Paste vs Dry Stacking Breakout work shop Presented by: Jerold Johnson Antonio Accioly Kib Huefner

Breakout Workshops: Paste vs. Dry Stack Advantages and disadvantage Paste Filtration Case Studies New WesTech Control –Pro thickener bed level and profile sensor Yield stress versus solids wt% relationship is dependent on many properties including, temperature, mineralogy, pH, and solids concentration. The affect of PSD variations is that the yield stress curve will shift to lower wt% or high wt% depending on if the PSD is coarser or finer. The PSD is the variable that changes the most at any mineral processing plant. This fact is less important to high rate thickeners but very critical for paste thickeners.

Yield Stress Curve: Sum of all parameters Paste Yield Stress Thickened Tailings The relationship between the internal resistance and the wt% solids of the suspension is referred to as the yield stress curve. The yield stress curve is a summation of all the variable’s affects. The yield stress curve is characterized with an extremely rapid increase in yield stress for relatively small increases in the solid concentration. A form-filter cake (no drying time so that water is still in the interstitial sites) concentration is used to estimate the boundary between suspensions that have character of paste (flow under enough pressure) and those with the character of solids (cracks under pressure) The magnitude of the yield stress is seen in the physical appearance of a paste. Newtonian slurries have relatively no or only minor yield stress (<20 Pa for high rate type thickener underflow). At higher solids concentration the yield stress becomes sufficiently high to support the particles (30 – 50 Pa) and hold them in suspension. This low yield stress suspension is known as “thickened tailings” in the mineral industry (generally 30 – 80 Pa). As the solid concentration increase the angle of repose increases, it becomes more non-settling and can support larger particles, and the slope on a surface stack increases. The high yield stress suspensions are referred to as “paste” (generally 150+ Pa). % solids Filter Cake

PSD Effect: Shifts Yield Stress Curve These yield stress curves are from one glass sand plant. This plant had two tailing streams; one containing the clays (fine PSD) and the other is the reject sand stream. These streams are blended as they are produced and the ratio varies greatly. The green dash curve on the left is mainly the clays stream. The blue dashed line on the right has a large percent of the sand stream. The others are varying ratios of clay to sand. You can see that the yield stress shifts to the left when the PSD is finer and to the right when the PSD is more coarse. A paste thickener is designed to produce a target yield stress. So if a thickener is designed to produce a 50 Pa yield stress the underflow wt% would vary between about 42 wt% to 60 wt%. The underflow would look the same – you would not be able to see the if it was low or high wt%. It would flow the same, pile the same, and generally pump the same.

The effect of PSD on Underflow density Example #2 China gold tailings startup Cylinder slump (10-days) Relatively constant yield stress Solids wt% varies

Quick Drying Water is wicked up through the closely packed particles. No free water on surface One of the unique characteristics of paste / thickened tailings is the speed the material dries. Essentially as quickly as it can evaporate the water is wicked up through the closely packed particles. Note the graph is for a drying study for a bauxite paste. The very fine PSD shows that a paste is produced at 30-35 wt%. After just one week the wt% increases about 50%. There is an initial compaction that occurs in the first couple of days. This tests were conducted during the rain season and the dry season and showed little difference. This is due to the angle of repose and drains rain from the surface to keep the drying to continue. The photo with the truck on the deposit is a central riser and the flow has been headed in to the photo for a couple of month and it has dried enough to support the truck weight.

Advantages of dry stacking Water recovery Public perception (no dam and pond) Smaller volume Others Today we will cover topics ranging from basic system information to applications and installations.

Disadvantages to filtered tailings Environmental dust factor Rain events can re-suspend solids and fines. High operator attention Maintenance costs

Golder presentation

Opportunities to Sell A Paste Thickener Established Plants With Short Life in Their Ponds Difficulty of Site to Receive Permit to Raise Dyke or Build New Pond Plant Currently Using Competing Equipment Belt Press (high flocculant operation costs) High Rate /Filter Combination for Backfill - Reduce Load on The Current Filters. High Rate Thickeners Greenfield Plants Ore Type in Our Installation List High Rate Thickener CCD

Subaqueous Deposition Sub-aqueous deposition is feasible with paste/thickened tailing. Sub-aqueous deposition is when the tailings is placed on a slope to flow into water. The paste does not re-suspend when it enters the water like a slurry does. Bench demonstration photo shows the non-Newtonian flow on a sloped surface and the Newtonian slurry going down the same sloped floor. Due to the yield stress the paste/thickened tailings does not re-suspend and will deposit on the bottom displacing the water.

Subaqueous – Coal Refuse Re-Deposit Subaqueous re-deposited delta Paste thickener Fine Coal Reprocessing plant Sub-aqueous deposition is feasible with paste/thickened tailing. Sub-aqueous deposition is when the tailings is placed on a slope to flow into water. The paste does not re-suspend when it enters the water like a slurry does. This site is a great example. This is a coal plant that is reprocessing their refuse pond. The previous slurry deposition filled the pond from the upper right with the dam in the lower left. The slurry segregated with the coarse depositing first, so the fines are down by the dam. The fines is where the coal is and is being dredged, pumped to the plant and the new refuse (tailings) is thickened and pump back to the same pond. This plant started with using a high rate thickener to produce a slurry discharge. The reprocessed tailings re-suspended creating a fine particle recirculating load that shut the plant down. The paste thickener was installed and the thickened tailings can be deposited sub-aqueous. The photo shows the delta formed as the thickened tailings first flowed under the water and then over time the stack displaced the water and dried. They had to move the discharge point so as to not isolate the top of the pond.

Example: Deep Bed – Coal Refuse Switched to Paste after release from pond and water balance needs Started with Belt Presses Replaced with 15m diameter Deep Bed More than $1.5 million savings each year in operating costs Coal refuse (tailings) pond converted to a surface stack. This site uses a Deep Bed paste thickener to produce the low-yield stress underflow. This site had problems with tailings releasing from the pond through unknown underground workings. They changed to a paste disposal. They originally used belt press filters to dewater to a paste and truck the refuse to the pond. The belt press filters were replaced by the paste thickener which can produce relatively the same wt% solids. Using a paste thickener saved over $ 1.5 million/yr in operating cost (flocculant, spare parts for the filters, trucking, etc). These savings allowed for pay back period for the project of about 2 years.

Economic Comparisons Each Site has different objectives and obstacles Article from Paste 2013 conference “Filter tailings versus thickened slurry: four case studies” by T.G. Fitton et.al. Items included are: Site selection Process plant dry Mtpa Mine life Water return infrastructure Embankments, Mechanical and electrical infrastructure Operation There are several excellent articles published providing comparisons between the basic tailings disposal options. One of the best conferences in the Paste 2015. It is held yearly at location rotated around the world. It was started in the mid-1990’s and provides a wide review of paste system. I would like to use one of the papers to show economic comparison of the tailings methods. Fitton discusses four case studies, the second one compares the conventional slurry, paste and filtration.

Water recovery Tailings ponds vary but may contain 45% moisture (55% solids). Paste 35% moisture (65% solids) Water recovery comparison Filter press typically 15% moisture (85% solids) Horizontal belt filter 22% moisture (78% solids) Disc filters 22% moisture (78% solids)

Copper Case Study 2 of 4 - Paste 2013 Option 1 Option 2 Option 3 Option 4 Option 5 10km 10m Case study 2 discusses the economics of 5 options shown here. Option 1 = high rate thickener and slurry pump 10km to pond Option 2 = paste thickener and pump 10km to pond Option 3 = un-thickened slurry pumped to pond where the paste thickener is and pumped to head Option 4 = a high rate thickener at the plant, slurry pumped to paste thickener at the site Options 5 = thickener/filter combination and convey to stack where earth moving is done to manage the stack “Filter Tailings Versus Thickened Slurry: Four Case Studies” TG Fitton etal; Paste 2013

NPC* for Case Study 2 Option Plant 10km transport Dewater at TSF** Dist. at TSF** Capex Annual Opex NPC 1 High Rate Pumped -------- 294 3.64 375 2 Paste 230 6.49 345 3 None 232 6.64 347 4 241 7.71 356 5 Filtration Conveyed Equipment 249 67.2 1074 Using a paste thickener to produce a stackable tailings was the least expensive. The filter was very high NPC comparatively. * - Net Present Cost ** - Tailings Storage Facility

Cost of Water Significant Variable For each of the studies, using the current plant conditions; filtration always had the highest NPC. However, this could change if the price of water was high enough. The table shows the water cost where the filtration became the lowest NPC Mine Number Break-even cost of water (USD/m3) 1 3.16 2 8.40 3 2.23 4 0.76 Fitton concludes that the cost of water at any particular site is a very critical variable. All four of the case studies show that filtration can have the lowest NPC, if the water is expensive enough. Note that the case studies 1, 3 and 4 had different location for the slurry pond and the stack. This is because each type of stack has different requirements so separate sites can be selected. This plays a significant part in these other studies. Note: The high rate thickener pond for Mines 1, 3, 4 were 3 – 5 times farther distant from the plant than the surface stacking site.

Introducing: WT Control-Pro Level Sensors Application: Thickener control is a large problem in the industry Thickener operation (U/F wt%) improves with steady state operation Paste thickeners – better control = more consistent opeartion

Picket mount Trans-Reserve Differential pressure Sensors Type 1 Type 2

Sensor Capability Application: New and Retro-fit Thickener Control Diagnostic Tool – Identify operating problems Process Optimization

Correlation with Manual Bed Level

Diagnostic: Oscillations in Data Test Thickener had short circuiting flow due to the bifurcated feedwell design.

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