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

2. 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.

3. 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

4. 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.

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

6. 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 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.

7. 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.

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

9. Golder presentation

10. 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

11. 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.

12. 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.

13. 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.

14. 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 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.

15. 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)

16. Copper Case Study 2 of 4 - Paste 2013
Option 1
Option 2
Option 3
Option 4
Option 5
10km m
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

17. 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

18. 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.

19. 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

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

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

22. Correlation with Manual Bed Level

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

24. © WesTech Engineering, Inc. 2016