Tailings Dust Emissions (TPD1705032)

Chino Mine Visit

The Freeport-McMoRan Chino Mine is a copper mine located 15 miles east of Silver City, New Mexico. Chino’s TSF (Fig. 3), which the Dust Busters had the opportunity to tour, is constructed with an upstream design. In this design, the underflow stream from the hydrocyclone is deposited upon the embankment, which gives rise to a new wall that supports the top of the slope to the beach. As the elevation of the embankment increases over time, the embankment shifts toward the center of the tailings storage facility.

At the Chino mining facility, a 45 weight percent solids tailings slurry is pumped 16 miles from the thickener to the TSF at a rate of 12,000gpm. A process flow schematic of the

 

Figure 3. Chino Mine Tailings Facility

hydrocyclone skids is given in Figure 4. Once the slurry has reached the TSF, it is divided and fed to four hydrocyclone skids that produce two streams: an underflow stream and an overflow stream. The underflow streams, with flow rates of 470 gpm per skid, are used in the building of the dam and have a coarse solids content of 80 weight percent. The overflow is discharged to the beach at a rate of 2530 gpm per skid and consists of 37 weight percent fine solids. The Dust Busters were given the opportunity to take samples of the underflow to use for bench scale testing. Therefore, all experimentation was conducted using actual copper mine tailings, improving the accuracy of the testing results.

The majority of the dust emissions are from the 400 acre area of the dam comprised of the outer berm and the road. The Chino mine builds its dam at a rate of 3-5 feet per year while only working on half of the dam at a time. The hydrocyclones are elevated on cranes, which allows for them to be easily relocated along the 4-mile long dam face [2].

Figure 4. Process Flow Schematic of Hydrocyclone System

Nature of the Material

Chief concerns with tailings are its abrasive, incompressible, and poor flow characteristics. Using the environmental scanning electron microscope and energy-dispersive X- ray (ESEM-EDX) available through the nanotechnology department at the University of Arkansas, the composition of the dry tailings sample was confirmed to consist of primarily silica. Another complication that arises when dealing with mine tailings is the variability in the granule size, as evidenced by the images from the SEM as shown in Figure 5. The copper wire in Figure 5 is 152.4 microns. The largest particle size is about 600 microns and the smallest particle size is about 70 microns.

Figure 5. Copper Mine’s Underflow Tailings under SEM (left) and light microscope (right)

The behavior of the tailings slurry emulates the material’s handling difficulties. The underflow solids quickly dewater, resulting in a sand-like sludge. However, the hydrocyclone underflow will flow freely as long as it keeps moving and does not dewater. The slurry material exits the hydrocyclone and freely flows down the slope of the beach. The angle of repose of the underflow material poured on a flat surface was found to be 20 degrees as shown in Figure 6. A slump test (Fig. 6), a common test used to characterize concrete, was conducted on a settled sample of the underflow slurry. The resulting angle of repose was essentially 90 degrees. As the angle of repose increases, the slurry’s ability to flow decreases.

Figure 6. Angle of repose of the tailings underflow slurry

Current Common Dust Mitigation Methods

The most commonly used methods to mitigate dust at mine tailings storage facilities is the topical spray application of magnesium chloride or other salt solutions. The salt solutions are applied with a composition of 35 volume percent salt and 65 volume percent water [3]. The liquid mixture is sprayed via spray trucks at approximately 0.5 gal/yd2 as needed. Although the salts are effective dust suppressants, they have a short effective life span when compared to other dust suppressants. Salt solutions are reapplied 2 to 3 times per year or as needed.

Hay crimping controls erosion and prevents fugitive dust emissions [4]. A crimper, tractor attachment, uses a serrated disk with blades that are approximately 20 cm apart from one another. The crimper pushes the hay into the tailings and causes the hay to be locked in a vertical position. Anchoring the tailings with hay reduces the dust emissions. Hay’s cellular structure allows for the retention of water within the cell wall. With prolonged water retention, the tailings remain wet for longer periods of time and tailings dust emissions are reduced. The recommended application rate is 2 tons of hay per acre.

Lignosulfonate:Lignosulfonate is a water-soluble anionic polymer byproduct of the paper manufacturing process. It acts as a dust suppressant due to its large size and affinity for binding with other polar and nonpolar compounds. The smaller dust compounds adsorb to the lignosulfonate and form a larger, heavier complex that is not as friable, suppressing the dust. While this compound is water soluble, it becomes insoluble after prolonged exposure to solar heating. This polymer is effective at dust suppression in arid environments and is inexpensive. However, the increased demand in other industries for lignosulfonate is causing the price to rise over time and makes the product increasingly difficult to obtain. Lignosulfonate negatively affects the environment if poorly handled or applied. If large quantities of lignosulfonate were to be released in surface water, it would decrease the oxygen levels reducing biological activity of the applied area. Other considerations are that it is corrosive to aluminum, odorous and sticky, and does not bind well with areas that are previously treated with a chloride solution [14].

Asphalt:Asphalt is the bottom of the barrel product of crude oil and is relatively inexpensive. One concern with the use of asphalt material is the possibility of leaching to occur. Polycyclic aromatic hydrocarbons (PAH) and heavy metals found in asphalt have had the tendency to leach out of the asphalt during rain. The PAH can contain carcinogens and other toxic compounds. Tests have been conducted to determine how much leaching occurs, and most studies have shown that the amount leached into the water is within the regulatory limits [15].

 

PELLETIZATION AND AGGLOMERATION

Process Considerations

One practical way to mold mine tailings into natural clods or peds would be to use an industrial sized pelletizer or briquetting machine. Pelletizers are commonly used in the iron, feed, and wood industries to create pellets. Briquetting machines are used to form coal and charcoal briquettes (Fig. 7). The various types of pelletizers found in industry are ring die (Fig. 7), extrusion, piston press, and flat die pelletizers.

Figure 7. Ring Die (left) for a Pelletizer and Briquetting Machine (Right) [16]

However, large scale pelletizers can generate a force much larger than this. To simulate an industrial pelletizer, and to minimize the amount of binder required, the pressure was increased by transitioning to a twelve ton hydraulic press (30,000 psi).

The next type of pelletizer that was created to simulate a large scale continuous design was an extruder. The extruder forces the tailings through a ½ inch orifice from a 1 inch ID feed chamber (Fig. 8). A knife was used to cut the extrusion into segments.

Figure 8. Extrusion-Style

The extrusion process was unsuccessful. When pressure was applied, free water would drain out of the bottom, and moist tailings would clog the orifice of the extruder.

The hydraulic press was then used to generate one inch diameter pellets with the apparatus in Figure 9. This method generated suitable pellets, but does not mimic any large-scale practical processes.

Figure 9. Hand Press (left) and Hydraulic Press (right)

Experimental Procedure and Results

The binding agents that were considered for pelletizing were a commercial lignosulfonate/bitumen blend, bentonite, cement, asphalt, corn starch, and spray starch. Experiments were conducted using these pellets to simulate the environment at a TSF. These tests included simulating rain, wind, and the hot, arid environment. The Southwest United States, although arid, is prone to short, heavy rainstorms. To simulate the worst case scenario of this environment, the pellets were tested under a hose with a nozzle attachment at a rate of 1 L/min. The pellets were exposed to the rain simulation until the pellet dissociated or an hour passed. The friability of the pellets was qualitatively tested in conjunction with the rain simulation, and the friability was rated on a scale of 1 to 5 with 5 being the most friable.

Table 1. Dissociation and Friability Test Results

A qualitative test was conducted to determine a 3 inch layer of pellets’ ability to mitigate fugitive dust emissions at elevated wind speeds using a leaf blower (approximately 60 mph winds). A wind tunnel was built with plywood and polycarbonate (Fig. 10). This design produced a uniform wind profile.

Figure 10. Wind Tunnel Apparatus

A 3 inch layer of the various pellets was placed atop a 500 g sample of dry tailings. The leaf blower was used to simulate up to 60 mph winds for a period of approximately 1 minute. This test proved that at wind speeds of approximately 30 mph pellets were effective. However, as the wind speed was increased, the pellet bed was disturbed and the concealed dust was entrained. The final test that was conducted on the pellets was the thermal resistance test. Since most mines are located in arid and hot environments, the pellets would be exposed to temperatures of over 100 °F for long periods of time. These conditions were simulated on the pellets by placing them in an oven at 150 °F for 8 hours and observing the effect on the friability and strength of the pellets. The results proved that all the pellets tested were thermally resistant.

Bentonite was a viable option for binding the mine tailings. Pellets formed using bentonite at compositions as low as 0.5 weight percent were strong and not friable. During the rain test the bentonite pellets disassociated after a few minutes. Like the bentonite, asphalt proved to be an effective binder and created a non-friable, strong pellet. The asphalt pellets lasted for seven minutes during the rain simulation. Corn starch and spray starches were also considered for testing. The starches required heating before addition with the tailings, and this is not viable for the full scale process. Another viable option that was considered was a pellet with

0.5weight percent of a commercial lignosulfonate/bitumen blend. The main concern with the blend and lignosulfonate in general stems from the availability limitations. No supplier was found to provide the lignosulfonate at a competitive cost compared to the other binders considered. Portland cement was the final and most cost effective binder. Cement created a strong and non-friable pellet that did not disassociate during the rain test. Concerns associated with cement include its pH effects on the retention pond water and its carbon dioxide emissions. However, these effects were deemed insignificant in comparison to the environmental benefits of dust control. Regardless of the effectiveness of the binder in producing pellets, the pelletizing process as a whole is not economically feasible. A commercial pelletizer does not exist that is able to effectively process the high volume and abrasive nature of the tailings slurry without substantial operating cost.

Bench Scale Apparatus

Dust Busters designed and fabricated a single cavity bench scale apparatus (Fig. 11). The inner semicircle allows the hydraulic piston to compress the tailings and form a pellet. Removal of the inner semicircle allows for the ejection of the pellet by the hydraulic piston.

Figure 11. Bench Scale Pelletizer Design

To represent the full scale production of the hydraulic press, Dust Busters designed a hydraulic piston press. This design is able to process the abrasive material and is rated for the required high pressure.

Full Scale Design

To create these pellets, 2 percent of the underflow from one hydrocyclone skid will be utilized. The underflow stream will be fed to a pugmill mixer, with counter-rotating blades, where it will be mixed to a composition of 1.5 weight percent portland cement. The blended stream will then be sent to one of six pelletizers (Fig. 12). To feed this machine, a hopper with mechanical agitation is positioned over the die to ensure that the slurry continuously moves, preventing clogging of the hopper and completely filling the cavities beneath them.

Figure 12. Process Flow Diagram for Full Scale Pelletization Process

A rotating die with cavities will be filled with tailings from this hopper. As the die rotates, one piston will compress the material in the cavity, and another piston will discharge the pellet from the die. Pellets that are 1 inch in diameter will be created and the free water present in the slurry will be distributed onto the free draining surface of the dam (Fig. 13).

Figure 13. Full Scale Pelletizer Schematic

Economic Analysis

The pelletization solution was found to have high capital and operating costs. The abrasive nature of the tailings wears out the dies in one month. Maintenance, materials and capital costs for pelletization process can be found in Table 2. The capital cost includes the six pelletizers and six pug mill mixers. Four separate commercial vendors were consulted about the economic feasibility of the pelletization process. All vendors confirmed the Dust Busters’ conclusion that the material was too abrasive in nature to operate normally in a commercial pelletizer, resulting in extreme maintenance costs. Two operators would be required for the operation of the pugmill mixers and pelletizers. The cost listed for the portland cement includes both raw material cost and the cost of transportation to the mining facility.

Table 2. Pelletizer Cost Analysis [18]

Slurry Wind Resistance Testing Procedure:Dry tailings were measured and loaded onto 8 inch by 11 inch plots of sandpaper trays to mimic the rough texture of the dam surface. A 500 g underflow slurry mixture and each binder were poured on the dry tailings to mimic the slurry cap. The slurry samples were allowed 24 hours to dry and were then loaded into the same wind apparatus used with the pellet wind testing (Fig. 10). The plots were subjected to 30, 45, and 60 mph wind speeds. The remaining mass on the plots were recorded and the mass retained was calculated. The visibility of the dust emissions was rated on a scale from 1 (holding together) to 5 (immediately blowing off).

Water Longevity Testing:Slurry samples were placed on testing trays and subjected to a simulation that replicated 1 inch of rain. Three rain cycles were used to analyze the longevity of the slurry solutions. Each sample was given a rating from 1 (crust integrity maintained) to 5 (complete loss of crust integrity). The goal of this performance test was to determine the longevity of each proposed mixture. By simulating multiple rain events, the Dust Busters were able to effectively model the durability and estimated lifespan of each method of dust control.

Thermal Resistance:To determine the slurries ability to withstand high temperatures, the slurries were mixed and placed in a heating oven at 150 °F for 8 hours. A rating system based on crust integrity was used to judge the slurries ability to withstand high thermal exposures. The system ranges from 1 (no observable changes) to 5 (complete loss of crust integrity).

 Results: During testing, the angle of the jet stream lifted a portion of the tailings. These portions flew off as chunks. The chunks were collected because they retained their shape, and their masses were recorded separately from the rest of the sample. At a TSF, the tailings would not be susceptible to a direct jet stream shearing off except for at the berm near the top of the tailings dam. The average humidity in Arkansas is greater than the expected humidity in New Mexico, which skews the results of deliquescent materials such as MgCl2. The *Gorilla-Snot^® vinyl copolymer outperformed all other considered binders based on cost and effectiveness.

Table 3. Results of underflow slurry injection testing

Bench Scale Mixer:To effectively mix the binders with the slurry, several industry experts and vendors recommended that a pugmill mixer would offer the best performance [19]. A bench scale pugmill was fabricated to examine how it handled the underflow slurry (Fig. 14).

Figure 14. Bench scale injection mixer

Full Scale Design

The underflow stream exiting the hydrocyclone will be sent to a portable pugmill mixer (Fig. 15). The binder blended slurry will exit the mixer and be spread on the dam of the TSF using the proper earth moving equipment. This process will only be used to create a cap over the tailings dam and will not run continuously.

Figure 15. Commercial Portable Pugmill Mixer [19]

The area of the dam that the underflow slurry would cap is 400 acres. The underflow volumetric flow rate for each hydrocyclone is 63 ft3/min. Assuming that the four hydrocyclone skids operate in six month cycles, each hydrocyclone is responsible for 50 acres per cycle. A minimum 2 inch layer is desirable. Assuming that the hydrocyclone skids are moved once a week, each hydrocyclone is responsible for 2 acres/week. The volume required for one hydrocyclone to form a 2 inch layer per week is 14,000 ft3. At these process conditions, it would require 3.73 hours to create a 2 inch layer. The mass flow rate of the underflow is 9,850 lbm/min. 2,206,400 lbm of underflow would be treated with a binder for the last 4 hours of operation in a week. The full scale slurry process is illustrated in Figure 16 below.

Figure 16. Process flow diagram for full scale underflow slurry injection

Table 4. Injection Binder Cost Comparison

Table 5. Final Injection Solution Cost Analysis

TOPICAL SPRAY

The final solution method considered for mitigating the fugitive dust emissions was topical spray solutions.

Bench Scale Experimentation

The bench scale experimentation for the topical spray solution was performed using the same methodology as the slurry experimentation, utilizing the wind resistance test, water longevity test, and thermal resistance test. Sample sizes of 88 in2 were also used for consistency among the samples. Table 6 below illustrates the results of the bench scale testing.

Table 6. Results of topical spray testing

The diluted *Gorilla-Snot® vinyl copolymer was selected as the most effective binder to be used in the topical spray, as it performed well under all three performance tests. An additional benefit of the *Gorilla-Snot® is its lack of environmental concerns. The polymer is not harmful to biota and is entirely biodegradable, eliminating the possibility of further waste generation. This means that the treated tailings can easily be reclaimed without concern of the binder eroding machinery or further contaminating the tailings waste.

Full Scale Design

The full scale implementation plan maintains simplicity as shown in Figure 17. The proposed topical spray is *Gorilla-Snot® vinyl copolymer which would be delivered to the TSF as a concentrated liquid. This would be provided via a 5,000 gallon tanker truck. The *Gorilla- Snot® could be stored in bladder tanks on site and would be spread on the tailings using a commercially available hydroseeder vehicle with a 3,000 gallon working capacity.

Figure 17. Full Scale Topical Spray Process Flow Diagram

Economic Analysis

The key advantage of this solution method is the low cost of the process in comparison to the high capital costs associated with the pelletization and injected slurry solutions. As shown in Table 7, the majority of the cost comes directly from the raw materials.

Table 7. Cost Comparison for Topical Spray [20]

CONCLUSIONS AND RECOMMENDATIONS

1)  The most effective, environmentally safe, and economical solution for tailings dust control is a topical spray solution consisting of *Gorilla-Snot® that forms a permeable but rigid crust atop the tailings.

2)  The efficacy of the *Gorilla-Snot®, compared to current methods, is higher due to its longevity and economic value.

3)  The *Gorilla-Snot® spray will save the Chino mine approximately $130,000-$390,000 per year on dust treatment material and will not create additional capital costs due to the presence of available on-site spraying equipment.

4)  The *Gorilla-Snot® is a sustainable and environmentally friendly solution due to its compatibility with biota and ability to be reclaimed in the mining process.

5)  The Dust Busters recommend that TSFs apply the *Gorilla-Snot® vinyl copolymer product at 11% by volume in water at a rate of 1,250 gal/acre. If this treatment does not prove adequate for the particular mine, the application should be increased by 50 percent.

6)  Pelletizing mine tailings should not be pursued due to not being a viable option fiscally, logistically, or as an effective measure for controlling the dust.

[9]   National Ready Mixed Concrete Association. Concrete CO2 Fact Sheet. National Ready Mixed Concrete Association, 2012, Accessed 9 Mar. 2017.

[10]   “Poly(vinyl Acetate).” Polymer Science Learning Center. University of Southern Mississippi, n.d. Web. 02 Feb. 2017

[11]   Golden, Mark, et al. “Understanding How to Control the Drying Process of Acrylics.” Just Paint, Mar. 1996, Accessed 9 Mar. 2017.

[12]   Hegenbart, Scott. “Understanding Starch Functionality.” Natural Products Insider. Global Health and Nutrition Network, 01 Jan. 1996. Web. 21 Feb. 2017.

[13]   Sidley Chemical. “Sodium Carboxymethyl Cellulose (CMC).” Cellulose Ethers Manufacturer & Supplier, Sidley Chemical Co. LTD, Accessed 16 Mar. 2017.

[14]   Lignin Institute. “Lignins: A Safe Solution for Roads.” Dialogue, vol. 1, July 1992.

[15]   Shedivy, Ryan, and Amara Meier. Leaching Characteristics of Recycled Asphalt Pavement Used as Unbound Road Base. University of Wisconsin, 2012, Leaching Characteristics of Recycled Asphalt Pavement Used as Unbound Road Base, Accessed 9 Mar. 2017.

[16]   “High Pressure Briquetting Machine.” Henan Fote Machinery Co., Ltd., 2013. Web. 09 Mar. 2017.

[17]   Hawkins, Walter, Process Sales Engineer, Feeco, personal communication.

[18]   Hubener, Robert, Sales Manager, Colorado Mill Equipment, personal communication.

[19]   Allen, Duane, President, Pugmill Systems Inc., personal communication.

[20]   Falkenberg, Chad, and Fabio Neto, President and Technical Director, Soilworks, personal communication.

[21]   United States of America. New Mexico Environment Department. Air Quality Bureau. Title V Operating Permit. By Joseph Kimbrell. N.p.: n.p., n.d. Print.

[22]   Kimbrell, Joseph, Permit Specialist, NMED-Air Quality Bureau, personal communication.

[23]   “Metal And Nonmetal Mines.” Program Policy Manual, vol. 4, Mine Safety and Health Administration, Arlington, VA, 2003, Accessed 9 Mar. 2017.

[24]   United States of America. Town of Silver City. Town of Silver City Sustainability Plan. By Nick Sussillo. Silver City Sustainability Task Force, Oct. 2013. Web. 9 Mar. 2017.

[25]   “State Plans Steps to Reduce Blowing Dust on US 180.” Silver City Sun-News. The Associated Press, 29 Nov. 2016. Web. 9 Mar. 2017.

PROFESSIONAL AUDITS

Duane Allen, President at Pugmill Systems Inc.

Don Jameson, Principal Technical Advisor, Halliburton

Marlene Crosby, Director of Gunnison County, CO Public Works

Walter Hawkins, Chemical Engineer, FEECO

 

Fabio Neto, Technical Director at Soilworks

* For purposes of clarification, some references to vinyl polymer and/or vinyl copolymer have been edited to reflect the actual commercial product brand, Gorilla-Snot®.

– Soilworks LLC, 2019 www.soilworks.com