Introduction

Sea salt has been produced for centuries in saltworks, evaporating basins where seawater is concentrated in salt, until reaching saturation and crystallisation of food-grade NaCl. As a by-product, saltworks also generate exhausted waste brine with a concentration more than 20 times higher than seawater, commonly called bittern. This can be an important source of valuable minerals (Mg, K, Br, Li) recoverable through different integrated approaches, generally requiring a first purification step using Ultrafiltration (UF) for polishing and organics removal (www.searcualrmine.eu). However, due to the unexplored nature of such application, the scientific literature is lacking of experimental information on the behaviour of ultrafiltration systems used as a pre-treatment for bittern processing.

The aim of this study was to characterise for the first time in the literature the peculiar behaviour of an UF pilot plant processing real bitterns, highlighting operating difficulties, flux enhancement possibilities and also collecting useful rheological data on real bitterns.

Materials and Methods

Different volumes of feed bittern from Trapani saltworks in Italy were treated in different UF tests at pressures ranging from 2 to 4 bar.

Tests were carried out in a UF pilot unit (Sepra, Italy, see Figure 1) equipped with a load centrifugal pump (Schmith) and a booster centrifugal pump (Lowara). Two asymmetric ceramic membranes with a total area of 0.7 m² with a nominal cut-off of 300kDa were used. Pressures was monitored by two gauges. During the experiments, the flow rate, conductivity and temperature (on both permeate and retentate side) were measured.

To monitor the composition of the feed inlet, chromatograph analyses (Metrohm 882 compact IC plus end Methohm 930 compact IC plus) were performed periodically to determine any possible change in the ionic composition of feed bittern and permeate. To complete the characterization of the bittern, a novel study was conducted on the variation of bittern viscosity as ionic composition and temperature changed. To do this, temperature-varying viscosity tests were performed using an Ubbelohde viscometer and a thermostat bath.

To determine the organic compound inside the bitterns, COD analyses were carried out.

Results and Discussion

In this study, the influence of the main properties of the bitterns (chemical composition, organic content and viscosity) and operating pressure on the main process parameters of the pilot plant, i.e. flow and permeate flow rate, was investigated. These case studies are representative of the bitterns available during the different stages of salt production. To better investigate the process, 3 case study made up as different bittern having different ionic and organic composition, are investigated. For each case study, the main process parameters were analysed, such as variation in flux, viscosity, fouling and variation in treatment time for each batch as the operating pressure varied...