IM 2021 July 21 | Page 30

PASTE & TAILINGS 2021

Filter aids to improve iron ore tailings filtration

A team * from Clariant and mining customer Vale outline how filter aids are enabling enhanced tailings filtration as the industry moves towards more use of dry stacking

Clariant Mining Solutions is focused on developing chemical-based technologies that support a more sustainable mining process . As the iron ore mining industry is focusing its efforts on developing safer tailings disposal alternatives , the demand has increased for dewatering processes of waste streams generated in the iron ore concentration process . As mining companies , particularly in Brazil , are targeting the replacement of tailings dam disposal with safer and more sustainable solutions like paste or dry stacking , these companies are investing in new filtration plants to dewater both coarse and ultrafine tailings .

The filtration rate in these plants depends on the pressure difference across the filter cake and its resistance to the flow , and a high content of fine mineral particles , particularly in the ultrafine tailings , reduces filter cake permeability leading to increased complexity in the filtration processes . As a result , tailings filtration has become a common bottleneck with low filtration rates , high cake moisture and other operational issues limiting plant productivity .
Filter aids are chemical reagents that may assist in this process , enabling better drainage of water out of the filter cake . For the proper selection of the ideal filter aid chemical technology it is important to evaluate not just the mineral particle size distribution , but also other aspects of the slurry such as the solids content , rheological behaviour and the equipment used for filtration .
For the dewatering process of coarse and ultrafine tailings , two different types of filtration equipment are usually considered : rotatory vacuum disc filter and filter press . Vacuum filtration consists of the application of a vacuum pressure on the system , allowing the liquid to pass through the filter cloth while particles are retained . For filter presses , the slurry is pumped to the filter plates using positive displacement pumps , and when the plates are filled by the slurry the system , pressure is increased which allows the water to be drained away from the filter cake . Vacuum filtration is usually applied for coarse tailings which require low pressure to dewater . For ultrafine tailings , filter presses are typically required instead as high pressure is needed . The pressure and the filter cloth type are also factors that affect the filtration process . Regardless of the type of equipment used , filter aids can generally improve the efficiency of the filtering process .
How do filter aids work ?
Capillary pressure tends to keep liquid trapped in the cake . If the pressure difference through the liquid is greater than the capillary pressure , the water will flow . Increased cake thickness , higher mineral particle specific surface area and higher liquid viscosity will reduce water flow . Considering these conditions and their effects on water flow , different chemical technologies can be evaluated with the capability to change these parameters and improve the filtration process . Surfactant based filter aids adsorb on the surfaces of mineral particles thus turning them hydrophobic and repelling water . These chemicals also act on the liquid-air interface by reducing the superficial tension , facilitating the water to flow out of the filter cake . On the other hand , polymers , like coagulants or flocculants , aggregate the fine particles that tend to clog the channels , allowing a for better water flow as it is drained away . Tailormade
P6 International Mining SUPPLEMENT | JULY 2021