Selective separation of very small particles by flotation : in relation to soil and sediment remediation

Next to numerous contaminated sites, also large quantities of polluted dredged sediments have to be cleaned in the near future in the Netherlands. Soil washing (classification) is one of the most common remediation techniques to remediate contaminated dredged sediment and excavated soil. It uses chemical and physical particle properties to separate contaminated particles from clean particles. The process is less successful when a large fraction of soil particles smaller than 63μm is present (the fines), which is very likely in dredged sediments. Classification is also not suitable when polluted and clean particles hardly differ in density or size.<br/>In cases where classification techniques fails, flotation can be a successful alternative remediation technique. Flotation is a separation technique based on the capture of particles by bubbles and their collection in a froth layer. It can also be used to succeed the classification techniques in order to clean the fines fraction.<br/>Also in flotation the separation of small particles cause problems. Factors like entrapment and entrainment lead to reduction in efficiency. Entrapment problems occur when fine hydrophilic particles become physically trapped by hydrophobic particles or bubble-particle aggregates. Hydraulic entrainment occurs when hydrophilic particles are recovered in the liquid film between air and bubbles in the froth. Entrainment becomes prominent when large quantities of fine particles are treated.<br/>This thesis investigates the possibility of the selective separation of very small particles by flotation in relation to the remediation of contaminated soil and sediment. The influence of some flotation reagents is investigated and the importance of the contact angle is studied.<br/>A new flotation device was developed which uses dissolved air, as small bubble source, together with a sedimentation zone. The mixing and sedimentation zone are separated by a baffle to prevent settled particles falling back into the mixing zone. This creates a flotation reactor that is capable of separating even the smallest particles whilst minimizing the effects of entrainment and entrapment. For contaminated soil it means that when the contamination is present in a hydrophobic fraction and the other fractions consists out of clean hydrophilic particles, there is no limitation for the separation of these particles by flotation.<br/>The new flotation technique was used to clean fractions of contaminated soil and sediment. The remediation of a sandy soil sludge contaminated with PAH was very successful and PAH removal could be further increased with flotation agents as Diesel and Montanol. The fraction of 45-90μm of PAH contaminated dredged sediment from Overschie and the Petrol Harbor was also successfully treated with flotation. The remediation was less efficient for the smallest fractions (<32μm) of these sediments, but PAH was still selectively removed.<br/>The counter current flotation sedimentation cell is a practical tool to perform a quick laboratory test to investigate if contaminated soil or sediment could be remediated by flotation. Next to that, on basis of the laboratory experiments a prototype reactor could be developed to remediate even the smallest fractions of contaminated soil and sediment in practice. The flotation reactor could succeed other classification processes, but could also be applied as a single (mobile) remediation step. Moreover, this new developed flotation process might even be more useful to selectively float more value valuable particles or powders in other industrial fields.