|
Resumen:
|
Although great efforts have been carried out during the last decades, the reduction of water use continues being a key issue for papermakers due to the stringent environmental legislation, the increase in water prices, the treatment costs, the development of sustainable processes, or simply due to the lack of water resources. Closure of water circuits by traditional internal treatments (e.g. dissolved air flotation) and reuse of the process waters is a common practice in paper mills; however, a high closure of the water circuits produces an exponential accumulation of dissolved and colloidal material (DCM) which affects the production process and the quality of the final product. One of the most applied strategies to further reduce the fresh water consumption is the reuse of the final effluent after an advanced treatment, usually involving a final reverse osmosis (RO) step. In deinking paper mills, the reuse of the effluent is limited by silica fouling of RO membranes. If silica is not previously removed, the maximum recovery in RO membranes is limited to 20%. The main source of these high silica levels (150?250 mg/L SiO2) is the sodium silicate used as process additive, which cannot be substituted due to its variety of functions and low price. For an economically feasible process, the RO recovery must be increased from 20% to 60?80%. Consequently, it is necessary to find a silica removal technique that allows treating large volumes of water with high silica contents. Silica concentration should be reduced to around 20?60 mg/L while maintaining the conductivity increase at minimum, without causing side effects on the membrane and at low cost. Silica removal is usually carried out by coagulation or during precipitative softening processes at high pH. These techniques have been successfully applied in waters with low silica levels and high hardness content. However, there are only a few studies in the literature focused on silica removal in waters with high silica and low hardness contents such as those from the paper industry. Therefore, the main objective of this doctoral thesis is to generate new knowledge on the applicability of these two techniques (coagulation?flocculation and silica removal during precipitative softening) in the waters from the paper industry. Moreover, the potential use of adsorption onto activated alumina and hydrotalcites for silica removal is also studied...
|
