Dry Stack Tailings: Air-Dried as an Alternative to Filtered Tailings
Due to the considerable expense of using filtration to achieve dry stack tailings, air-drying of paste or thickened tails is an option worth evaluating. In certain situations, paste technology combined with an inexpensive, widely applicable air-drying scheme can permit bulk handling of the tailings.
This proposition is not a solution for every site. However, as the following case study illustrates, air-drying of paste or thickened tails can be a viable option for dry stacking and an alternative to filtered tailings.
Dry Stacking With Relocation in Mind
First, let’s define the term “dry stacking.” In tailings management, it is most often interpreted as filtered tailings or possibly as tailings that cannot be pumped (Davies & Rice 2001). While it is not a loosely defined term, there is certainly flexibility of the meaning since the Merriam-Webster Dictionary doesn’t help us; at least not yet.
What is clear is that paste underflow from a high density thickener is not considered “dry stack” material; with the possible exception of few creative paste thickener sales efforts.
Certainly the argument can be made that a primary goal of dry stacking is to minimize free water in the tailings. Paste and thickened tails accomplish that goal quite effectively. They do however produce a non-Newtonian slurry that can be pumped, and transporting paste thickener underflow via truck or conveyor belt is more challenging than with filtered tails.
Leaning on the definition of dry stacking as “tailings that cannot be pumped,” we present a case study in which thickened tails are air-dried to where they may be easily relocated using bulk handling equipment. The method requires low capital expenditure (CAPEX) and operational expenditure (OPEX), substantially less than required by filtration.
Admittedly, few installations outside of our case study have considered relocating the stack after it has dried. Nonetheless, the concept is intriguing enough to share.
Shri Bajrang Power and Ispat Limited (SBPIL) is located near Tilda, about 35 km (22 miles) from Raipur, Chhattisgarh, India. SBPIL’s parent is GOEL Group, a leading producer of steel, sponge iron, pellets and power in Chhattisgarh.
The Tilda site is an integrated steel facility producing 1.4 million metric tons per year (1.5 million short tons per year) of iron pellet and 500 tons per day (551 short tons per day) of sponge. The complex includes a 16 megawatt power plant. The mill processes 2.0 million metric tons per year (2.2 million short tons per year) of high and low grade iron ore that results in 60-70 dry metric tons per hour (mT/hr) (66-77 short tons per hour) of tailings. Original design of the system was to deal with as much as 150 mT/hr of solids. (Misra, et al, 2016)
Mill tailings at SBPIL contain enough fines to support thickened tails with yield stresses of 50-80 Pascals (Pa); high enough that there is little to no free water in the underflow, but low enough for deposition with 1-3% angle of repose. The latter means monsoonal rains will drain away without eroding the stack. Precipitation averages 1,188 mm (47 inches) annually, but over 95% of that comes in a four month period between June and September.
In 2015, SBPIL replaced their high rate tailings thickener with a 14 meter (46 foot) WesTech Deep Bed™ paste thickener. Deposition is in an existing 3.6 hectare (9 acre) tailings pond, accomplished by pumping paste to three towers located in different zones of the impoundment as illustrated in Figure 1 (small version below; full-size at the top of the page).
The towers in Figure 1, and their surrounding stacks, are marked with red arrows. Rain runoff is clearly visible in the background, as the photograph was taken near the end of the monsoon season. The stacks have suffered no visible erosion, attesting to the success of the low-angle deposition.
Figure 2 (below left) shows a tower with newly deposited paste and Figure 3 (below right) a second tower where the paste stack has dried.
Alternate Method to Filtered Tails
SBPIL sequences deposition of paste through one tower after the other. Their plans were to seek a market for the tailings and ship the solids by reclaiming them from dried stacks.
A suitable buyer for the tailings has not been found, but SBPIL is currently considering blending the dried tailings with high grade run of mine ore for reprocessing. Meanwhile, conversion to paste has substantially extended the theoretical life of the tailings impoundment.
Years of experience with paste and thickened tails has shown that lack of surface water means the stacks dry very quickly. This is illustrated by the series of photos below, showing a paste sample air-drying over four days. The first image is Day One, the second is Day 2, and the final image shows Day 4.
SBPIL has found that during the dry season, once paste deposition has been moved to another tower, stacks will support personnel and some equipment after about a week. Those studies correlate with observations at other installations, where experience has shown that dry-time is much more dependent on depth of deposited layers than it is on other factors, such as rain events.
Further study will be needed to:
- Determine dry time during the monsoons.
- Optimize the deposition and dry cycle.
- Address the inevitable unintended consequences of a new process.
However, SBPIL’s successes in their first year of operation, coupled with experience at other facilities, suggest that their original plans are functionally sound.
Other sites may find that the required size of the deposition area is a drawback to air-drying thickened tails as a method for dry stacking. In SBPIL’s case, the area is 3.6 hectares (9 acres); but the old tailings pond was a space readily available. Deposition methods could be optimized and the deposition towers, which were designed and built locally, are an atypical configuration. Addressing those issues would offset some of the space concerns.
“Functionally sound” does not always translate to economically sound. Therefore, we attempted a financial assessment of the proposed method.
Capital cost for SBPIL to design, build, and install the entire system was approximately US$700,000. A thorough study for a filter plant has not been done; but purchase of local pressure filters for the same tonnage would be perhaps US$600,000 to US$900,000. Attendant filter plant costs for auxiliary equipment, design and construction are assumed to be another US$1,000,000.
So using filtration to achieve dry stacking calculates to about 2½ times the CAPEX. Given the accuracy of estimates, a range of 2-3 times would be more appropriate.
Designing the system around filtered tailings would likely require some sort of intermediate transport and storage of filtered solids, where that is not necessary for the air-dried tailings at SBPIL. However, a different air-dry configuration at another site might also require intermediate transport and storage; so we will simply acknowledge that there is no one-size-fits-all answer to such a complex issue and consider the cost of the filter plant to be the major variable in this analysis.
Because India is one of the most cost-effective places in the world to build such a system, we also deemed it appropriate to do a CAPEX evaluation for design, build, and construct in North America. Table 1 has the results of the alternate comparison of a paste thickener circuit followed by air-drying versus a high-rate thickener followed by filtration.
An attempt was made to estimate OPEX (see Table 1). While those values are also approximate, they are sufficient to illustrate the point; operating costs for a filter plant would be much higher than for air-dried tailings.
|Table 1 - Comparative costs based on North American estimates (approximate values only)|
The numbers are general in nature and the economic analysis clearly incomplete, but it illustrates that there must be substantial costs saved or revenue generated to economically offset the filtration step.
Loss of Process Water
Perhaps the greatest disadvantage to air-drying thickened tails is the loss of process water due to evaporation. For sites where water costs are high, a filtration plant may be economically justifiable compared to air-dried tailings.
Although each site must be evaluated for its own challenges, we have outlined a generalized assessment to show differences in water loss for different options.
If one considers the SBPIL yield stress curve in Figure 1, and knowing that the planned operating range was 50-80 Pa, then solids in the thickener underflow would range 55-62% by weight; or conversely residual water would be 45-38%. SBPIL processes 60-70 mT/hr (66-77 sT/hr) of dry solids. As can be seen in Table 2, the low range estimate of water in the underflow would be 37 mT/hr (41 sT/hr) and the high range would be 57 mT/hr (63 sT/hr).
|Table 2 - Estimated water losses with air-dried thickened tails|
Residual cake moisture in filtered tailings can vary widely. A range of 6-50% by weight, or 94-50% solids, covers most operations (Robertson, et al 1982), (Watson 2010) and (Fiscor 2010). However moisture content less than 10 or 15% is rare and, in the event of high moisture filter cake, it is unlikely that filters would be selected; so for comparative purposes we will assume a practical range of 10-35% moisture. Applied to the slurry at SBPIL, the hypothetical range of cake moistures would be 7-38 mT/hr (7-42 sT/hr), as can be seen in Table 3.
|Table 3 - Estimated water losses with filtered tails|
Subtracting residual moistures in Table 3 from Table 2 suggests that a filter plant could recoup 20-30 mT/hr (22-33 sT/hr) more water than the paste thickener.
At SBPIL, the influent to the thickener varies from 7% to 22% solids by weight, which means 248-797 mT/hr (274-879 sT/hr) of water in the thickener feed (see Table 4). Compared to values from Table 2 and Table 3, we can see in Table 5 that the air-dried tailings lose 5-23% of the total water and filters could be expected to lose only 1-15%.
|Table 4 - Estimated water in thickener feed|
|Losses||mT/hr||Percent of Inflow||mT/hr||Percent of Inflow|
|Out with Paste||36.8||4.6%||57.3||23.1%|
|Out with Filter||6.7||0.8%||37.7||15.2%|
|Table 5 - Water loss from different options|
Like any engineering decision, one must consider the compromises involved. However, air-drying thickened tails to achieve dry stacking appears to be a practical option in many situations.
Davies, M.P. and Rice, S. (2001). An alternative to conventional tailing management – “dry stack” filtered tailings, AMEC Earth & Environmental, Vancouver, Canada
Misra, B.P., Upadhyaya, P.S., Biesinger, Mark T., and Goel, Pawan (2016), Start-up of India’s first ever Paste Disposal System for Iron Ore Tailings Management at Shri Bajrang Power and Ispat Ltd. using a WesTech Deep Bed™ Paste Thickener, Federation of Indian Mineral Industries Conference – FIMI-2016
Robertson, A. MacG., Fisher, J.W. and van Zyl, D. (1982). The Production and Handling of Dry Uranium and Other Tailings, Symposium on Uranium Mill Tailings Management, Colorado, December.
Watson, Andrew (2010), Alternative Tailing Disposal – Fact and Fiction, International Mining Supplement, April
Fiscor, Steve (2010), Filtration Becomes a Viable Option for Environmental Compliance, Engineering and Mining Journal, 11 August 2010