A new approach to handling fines could improve the strength of tailings storage facilities and solve acid mine drainage problems
By George Rodger
The acid mine drainage that leaches from historic tailings dumps are a constant reminder of the lack of emphasis the mining companies that extracted metals from sulphide ores placed on tailings storage. Even today, as the industry knows well, the design of tailings storage facilities (TSFs) and the consistency of the materials placed within them also present substantial risks. Could we kill two birds with one stone? Could we pelletize sulphide tailings, use them structurally and let the alkali nature of the binder negate the acidic water that flows from the TSF?
To accomplish this, one must first understand the nature of fines in mineral processing. Most labs do not normally perform 1% end-point checks when conducting particle size analysis on very fine materials. A fine dry sieve blinds easily and it can take one person a day to perform a particle size analysis. Instead, they will wash the fine material through the percent passing sieve. Then, vacuum filter the coarse product and pressure filter the fines. Both products are dried and the size analysis continues on from there.
During vacuum filtration of the coarse fractions, the plus 100 mesh fraction has been observed as almost self-draining. Vacuum filtration of coarse fractions down to 325 mesh (45 microns) requires increasingly more time but generally does not take more than 30 minutes. On the other hand, fine fractions in the pressure filter (at 6 Bar or 80 psi) generally require an overnight period to produce wet cakes with 20% to 25% moistures.
The proposed dry, stable and self-fluxing tailings concept follows the same steps as the front end of the wet screening procedure. First the tailings product would be deslimed at 325 mesh (45 microns). The coarse fraction would be vacuum filtered. The slimes or fines fraction would employ a two-step solid/liquid separation to produce a wet cake. Both cakes would be partially dried and combined. Then iron ore pelletizing practices would be employed to produce cold bonded pellets.
Concept Development
The 325 mesh sieve size (45 microns) was selected because of experience with iron pelletization. When pelletizing iron ore, the feeds generally average 65% minus 325 mesh. At that product size, the Blaine test surface areas vary between 1,100 and 2,500 square centimeters per gram (cm2/g), a range that is ideal for pelletizing.
Plus 325 mesh products can be filtered on horizontal vacuum filters, partially dried prior to pelletizing; or sent for permanent storage if they are too coarse. After the minus 325 mesh product has gone through two-stages of solid-liquid separation, the fines cakes are partially dried, and then pug milled prior to pelletizing.
At the pug mill stage, swelling sodium (sodium bentonites) and type 10 portland cement would be mixed into the tailings. Sodium bentonite provide the pellets with green strength prior to firing in a furnace. In this tailings application, bentonite would provide the required green ball strength until the time it takes (30 to 40 hours) for cement to cure to 20% to 30% of its final strength. Reagent quantities shown on the flow sheet, for bentonite are close to the averages for iron ore pelletizing practice. For cement, the quantities would be about the same as what underground mines employ for ground support, but not stope floors.
Blaine Surface Area Estimates
The Blaine test is a method used for the evaluation of the fineness of a powder, on the basis of the permeability to air. It is chiefly used in testing the fineness of portland cement. That test has been adapted for the practice of iron ore balling and pelletizing.
In the iron ore field, lower Blaine numbers in the range of 1,100 to 1,400 cm2/g require lower moistures (8% to 10%) to form pellet seeds and green balls. Below this Blaine range, pellets and balls become difficult to form. Blaine numbers of 2,000 plus require moistures of 11% to 12%. Above these moisture levels, the balls become plastic and deform easily.
A way to estimate Blaine values was set up by employing particle size distributions, dry-solids specific gravity and selected particle-shape prisms. Reduced height prisms for cubic, cylindrical and rectangular prisms provided excellent agreement with published data. The data for six iron ore pellet plants was employed to establish this estimating method.1 In general, those estimates show that balling level Blaine numbers occur in the particle size range between 45 microns (325 mesh) and 10 microns (1,250 mesh).
Experience With Canadian Mills and Sulphide Tailings
Particle size distributions for 12 base metal and gold mill sulphide tailings were tabulated.2 The objective was to determine the range of minus 325 mesh (45 micron) values, which would be feed for a tailings pelletizing flow sheet. The 12 mill average was 61% minus 325 mesh and those values ranged from low of 37.6% to a high of 85.8%.
The 61% minus 325 mesh average is a close match to the minus 65% value employed for iron ore balling practice. In the low, 37.6% minus 325 mesh size distribution range, the coarse particle size fractions would not be candidates for balling feed. But they could be filtered on a horizontal vacuum filter and sent directly to the tailings storage facility (TSF). In the high 85.8% minus 325 size distribution range, coarse material may have to be blended in to lower Blaine values to a range that avoids plastic ball formation.
Proposed Flow Sheet
The proposed flow sheet would start by processing tailings in a cyclone separator. The type of cyclone employed would be a unit with a spigot underflow regulator and a siphon controlled overflow. Those cyclone units provide high (75%) solids underflows; and an almost true slime product overflow, in the 45 micron particle size range.3
The approximately 75% solids, cyclone separator underflows will feed horizontal vacuum filters where the tonnage rate unit area filtration factors will be high, due to the absence of fines. The slime overflow products would be subjected to two solid-liquid separation stages, first in a Lamella type thickener, then in a three-stage belt press.
Lamella thickeners are primarily clarifiers with rake mechanisms. Their reclaim waters can be returned to the mill without fear of fine solids building up in mill reclaim water lines.3 Lamella thickeners have a disadvantage of not being able to produce high (55%-60%) solids underflows. So, their approximately 40% solids underflows would be processed in a three belt press to produce solids with cake moistures between 20%-25%.
Both cake products (the plus and minus 325 mesh cakes) will still contain too much moisture to initiate the pelletizing process. Those cakes will need to be partially dried in hollow-flight driers, where moisture levels can be reduced to the 8%-12% moisture range set by Blaine surface area requirements. The hollow flight driers will provide drying and the initiation of tumbling and mixing to form seed pellets. Then high horsepower pug mills will continue the seed forming stage when dry balling reagents (sodium bentonite and portland cement) are added prior to being fed to a balling drum.
Between the pug mill and balling drum, a recycled seed product is added to initiate and augment the balling process.
The flow sheet shows balling drums being employed to form the dry self-fluxed green pellets. In iron ore pellet practice, both balling discs and balling drums are employed to form green pellets. Current iron ore practice favors the use of balling discs over balling drums. In iron ore pellet practice, the end product is a closely sized (6.25 mm or 0.5 in. diameter) pellet, a size that expedites blast furnace meltdown rates. To form pellets, balling discs have shorter retention times than balling drums. Balling drums with longer retention times are subject to producing “goonies” or oversized pellets, which can be up to 10 cm in diameter. In a tailings application, it would be desirable if the balling process produces a product that has or is close to a natural particle size distribution. The target size distribution for a final dry tailings product should be clean gravel with no fines or clean sand with little or no fines.
For this reason, drum filters were selected to form the green tailings pellets. Drum discharge can be over an end trommel, or from an end with a spiral configuration, which will distribute product evenly over the feed end of a vibrating screen. The flow sheet shows a roller screen accepting fines from a trommel or vibrating screen. There is a need to provide the balling drum with seed pellets to augment pellet growth.
Roller screens will provide consistent sized seed products, in this case 0.6 to 1 cm (0.2 to 0.375 in.). The sizing action of roller screens will maintain the spherical form of the seed pellets. Iron ore pellet-plant roller screens generally have chrome-plated rollers because at this point in the process, the material is “sticky” and chrome-plated surfaces are able to resist that problem.
Final Tailings Product
Pelletized tailings would be transported to a permanent storage site via overland conveyors and discharged at the final site with stacking conveyors. In this application, time and distance to the final disposal site will be much longer. Longer than an iron ore pellet plant where pellets go from balling and sizing directly to an induration furnace. With a series of conveyors, there will be a large number of transfer points, and a final trajectory discharge. The increased handling on a long journey will increase green ball binder requirements. More bentonite and cement binder will be required for the green pellets to arrive intact at their final destination. Should binder requirements become excessive, a temporary adjacent “aging” site might be considered.
With tailings side banks sloped at slightly below the angle of repose, a dry stable long-term TSF will be formed. In addition, with tailings fines formed into pellets, there should be no water saturated areas and the phreatic line, if there is one, will be close to the base of the tailings structure. And, by completion of cement hydration, most of the water employed to form pellets should be absorbed. Rain will drain through the tailings structure, and retained moisture will be limited to interstitial water that surface tension forces employ to bind particles together.
As previously noted, bentonite is added to provide the pellets with a green strength until the cement has time to hydrate to a suitable hardness. In general, sulphide minerals are brittle; so most may end up in the fine particle-size fractions. Cement will provide an in-situ alkali to neutralize acid rock drainage that could eventually form from a breakdown of sulphide minerals.
Concept Proof
Working with bulk tailings samples, the concepts described above can be confirmed in a laboratory and pilot plant. In a laboratory, the amount of sodium bentonite required to form pellets can be established with an airplane tire apparatus or with a 6-ft diameter balling disc. Also required will be a series of physical tests to evaluate pellet strengths. Control of acid rock generation properties can be evaluated by determining the amount of alkali required to neutralize acid soluble sulphides. That may or may not be in excess of the amount of cement required for cold bonding pellets. Finally, in a pilot plant, a full-scale pelletized tailings size distribution can be generated upon which engineering and design values can be established.
There are two U.S. equipment companies with years of experience in balling and pelletizing. The state of pelletizing knowledge has advanced since 1977 to the point that horse manure is now sold as fertilizer pellets; although the binding material is a trade secret. And finally, process equipment required for full-scale pelletizing operations already exists.
Tailings are often referred to as the Achilles’ heel for mining companies. Viewing mines through the ESG lens, savvy mine investors look for fatal flaws before providing capital to build mines. For sulphide ores, dry stable self-fluxing tailings could present a new advantage for TSFs.
Footnotes
- Data from a paper by R.A. Koski, chief metallurgist for Cleveland Cliffs, was found in the SME “Agglomeration 77” books, Volume 1, page 49.
- CIM-Special Volume 49 “Canadian Milling Practice.”
- Personal experience.
George Rodger retired after a 40-plus year mineral processing career in the iron ore, base metals, gold and industrial minerals. He has taken up the mitigation of sulphide acid generation in tailings as a challenge to stay focused. He can be reached at: george.ernest.rodger@gmail.com.
