COOLING POND WATER RECOVERY TRENCH UP-GRADE PART 2:

QUALITY CONTROL AND CONSTRUCTION CHALLENGES

FINAL REPORT

Christine Osborne

National Sales Manager

Corrosion Engineering, a division of

ERGONARMOR

Jackson, Mississippi 39232

Prepared for

AMERICAN INSTITUTE OF CHEMICAL ENGINEERS

4798 S. Florida Ave. #253

Lakeland, Florida 33813 USA

June 2016

v

ABSTRACT

The generation of a large volume of pond water is inherent to the production of

phosphoric acid via the wet hydrate process. Beginning in 2014, one producer undertook

reconstruction of its 1,200-lf (365 m) evaporator cooling pond water recovery trench

system to expand capacity and enhance maintenance access. After considering alloy,

thermoplastic, and polymer concrete lining alternatives, the design team selected

anchored polypropylene concrete protection liners. This paper describes the challenges

the project team faced in implementing this approach.

vi

ACKNOWLDGEMENTS

The author would like to thank the following individuals and companies for their

support and assistance in sharing this story:

Thomas Alligood, PCS Phosphate, for his account of the challenges faced

during construction.

Gary Hopkins, Plastek Werks, for his account of the challenges faced

during construction, photos, and his company’s commitment to

thermoplastic welding quality.

vii

TABLE OF CONTENTS

ABSTRACT v

ACKNOWLEDGEMENTS vi

TABLE OF CONTENTS vii

LIST OF FIGURES AND TABLES viii

BACKGROUND 1

MATERIALS OF CONSTRUCTION 1

FOAM CONCRETE FORMWORK DISPOSAL 3

FORMWORK DISPLACEMENT DURING CONCRETE PLACEMENT 4

DAMS BETWEEN ACTIVE TRENCHES AND CONSTRUCTION AREAS 5

NONDESTRUCTIVE LEAK LOCATION AND REPAIR WORK 6

CONCLUSION 9

REFERENCES 9

viii

LIST OF FIGURES AND TABLES

Figure Page

1. Partial schematic of the Aurora mill flowsheet (bucket wheel excavator has since

been replaced with other technology). (Kable 2014) 2

2. Conceptual illustration of anchored thermoplastic trench liner with cutaway detail. 3

3. Inverted trench liner sections with foam forms in fabrication shop 3

4. Trench liner section with foam formwork ready for concrete backfill 4

5. Competed trench section before solid cover was installed. 5

6. Knife gates in existing pond water trench. 5

7. Old block gates. New gates will be covered in anchored plastic. 6

8. Recessed channels in trench ID, designed to accommodate the inflatable bladder

between the trench ID and block gate OD. 6

9. Liner installation detail – section view through wall showing attachment of liners to

formwork at butted joints between liner panels. 7

10. Liner installation detail – section view through wall showing mechanism for

hanging liners from the top of the formwork. 7

11. Anchored liner panels, attached to concrete formwork for pump pit walls 8

12. The locations of sparks were marked for repair with a white pen. 8

13. Holidays were sealed by means of extrusion welding. 9

1

BACKGROUND

Potash Corporation operates an integrated phosphate mine and chemical plant just

outside the town of Aurora, North Carolina, USA. The chemical processing facility

includes three sulfuric acid plants, four phosphoric acid plants, a liquid fertilizer (11-37-0)

plant, a superphosphoric acid plant, four purified acid plants, a silicon tetrafluoride plant,

and a solid fertilizer plant producing DAP and MAP.

This facility utilizes the wet process for converting calcium phosphate to phosphoric

acid. All process waters including cooling pond water are recycled and contained on site

(Kable 2014). An extensive system of channels and pits collect and recycle cooling water

from the evaporators in the phosphoric acid plant.

In 2012, PCS Phosphate undertook comprehensive reconstruction of its 1,200-lf (365

m) cooling pond water recovery trench and sump system to expand capacity and enhance

reliability and maintenance access. Some existing trenches and sumps were to be relined

and additional trenches constructed. The changes would accommodate larger capacity

requirements and provide more flexible flow control.

Typically cooling pond water is acidic, containing traces of phosphoric and

fluorosilicic acids. The maximum pond water temperature at isolated locations during

summer months reaches 140°F (60°C). This temperature was used for design purposes.

MATERIALS OF CONSTRUCTION

The project team considered several chemical containment and corrosion protection

lining alternatives for the new cast-in-place Portland cement concrete trenches and sumps

including metal alloy, organic polymer concrete, and thermoplastic

Favorable past experience with Acroline™ System anchored thermoplastic liner in

trenches in the Evaporator area lead the project team to select the same technology for the

cooling pond water trenches.1 Polypropylene (PP) liner, which is rated for service up to

203°F (95°C), was chosen to handle the 140°F (60°C) effluent design temperature and

provide a comfortable factor of safety. Figure 2 shows a conceptual illustration of this

anchored trench liner system.

A discussion of the advantages and limitations of these materials for this service was

presented in “Cooling Pond Water Recovery Trench Up-Grade Part 1: Drivers and Design

Considerations,” by this author at the AIChE Central Florida Chapter Clearwater

Convention in June 2014.

1 ACROLINE is a trade mark of Ergon A&E Inc. in the USA.

2

Figure 1. Partial schematic of the Aurora mill flowsheet (bucket wheel excavator

has since been replaced with other technology). (Kable 2014)

3

Figure 2. Conceptual illustration of anchored thermoplastic trench liner with

cutaway detail.

FOAM CONCRETE FORMWORK DISPOSAL

The new pond water recovery trenches were designed with rounded rather than

square bottoms. The shape was chosen for three reasons:

1. Minimize build-up of gypsum-laden sediment in the corners of the trenches

2. Simplify the liner fabrication – minimize seams and corner welds

3. Reduce the risk of air entrapment during concrete back-filling

Concrete casting forms were cut from light-weight expanded polystyrene foam

rather than wood or metal as shown in Figures 3 and 4, because they could easily be cut to

fit the shape of the round-bottom trenches and provided uniform resistance to deflection, top

to bottom.

Figure 3. Inverted trench liner sections with foam forms in fabrication shop2

2 Photo courtesy of Plastek Werks, Inc., Cleveland, Georgia.

4

Figure 4. Trench liner section with foam formwork ready for concrete backfill3

Recycling the large volume of expanded polystyrene foam presented unanticipated

challenges. Because the foam was contaminated with concrete residue and chemicals with

which in came into contact during use, foam recycling facilities declined to accept it. In

addition, the logistics of transporting the large volume of foam to a distant recycling facility

were economically unfavorable. Alternative disposal arrangements were made.

FORMWORK DISPLACEMENT DURING CONCRETE PLACEMENT

The combined weight of the liners and formwork was lighter than the volume of

concrete they displaced creating an unbalanced buoyant force. The liners and formwork

tended to float on the fresh concrete backfill.

While the rounded shape of the trench bottoms facilitated the escape of air caught

under the liners during placement of the concrete, it did not offer substantial lateral

anchoring to help prevent the liners’ floating. In some cases, the walls of the liners were

well restrained, while the bottoms of the liners freely deflected, vertically displacing the

foam formwork.

Once these dynamics were well understood, the floating risk was easily managed.

Placing the concrete in 2 or 3 lifts provided adequate anchoring to prevent floating. Figure

5 shows a successfully installed section of the trench liner.

3 Photo courtesy of Ergon A&E Inc., Jackson, Mississippi.

5

Figure 5. Competed trench section before solid cover was installed.4

DAMS BETWEEN ACTIVE TRENCHES AND CONSTRUCTION AREAS

The project’s progression depends upon proper functioning of the knife gates (Figure

6) and removable dams (Figures 7 and 8), which will be used to isolate sections of the active

trench system as the new system is integrated with the old system and the old system is

phased out.

Figure 6. Knife gates in existing pond water trench.5

Both the knife gates and block gates were as effective as they were before the trench

was reconstructed and lined with polypropylene embedment liners, but no more so. They

routinely leaked a tolerable amount. Visual monitoring and passive hydrogen sulfide

monitors were used to manage risk of personnel exposure

4 Photo courtesy of Plastek Werks Inc., Cleveland, Georgia. 5 Photo courtesy of PCS Phosphates, Aurora, North Carolina.

6

Figure 7. Old block gates. New gates will be covered in anchored plastic.6

Figure 8. Recessed channels in trench ID, designed to accommodate the inflatable

bladder between the trench ID and block gate OD.7

NONDESTRUCTIVE LEAK LOCATION AND REPAIR WORK

The liner installation procedures and construction details (Figures 9 and 10) indicate

that the liners are to be secured to the concrete casting forms for the pump pit walls so as to

avoid making holes in the liners that would be difficult to locate and repair once the

formwork is stripped. In practice, this ideal proved difficult to attain, as the black liners

6 Photo courtesy of PCS Phosphate, Aurora, North Carolina. 7 Photo courtesy of Plastek Werks Inc., Cleveland, Georgia.

7

grew larger and smaller as the sun rose and set each day in the expansive, unshaded

construction area.

Figure 9. Liner installation detail – section view through wall showing attachment

of liners to formwork at butted joints between liner panels.

During the heat of the day when the liners were largest, they would deflect away

from the formwork, as show in Figure 11. As temperatures cooled, the liners would

sometimes detach from the formwork. An abundance of nails were used to secure the liners

and prevent concrete from getting between the liners and formwork as it was placed.

Figure 10. Liner installation detail – section view through wall showing mechanism

for hanging liners from the top of the formwork.

WIRE LOOP NAIL

WOODEN CON-CRETE FORM

LINER

CONCRETE

WIRE LOOP

NAIL

END PROFILE

EXTRUSION WELD

LINER

CONCRETE

WOODEN CONCRETE FORM

8

Figure 11. Anchored liner panels, attached to concrete formwork for pump pit

walls8

To locate holes that were invisible, a 32 kV DC holiday detector proved very

effective. In accordance with DVS 2206-4 – Non-destructive tests on tanks, apparatus and

piping made of thermoplastics – Testing with electrical high voltage, the liner was spark

tested with 75-125 kV output, 15-25 kV per mil of liner thickness. Locations producing a

spark were marked for repair with white circles drawn on the black liner as shown in Figure

12. Holes were sealed by means of extrusion welding as shown in Figure 13.

Figure 12. The locations of sparks were marked for repair with a white pen.9

8 Photo courtesy of Ergon A&E Inc., Jackson, Mississippi. 9 Photo courtesy of Plastek Werks Inc., Cleveland, Georgia.

9

Figure 13. Holidays were sealed by means of extrusion welding.10

CONCLUSION

The PCS Phosphate encountered a number of challenges during reconstruction of its

phosphoric acid pond water recovery trenches and sumps, some of which were specific to its

choice of anchored thermoplastic concrete protective liners. Difficulties were encountered

preventing the liners from floating during concrete placement; disposing of a large volume

of expanded polystyrene foam; isolating sections of the system while phasing in completed

sections and phasing out old sections; and locating and repairing nail holes through the liner.

None of these difficulties proved insurmountable. The occurrence of some of them

decreased as the crews gained experience working with the liners. Close collaboration

between the general civil contractor, Trader Construction, and plastic fabricator, Plastek

Werks, assured safety, quality, and functionality.

Final phases of construction were underway at the time of this paper’s publication.

Information about performance in service is not available at this time.

REFERENCES

Kable (2014). Retrieved from http://www.mining-technology.com/projects/aurora-

phosphate-mine/

Kable (2014). Retrieved from http://www.mining-technology.com/projects/aurora-

phosphate-mine/aurora-phosphate-mine4.html

10 Photo courtesy of Plastek Werks Inc., Cleveland, Georgia.