## 2 Characteristics of the Leaching Process in the Context of Risk Assessment  
The technology of oxidation leaching in autoclaves is a complicated process. Depending of leaching conditions several processes may run simultaneously in the closed autoclave, e.g., sorption of the gaseous oxygen in the solution, mass transport from the fluid volume to solid particles, diffusion of reagents through the boundary surface layers on the surface of solid particles, diffusion of the products of chemical reactions in the solution. Oxidation of sulfides runs by means of dissolved oxygen in a liquid phase whereby sorption of oxygen by leaching solution plays an important role [5; 6; 7].  
Decomposition of the real flotation sulfide concentrate is a goal of the process.  
Processes of dissolution, oxidation, diffusion, precipitation, and hydrolysis can be described by the following reactions:  
\[2\text{FeS}_{\text{2(s)}}+15/2\text{O}_{\text{2(g)}}+8\text{OH}^{-}_{\text{( aq)}}\rightarrow\text{Fe}_{\text{2}}\text{O}_{\text{3(s)}}+4\text{SO}^{2-}_{ \text{4(aq)}}+4\text{H}_{\text{2}}\text{O}_{\text{(l)}} \tag{1}\]  
\[2\text{FeAsS}_{\text{(s)}}+10\text{OH}^{-}_{\text{(aq)}}+7\text{O}_{\text{2(g) }}\rightarrow\text{Fe}_{\text{2}}\text{O}_{\text{3(s)}}+2\text{AsO}^{3-}_{ \text{4(aq)}}+2\text{SO}^{2-}_{\text{4(aq)}}+5\text{H}_{\text{2}}\text{O}_{ \text{(l)}} \tag{2}\]  
The flotation sulfide concentrate from Pezinok (Slovak Republic) was used for the experimental work. Chemical composition of the concentrate is listed in Table 1. As regards the mineralogical composition, it contained pyrite, arsenic pyrite, tailings, silica, and shale.  
Rotating autoclave LAMPART of 4 l volume was used for leaching; oxygen was introduced through a pressure reducer. The concentrate was mixed with a relevant volume of the solution of NaOH a slurry of different density with the ratio L:S (liquid/solid) between 5:1 and 20:1 was formed. The concentration of the leaching solution NaOH varied from 3.55 to 7.1 M and applied temperatures ranged between 120 and 185 \({}^{\circ}\)C. After the temperature reached the required values, the reaction time 90-300 min was measured. All experiments were carried out at a constant initial pressure of oxygen 0.9 MPa which was sufficient for the course of oxidizing reactions in the system. [PERSON] and [PERSON] proposed that in the equipment used for pressure leaching the highest efficiencies of the parallel decomposition of both sulfides was achieved at the following conditions: concentration of the leaching solution-3.55 M NaOH, the temperature of 185 \({}^{\circ}\)C, the initial pressure of O2-0.9 MPa, time-180 min [8].  
Due to the aggressive environment, increased pressure and temperature, the failure of the leaching process is possible. Its probability was calculated by Fault Tree Analysis (FTA).  
The term \"failure\" (fault = top event in FTA terminology) means the premature termination of leaching, the result of which is unusable and implies material and financial loss or damage to health.  
The goals of the work are:  
1. To calculate the probability of the top event-the failure (early termination) of the leaching process using the FTA.
2. To model the impact of the dynamic of the probability of base events on the top event using the Monte Carlo method.  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{Concentrate} & **Fe** & **As** & **S** & **SiO\({}_{\text{2}}\)** & **Al\({}_{\text{2}}\)O\({}_{\text{3}}\)** & **Au** & **Sb** & **Cu** \\ \cline{2-9}  & 27.00 & 7.78 & 23.76 & 15.6 & 9.88 & 0.0041 & 0.23 & 0.17 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Chemical composition of the concentrate (wt. %).  
Propose an effective improvement of the leaching process in order to effectively reduce the probability of the top event.