Clay Settling Areas 101

1
Clay Settling Areas 101

• Dams, Clay consolidation, hydrology, and
  long-term
• management

Stephen H. Partney, PE Program Administrator Techn
ical Support SectionBureau of Mine Reclamation
2
Can you pick out the clay settling areas?
3
Recipe for waste clay

• Dig up some phosphate matrix
• Add lots of water
• Screen out the 150 mesh solids

4
Settling Ponds
Waste Clay
Sand Tailings
COARSE PRODUCT
Washer underflow to feed preparation
Flotation
Matrix Line
FINE PRODUCT
BASIC PHOSPHATE PROCESS FLOW
5
What is 150 mesh solids?

• Particles passing a standard 150 mesh screen,
  that is, a screen with 150 openings in a square
  inch, or a particle size approximately 3/1000 of
  an inch
• About the size of a grain of flour

6
Everything Smaller than that is clay?

• No the fine fraction is composed of about the
  same percentages of sand, phosphate, and clay as
  the original matrix, down to about 600 mesh.
  Smaller than that, the silica and phosphate
  decline dramatically.

7
Why does it take up so much space?

• Natural clay in the matrix is about 50 solids by
  weight
• In the beneficiation process, the clay is diluted
  and agitated to separate it from the course
  material
• It is pumped away from the beneficiation plant at
  2 3 solids by weight
• With natural consolidation, 10 solids is reached
  in 2-3 days

8
Why does it take up so much space?

• 20 solids takes about one year
• 30 solids takes about five years
• 35 solids takes about ten years
• This is still only about 18 solids by volume, in
  other words, the contents of a mature pond is
  still 80 water.

9
Single fill and consolidation, no flocc
Natural grade
Days
10
Stage fill and consolidation, no flocc
Natural grade
Days
11
Why do the clays hold so much water?
Clays are very small, electrically charged
particles
In solution, the opposite charges attract. And
form open shapes, such as tetrahedrons

-
The open spaces Are filled with water

-
12
Desiccation neutralizes the charges

- - - - - - - - -
- - - - - - - - - - - - - - - - - - -

Uncharged particles stack loosely

- - - - - - - - - - - - - -
Particles form boxes when charged
13
Photomicrograph of illite, showing fibrous
structure
14
Photomicrograph of Attapulgite
15
SEM Kaolinite and Halloysite
5 nm
5 µ m

16
SEM Photomicrograph of Montmorillonite
17
SEM of Illite
PREVIOUS SEM
18
At what point is the waste clayno longer a spill
threat?

• Plastic clay will stay in place at about 30
  solids
• Desiccated clay is not a spill threat it is
  typically above 40 solids and is no longer
  charged
• When an adequate crust of two feet covers the
  entire pond, the dam can be abandoned
• In practice, this is rarely less than 10 years

19
Are the industries dams sufficient to last 10
years without failing?

• The current 62-672 rule elevates waste clay dam
  construction to the highest standard for earthen
  dam construction in the United States.
• Assuming the provisions of this rule are
  followed, the risk of failure is approximately
  one chance in 2 million operating days.

20
WASTE CLAY POND FAILURES IN FLORIDA
FIVE YEAR BLOCK
21
Clay Settling Areas after reclamation
Desiccation zone
Overburden
Plastic zone
Fine Sand
22
Proposed site for Mitigation on clays PC8
pond Looking from proposed Outfall location
23
Proposed site for wetland On clay mitigation
for Ona Mine FG7 pond Looking from proposed
outfall location
24
Fort Green Mine Mitigation site for Ona FG
H1A Looking across clays From proposed outfall
25
Wetland at discharge from Reclaimed FM5 Clay
settling area
26
Looking from edge of FM5 wetland
downstream Toward Bridge
27
(No Transcript)
28
MCC-FM-05
29
GAR-FM-03/04
30
AGR-PC-018/021
31
Schematic Diagram CSA Wetland Filter Basin
Direct Recharge
Diagram courtesy of Schreuder, inc.
32
Questions?