COOLING POND WATER RECOVERY TRENCH UP-GRADE PART 1: DRIVERS AND DESIGN CONSIDERATIONS

FINAL REPORT

Christine Osborne

National Sales Manager

Corrosion Engineering, a division of

ERGONARMOR Lester, Pennsylvania 19113

Prepared for

AMERICAN INSTITUTE OF CHEMICAL ENGINEERS

4798 S. Florida Ave. #253 Lakeland, Florida 33813 USA

June 2014

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ABSTRACT

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

phosphoric acid via the wet hydrate process. This paper recounts one producer’s experience evaluating materials of construction as it undertook comprehensive reconstruction of its 1,200-lf (365 m) evaporator cooling pond water recovery trench

system to expand capacity and enhance maintenance access. After considering several lining alternatives including alloy, thermoplastics, and polymer concrete, the design team

selected anchored polypropylene concrete protection liners. Factors leading to this choice are reviewed herein.

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ACKNOWLDGEMENTS

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

support and assistance in sharing this story:

Dale Rahuba, PCS Phosphate, for his account of the material selection

process, facts and figures pertaining to the plant facilities, and photos of existing flow control structures

Gary Hopkins, Plastek Werks, for excerpts of shop drawings and his company’s commitment to thermoplastic welding quality

Rob Simpson, Corrosion Engineering, for supporting my involvement in

contributing technical content to industry conferences like AIChE Clearwater Convention

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TABLE OF CONTENTS

ABSTRACT.........................................................................................................................v

ACKNOWLDGEMENTS .................................................................................................. vi

LIST OF FIGURES AND TABLES................................................................................... 2

BACKGROUND ................................................................................................................ 3

MATERIALS OF CONSTRUCTION................................................................................ 3

METAL ALLOY ................................................................................................ 3

ORGANIC POLYMER CONCRETE ................................................................ 5

THERMOPLASTIC ........................................................................................... 5

CONCLUSION ................................................................................................................. 10

REFERENCES ................................................................................................................. 10

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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)........................................4

2. Conceptual illustration of anchored thermoplastic trench liner with cutaway detail............................................................................................................................5

3. Integrally extruded concrete embedment anchor, shown in gray polypropylene..............................................................................................................6

4. Anchor arrangement (dimensions in mm). .................................................................6

5. Section view of anchored thermoplastic liner specified in 2014 pond water recovery trench reconstruction project (dimensions in mm). .....................................6

6. Prefabricated trench liner sections on foam concrete casting forms inverted on shop floor ...............................................................................................................7

7. Section view of anchored thermoplastic liner extrusion welded to

termination profile.......................................................................................................8

8. Knife gates in existing pond water trench. ..................................................................8

9. Block gates (removable dams) used to temporarily seal off sections of the existing pond water trench system. .............................................................................9

10. Existing knife gate track, isometric view....................................................................9

11. Proposed knife gate track, plan view. .........................................................................9

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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,

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

Thermoplastic

METAL ALLOY

The existing cooling pond water recovery trenches and sumps had been lined with 316L stainless steel. Over time, the liner material was replaced with 409 stainless steel.

Neither provided a satisfactory degree of reliability, and neither was considered for the new pond water recovery trenches and sumps.

While the project team felt new Hastelloy G30 liners would require less

maintenance than the existing 409 stainless liners, this alloy was ruled out for its high initial cost.

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

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

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ORGANIC POLYMER CONCRETE

The project team entertained a proposal to have the Portland cement concrete

trenches and sumps lined with a cast-in-place organic polymer concrete lining. While polymer concrete is widely used to line and construct structures exposed to corrosive

chemicals, the project team favored alternative materials with which it had more experience. There were unresolved concerns about the polymer concrete’s resistance to fluorosilicic acid, which is known to attack silica based fillers; expansion and contraction

behavior; and long-term reliability.

THERMOPLASTIC

Based on the satisfactory performance of high density polyethylene (HDPE) piping in similar chemical service in the plant, the idea to use HDPE as a liner material for the trenches and sumps gained traction. The initial HDPE liner design concept

featured thick HDPE plate mechanically anchored with countersunk metal bolts on 2-foot centers. The bolt holes were to be covered and sealed with HDPE.

While this concept satisfied the team’s desire for a robust liner, the team was apprehensive about its ability to accommodate thermal stresses without compromising its leak-tightness as a chemical containment and corrosion protection barrier. The

considerable spacing between the bolts, numerous bolt holes through the liner that would need to be sealed, and anticipated thermal deformation raised concerns.

In 2009, PCS Phosphate expanded its phosphoric acid plant. Part of this expansion project involved installation of trenches lined with Acroline® System1 5mm thick HDPE liners around equipment that contained stronger phosphoric and

hydrofluorosilicic acid than the pond water. Figure 2 shows a conceptual illustration of this anchored trench liner system.

Figure 2. Conceptual illustration of anchored thermoplastic trench liner with

cutaway detail.

1 ACROLINE is a registered trademark of Ergon Asphalt & Emulsions, Inc.

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In contrast with the bolted HDPE plate liner design concept, these liners featured integrally extruded embedment anchors shown in Figures 3-5, by which the liner was

attached to the concrete without penetrating the liner. The close spacing of the anchors shown in Figure 4 effectively distributed thermal strain and deformation.

Figure 3. Integrally extruded concrete embedment anchor, shown in gray

polypropylene.

Figure 4. Anchor arrangement (dimensions in mm).

After recalling the satisfactory performance of the phosphoric acid expansion project trench liners, the project team was inclined to consider a similar approach to

lining the pond water recovery trenches with two modifications. Polypropylene (PP) liner, which is rated for service up to 203°F (95°C) was chosen over HDPE to handle the 140°F (60°C) effluent design temperature and provide a comfortable factor of safety.

The concrete embedment anchor length was increased from 13 mm to 19 mm, the longest available from the manufacturer’s standard range, as shown in Figure 5.

Figure 5. Section view of anchored thermoplastic liner specified in 2014 pond

water recovery trench reconstruction project (dimensions in mm).

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TRENCH LINER DESIGN

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

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

Concrete casting forms were cut from foam rather than wood or metal as shown in

Figure 6, because foam could be easily shaped to fit variable dimensions and curves.

Figure 6. Prefabricated trench liner sections on foam concrete casting forms

inverted on shop floor2

To accommodate heavy traffic, including construction cranes, the trenches were designed with stainless steel frames around their covers, which extended a few inches out onto the adjacent apron and a few inches down over the plastic-lined trench walls. A

termination profile, shown in Figure 7, was welded to the top of the plastic trench liners and embedded in the concrete wall, forming an uninterrupted barrier to liquid migration

behind the liners.

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

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Figure 7. Section view of anchored thermoplastic liner extrusion welded to

termination profile.

Flow control devices including knife gates, such as those shown in Figure 8, and removable block gates, shown in Figure 9, presented interesting design challenges.

Custom fabrication details such as those shown in Figures 10 and 11 were developed to maintain a continuous polypropylene barrier between the pond water and the concrete walls of the conveyance and collection structures. The use of mechanical seals between

dissimilar materials was avoided.

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

removable dams, 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 8. Knife gates in existing pond water trench.

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Figure 9. Block gates (removable dams) used to temporarily seal off sections of

the existing pond water trench system.

Figure 10. Existing knife gate track, isometric view.

Figure 11. Proposed knife gate track, plan view.

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Fabricator qualification was a key part of the quality control plan. The selected fabricator had a favorable track record at PCS Phosphate and its thermoplastic welders

maintained current certification in accordance with AWS B2.4 Specification for Welding Procedure & Performance Qualification for Thermoplastics3. As part of the quality

control plan, the fabricator offered to supply a weld map to trace each weld to the corresponding welder, welding equipment, and weld rod lot.

Trench liner fabrication details around the flow control devices were finalized in

April 2014 with input from the liner supplier, plastic fabricator, construction contractor, engineering consultants, and project manager, at which time excavation work was

underway and the project was on schedule. Construction is forecast to continue into 2016. The earliest phases of implementation will involve construction of six pits.

CONCLUSION

The PCS Phosphate phosphoric acid pond water recovery system project team selected polypropylene liners, featuring an integrally extruded and homogeneous,

engineered anchoring system, over a custom-designed, mechanically anchored high-density polyethylene sheet liner system, alloy, and cast-in-place polymer concrete linings.

Though the need to increase hydraulic capacity and operational flexibility drove the project, the desire to improve reliability and reduce maintenance demand drove the selection of materials of construction. Recent favorable experience with similar anchored

plastic liners and the selected thermoplastic fabricator provided reassurance as to the efficacy of this choice.

Initial 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

3 American Welding Society. Specification for Welding Procedure & Performance Qualification for

Thermoplastics. AWS B2.4.