​A moving solution

As the environmen­tal guidelines for mining become stricter, and com­panies seek to become more sus­tain­able as part of their oper­­ating procedures, they are looking at smarter ways to approach minerals processing. One South Australian mineral sands operation recently worked with BASF on its project to develop a novel solution for the management of its tailings deposition and overall rehabilitation process, with the two highlighting the work in a paper at the recent PASTE conference. Martin Edgar, BASF Rheomax market manager for mining solutions Asia Pacific, and Lewis Utting, the global project and business development manager for mining solutions at BASF, explained to Australian Mining the steps in developing an innovative chemical solution for tailings. As part of the mining licence, the site has a maximum defined operational footprint of the open area of the mine. Extraction at the site follows the 'moving hole' concept where topsoil and overburden is removed; the ore is mined; and tailings are deposited in the mined out cells at an average rate of 10 metres per day, which is then capped by the overburden.  
This backfill tailings consisted of seven parts overburden to one part slurried PCP waste. 
The cells were filled with tailings one-by-one where possible, however if slimes levels were unusually high or if the end-of-line polymer dosing was incorrect, then slower dewatering led to an increase in consolidation time and a need to relocate discharge points to the next cell, before returning them to the previous cell to complete the deposition once further dewatering had occurred and sufficient space was available. 
This process resulted in significant time and machinery costs due to constant wall building and spigot relocation work. 
"The mine itself faced a unique challenge, as they essentially had to rehabilitate a moving hole," Edgar explained. 
"As they mined the narrow strandlines they had to then fill the hole behind them, so the challenge is not just putting the tailings in the void as you have the problem that the coarse and fine material separate," he said. 
Because of this it is important that dewatering, consolidation, and stabilisation of the back deposited tailings happen quickly and efficiently.  
According to Utting, BASF worked with the mineral sands operation to implement a tailored chemical tailings solution, rather than the more traditional methods: a mix of art as well as science. 
"Driven primarily by economic pressures, the decision to modify the tailings cell design can retrospectively be shown to be born out of a healthy mix of both operational art and science," BASF said. 
The tailings are treated using a high molecular weight Rheomax ETD 9050 polymer, which rapidly releases clean water for re-use in the concentrator plant.  
The polymer binds the coarse and fine particles together to create a non-segregating deposit, decreasing the time until onset of consolidation and accelerates the rate of consolidation itself. 
"As the polymer treatment released water quickly it provided faster and better consolidation to help the miner meet its 10 metre per day target," Edgar said. According to BASF, "the changes observed in the responses of the tailings however were larger than the authors have observed in similar test programs, and larger than most published examples in literature". 
In addition to these, another development for the mineral sands mine was to adjust the natural acidic pH of the tailings to a slightly alkaline level using lime. 
This was done to overcome a throughput bottleneck in the slimes thickener and reduce polymer dosing levels in the slimes thickener and tailings applications. 
"The addition of lime to take pH up from 4 to 8 has changed the properties of mineral sands slimes and given a 20 per cent increase in thickener throughput capacity and an 80 per cent reduction in flocculant dosing rate to the feedwell," BASF said. 
This impact of the lime addition on polymer dosing, together with the effect of lime on tailings consolidation properties, was also investigated.  
By redesigning the configuration of tailings cells at the operation it means that for every linear kilometre of strandline, only 300 metres of retaining walls are required, instead of 1.6 kilometres of wall as was previously required. 
However, while this methodology can't be implemented at every operation, with Utting stating "the value is where the tailings are fine grained and have a high clay content," and when you have high dissolved salts or permeability challenges with the tailings it provides a more sustainable option.  
"By approaching tailings in a smarter way, and tailoring an approach to the specific needs of an organisation you can increase the life of your tailings facility," Utting said. 
"That's our focus and our expertise, and why we look at how we can apply our chemical suite effectively, to an area that has historically been ignored." 

To keep up to date with Australian Mining, subscribe to our free email newsletters delivered straight to your inbox. Click here.