A novel approach to polymer dissolution
BASF and ProProcess * discuss how continuous, modular hydration and dissolution systems can be designed to remove the complexity of large batch polymer equipment in tailings thickening
Thickening of tailings material produced in the mining, chemical, water supply and sanitation industries started when John Dorr invented the Dorr Thickener in 1905( Mining Foundation of the Southwest, 1997- http:// www. miningfoundationsw. org / John _ Dorr). These conventional thickeners were used well into the 1960s until the development of polymer flocculants allowed for a denser thickener underflow and led to the development of high rate thickeners in the 1970s. This was followed by the invention of high density and deep cone paste thickeners a decade later( Paterson and Cooke Thickener Course Notes 2014, p. 13). The underflow and paste quality produced in these thickeners and the resultant tailings dam is strongly influenced by the selection of polymer.
In addition to selecting the correct polymer for optimum thickening and tailings deposition the preparation, transfer and injection of the polymer is of utmost importance to maximise the efficiency of the resultant polymer solution. BASF in conjunction with ProProcess has supplied one of the largest continuous polymer plants in the world, as well as one of the world’ s largest batch polymer plants for a mining application. Modular plants are ideal for use in remote locations. The off-site assembly greatly reduces construction costs on site. Making use of containerised plants further optimises the structural steel requirements of the plant and aids in the transportation of the modules to site. Additionally, the plant can be trial assembled and tested prior to shipping, reducing the risk of site modifications. The individual modules can be designed such that they stack on top of each other, thus reducing the plant footprint. This can aid in the reduction of civil costs in locations where unsuitable soil conditions( clay soil environment) prevail and supporting soil needs to be imported. Steel raft foundations have been developed that can be manufactured locally or transported to site.
Batch hydration versus continuous
Conventional polymer hydration has usually taken place in a batch type process whereby the hydration tank is separate from the polymer ageing and storage tanks. Where large volumes of polymers are required, tank sizing with adequate redundancy and duty standby arrangements presents challenges with regards to overall footprint, fabrication, shipping and overall costs. Utilising a modular continuous hydration
concept where the majority of components would fit inside the typical transportation window allows for complete fabrication of items off-site.
The following table shows a direct comparison of the capital requirements for a typical batch versus continuous hydration and dissolution system when the dry polymer design demand is about 1,600 kg / h at a design concentration of 0.65 % w / v with active dissolution time of 40 minutes.
The main benefit in operating a batch system is the ability to design in a high level of contingency through having a standalone storage system. However, there also many ancillary components that are not considered which contribute to challenges when economically designing a batching system at this capacity including: structural steel supports, access stairways and landings, civil works, agitator size, transfer pump housing etc.
Polymer off-loading
For operations that use large quantities of polymer, the preference is for the polymer powder to be transported to site in specially designed inter-modal containers( POD) to avoid the handling of bulk bags and improving the EHS conditions for the customer. Intermodal containers are able to reduce the transport footprint, yielding freight savings per container.
Once on site, the POD is loaded on to a tilt truck and is connected via a 10 in hose to a rotary feeder allowing the polymer to be gravity fed and transferred into the polymer storage system( PSS) using a pneumatic transfer system specially designed for the task, which ensures that the temperature is optimal for transfer. The silo headspace is humidity controlled to prevent clumping of the polymer within the silo. Similarly, moisture build-up in the transfer lines that could form blockages is negated by blowing through dry air. The lines are blown clean prior to and post the transfer process. A typical transfer of 20,000 kg of powder polymer can take around 60 minutes to complete.
A major advantage of the system is that it is self-contained. Site assembly is minimised once the POD is positioned, piped up and ready for operation. The off-loading system is fully automated and controlled using a specific start-up and shut down procedure and has been designed to utilise conventional transfer equipment from established and well known pneumatic transfer brands to minimise risk, ensure a trouble free start up, and allow for continued parts support.
The PSS is equipped with a dust extraction system fitted with anti-static filter bags that is used to mitigate the polymer content of the exiting conveying air. Reverse jet filters are preferred for cleaning the filter bags and these systems can be modified to suit any climate conditions. Inspection
Type of Polymer Mixing Tanks Transfer Pumps Storage Tanks Dosing Pumps
Preparation System Conventional |
3 x 60 m 3 |
3 x 180 m 3 |
3 x 90 m 3 |
2 x 128 m 3 / hr |
Batch System New Continuous |
4 x 42 m 3 |
0 |
0 |
2 x 128 m 3 / hr |
Polymer System |
|
|
|
|
hatches located at the top of the PSS are accessed via non-slip flooring. The PSS is also fitted with large manholes on the shell to enable cleaning and access and inside of the PSS an epoxy coating is applied to prevent static build up.
Dry polymer transfer
The PSS skirt is used to house the polymer dosing equipment and other ancillary equipment. The PSS is equipped with vibration pads to ensure no bridging of the dry polymer during use with the activation of the vibration pads integrated into the control sequence, which also ensures that the vibration pads are not activated too often, potentially resulting in polymer compaction and silo inoperability. Dehumidifier units are employed to maintain a low moisture content of the air in the PSS skirt.
Polymer is gravity fed from the PSS into intermediate storage silos which are equipped with BASF’ s proprietary dry polymer dosing system called the Alcotech Aerowet™. Each Aerowet is used for dilute phase transport of the dry polymer to the wetting apparatus at the controlled dose rates to meet the requested polymer concentrations. The units consist of a screw feeder with variable frequency drive( VFD) motors feeding a transfer line connected to a side-channel blower enabling pneumatic transfer of the polymer. During commissioning the VFD is calibrated to ratio the amount of powder dosed, resulting in a straight-line correlation.
Polymer hydration
The pneumatically conveyed polymer is contacted with water in the Jet Wet Head ®( JWH), BASFs
P4 International Mining | JUNE 2017 Supplement