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In the present work, we report the successful extraction of high purity silica from the Algerian siliceous sand. We improved the chemical extraction process of silica using acid pretreatment before purification and studied the effect of sodium hydroxide (NaOH) solution molarity on the purity of as-obtained silica. X-ray diffraction, Energy-dispersive X-ray spectroscopy, and Scanning Electron Microscopy analyses were carried out on the samples before, during, and after chemical purification. The results show the improvement of the purity of the resulting powder with the change of the alkaline concentration. The results showed a decrease in the impurity concentration of the sand after acid leaching. The extracted silica has a purity of more than 99% and has an amorphous and mesoporous nature, confirmed by the mentioned analysis techniques, together with Fourier Transform Infrared Spectroscopy, thermogravimetric analysis, and the Physical Adsorption Analysis, which makes our chemical extraction method very promising, and the resulted silica is a good source to elaborate solar grade silicon used in photovoltaic applications.

In this study, network simulation method is applied to solve a 1-D solute transfer problem governed by transient storage model in a mountain stream including dead zones. In this computational method, for each node of the discretized domain, the terms of governing equation are substituted by the equivalent electrical devices which are connected to each other based on Kirchhoff?s current law. Finally, the total electric circuit is solved using an appropriate electrical code to obtain the unknown value at the nodes. Because no analytical solutions for this model have been presented so far, to verify network simulation method, the problem is solved by finite volume method, as well. According to the results, estimations made by network simulation method and finite volume method are in good agreement. Further, network simulation method is easier in implementation, especially in implementation of boundary conditions, and faster than finite volume method in computation. Therefore, in the case of 1-D mass transfer problems with a set of coupled equations, network simulation method is recommended to be used as an efficient alternative to numerical methods.

In this paper, the use of a zero-mass extraction device has been simulated in the volcanic area of CampiFlegrei (Italy),one of the most promising geothermal districts of Italy.The sustainability of the heat extraction has been studied with a coupled model of the geothermal reservoir and the deep borehole heat exchanger. The reservoir model has been built using the SHEMAT software, the heat transfer in the deep borehole heat exchanger has been simulated using GEOPIPE, a pure conductive semi-analytical model. An iterative approach has been used to couple the two simulators. The work has demonstrated that the area of CampiFlegrei is a promising candidate to produce sustainable geothermal energy with a zero-mass extraction device. It is also demonstrated that the coupled model of reservoir and deep borehole heat exchanger is the best modelling approach when convective structures are present in the geothermal system, which can generate heat recovery effects.