2}\rightarrow\text{SO}_{2} \tag{1}\]  
\[\text{Conversion}:\;\text{SO}_{2}+1/2\text{O}_{2}\rightarrow\text{SO}_{3} \tag{2}\]  
\[\text{Absorption}:\;\text{SO}_{3}+\text{H}_{2}\text{O}\rightarrow\text{H}_{2} \text{SO}_{4} \tag{3}\]  
The sulfur comes in the form of solidified pellets or flakes that are first melted using low-pressure steam, as shown in Figure 1[6]. Most of the used natural sulfur contains salt impurities in the ppm (parts per million) levels [7], but elevated levels between 30 ppm and even up to 12,000 ppm have also been measured. [PERSON] [8] reported the impurity limits depicted in Table 1 for industrial sulfur in Canada. The impurities tend to form rubbery or even solid deposits upon heating [9; 10]. Furthermore, the relatively large quantities of solid sulfur are generally stored outdoors, and they can thus collect dust as well as dirt from the surrounding environment [11]. To produce the required quality of sulfuric acid, the liquid sulfur that is sent to the burner of the sulfuric acid plant must first undergo a pretreatment or filtration process to remove solid impurities, which are referred to as ashes. The presence of ashes in the liquid sulfur can plug the sulfur spray nozzles or accumulate in catalyst beds, leading to an increase in pressure and also to shorter operating lifetimes of the equipment, and this results in increased operational costs. Hence, the removal of these solid impurities is crucial to avoid the premature shutdown of a sulfuric acid plant [11]. Several techniques of separation are used for this purpose already. In the beginning, the removal of solids from molten sulfur was performed using settlers, but due \begin{table}
\begin{tabular}{c c} \hline \hline
**Impurities** & **Specification Limit** \\ \hline Ashes & 500 ppm \\ Carbon & 250 ppm \\ Residual H\({}_{2}\)S & 0–30 ppm \\ Arsenic & 0.25 ppm \\ Selenium & 1 ppm \\ Water & 1\% \\ Acidity & 0.01\% \\ Chloride & 50 ppm \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Impurity limits of solid sulfur as reported by [PERSON] [8]. Figure 1: Basic steps of sulfur fusion with filtration for phosphoric acid production in Morocco. to the high viscosity and density of the molten sulfur, the settling rate of the solids in the molten sulfur is relatively slow [11]. Molten sulfur filtration has been adopted in many plants to remove the settlers that proved to be less effective [9; 12; 13]. In this process, it is common to use horizontal tank-vertical pressure leaf filters made from AISI 316L stainless steel, which are schematically shown in Figure 2. The pressure leaf filters need to be precoated with a filter aid before usage [14]. This filter aid serves to protect the filter cloth from corrosion, prevent fouling of the wires, and improve the later filter cake release [11].