TECHNICAL MEMORANDUM

TO: Tina Ndoh, U.S. EPA/OAQPS/SPPD - Minerals and Manufacturing Group

FROM: Eastern Research Group, Inc.

DATE: August 25, 2014

SUBJECT: Analysis of Requirements for Gypsum Dewatering Stacks and Cooling Ponds at 

Phosphoric Acid Manufacturing Plants

1.0 INTRODUCTION

The U.S. Environmental Protection Agency (EPA), under section 112 of the Clean Air 

Act (CAA), is required to establish maximum achievable control technology (MACT) standards 

for phosphoric acid manufacturing plants (40 CFR part 63, subpart AA). Gypsum dewatering 

stacks and cooling ponds were not previously regulated in subpart AA, and the EPA is proposing 

to amend the rule. The amendments would require facilities to comply with work practice 

requirements for existing and new gypsum dewatering stacks and cooling ponds to minimize 

fugitive emissions. Gypsum dewatering stacks and cooling ponds are the largest source of HF 

emissions for the industry, accounting for approximately 96 percent of the HF emissions for 

phosphoric acid manufacturing facilities.

1 The work practices being proposed by the EPA are 

based on state requirements and current literature on the industry. The work practices include 

development of a gypsum dewatering stack and cooling pond management plan that specifies

which control measures will be implemented by the facility. Facilities with new gypsum 

dewatering stacks must also comply with a gypsum dewatering stack ratio, based on the total 

active gypsum dewatering stack area and phosphoric acid manufacturing capacity of a facility.

This memorandum documents the analyses performed to develop the gypsum dewatering stack

ratio and discusses the control measures of the management plan.

2.0 GYPSUM DEWATERING STACK RATIO

The proposed changes to the Phosphoric Acid Manufacturing NESHAP include a work 

practice that would require phosphoric acid manufacturing facilities to limit the total area of their 

active gypsum dewatering stacks in proportion to the phosphoric acid manufacturing capacity 

upon development of new gypsum dewatering stacks. This work practice would limit the total 

1
area that phosphoric acid facilities devote to gypsum dewatering stacks and will minimize

hydrogen fluoride emissions. The methodology used to determine the ratio of gypsum 

dewatering stack area versus phosphoric acid manufacturing capacity is presented below. 

As part of the section 114 survey, phosphoric acid manufacturing facilities identified 

their gypsum dewatering stacks and provided the total area of each, in acres. Phosphoric acid 

manufacturing capacity data was available in Title V permits, and corresponds to the maximum 

input (equivalent P2O5 feed) for each phosphoric acid process line. The gypsum dewatering stack

area and manufacturing capacity data for each phosphoric acid manufacturing facility are shown 

in Table 1. Table 1 also includes the calculated gypsum dewatering stack area versus

manufacturing capacity ratio for each facility.

Table 1. Data used to develop ratio of gypsum dewatering stack area versus the phosphoric acid 

manufacturing capacity for each facility.

Facility

Gypsum 

Dewatering 

Stack Area 

(acres)

Phosphoric Acid 

Manufacturing 

Capacity (tons of 

equivalent P2O5

feed per year)

Gypsum 

Dewatering Stack

Area to Capacity 

Ratio (acres /

100,000 tons P2O5

feed per year)

Mosaic New Wales 256 2,587,120 9.9

Mississippi Phosphates 76 511,000 14.9

Mosaic Uncle Sam 200 1,317,650 15.2

Mosaic Bartow 245 1,489,200 16.5

PCS Aurora 344 1,796,338 19.1

Mosaic Riverview 424 1,489,200 28.5

CFI Plant City 390 1,288,895 30.3

Agrium Nu-West 225 560,000 40.2

Simplot Rock Springs 425 880,000 48.3

PCS Geismar 246 443,256 55.5

PCS White Springs 1205 1,400,000 86.1

Simplot Don-Pocatello 440 471,000 93.4

We expected facilities with greater phosphoric acid manufacturing capacities to, in most 

cases, require larger gypsum dewatering stack areas, because higher phosphoric acid 

manufacturing rates result in higher gypsum generation rates; however, this was not the case. 

Based on the available data, we did not detect a correlation between gypsum dewatering stack 

area and phosphoric acid manufacturing capacity. We considered that the size of active gypsum 

2
dewatering stacks at a facility is dynamic and does not remain the same over time. We also 

considered other factors that influence gypsum dewatering stack size such as the actual area 

available for stack construction, closure of recently active stacks, and local permitting 

limitations. Gypsum dewatering stacks also serve the fertilizer manufacturing processes in 

addition to the phosphoric acid manufacturing processes as a source of cooling water, wash 

water, process water, and slurry water. As a result, we concluded that the size of gypsum 

dewatering stacks is a function of several factors, including process optimization. Nonetheless, 

we still believe that phosphoric acid manufacturing capacity has a significant impact on the size 

of gypsum dewatering stacks.

A review of the calculated gypsum dewatering stack versus phosphoric acid 

manufacturing capacity ratios for the 12 facilities indicated that a reasonable cutoff would be in 

the range of 60 to 80 acres per 100,000 tons of equivalent P2O5 capacity per year. There is a 

natural progression of ratios for the first ten facilities, but a jump in ratios from 55 to 86 occurred 

for the final two facilities. These two facilities with the highest ratios were considered outliers. A 

value of 80 acres per 100,000 tons of phosphoric acid manufacturing capacity (equivalent P2O5

feed basis) was selected as the limit, shown in Table 2. This value was selected because it is 

representative of the current operations of most facilities and provides a reasonable limit on the 

size of active stacks in the future. The proposed revisions to subpart AA include a provision for

facilities that construct a new gypsum dewatering stack which requires that the ratio of gypsum 

dewatering stack area to phosphoric acid manufacturing capacity must not exceed this value. If 

construction of a new gypsum dewatering stack would cause the facility to exceed this ratio 

value, then a portion (or all) of an existing gypsum dewatering stack must be closed.

Table 2. Gypsum dewatering stack ratio limit that facilities may not exceed when building new 

gypsum dewatering stacks.

Gypsum Dewatering Stack Ratio Limit

80 acres per 100,000 tons of equivalent P2O5 feed 

manufacturing capacity per year

Gypsum dewatering stacks are active for an extended period of time, and the EPA has no 

knowledge that there are any new gypsum dewatering stacks in the planning stages. Therefore, 

the EPA does not anticipate that there will be an economic impact on phosphoric acid 

manufacturing facilities within the next five years. However, an impact model for future 

3
expected gypsum dewatering stack closure area and emission reductions was developed. For this 

impact model, we assumed that each facility would build a new 200 acre gypsum dewatering 

stack. The ratio based on the new total active gypsum dewatering stack area (i.e., the existing 

gypsum dewatering stack area, from Table 1, plus 200 acres) was calculated and compared to the 

limit of 80 acres per 100,000 tons. Three facilities would exceed 80 acres per 100,000 tons, and 

an estimate of the gypsum dewatering stack area that would be closed in order to meet the limit 

was then calculated. The associated reduction in HF emissions is also presented for closure of 

these three gypsum dewatering stacks. An emission factor of 0.2 tons per acre per year was used, 

which is consistent with the estimates used for the air emissions modeling and risk assessment 

performed for this rulemaking.

1 See Table 3 for the results of the impact model. Note that the 

phosphoric acid manufacturing capacity was not changed for the model.

Table 3. Gypsum dewatering stack ratio impact model results based on a new 200 acre gypsum 

dewatering stack, showing the existing gypsum dewatering stack area that would be closed and the 

corresponding HF emission reductions.

Facility Name

Calculated Gypsum 

Dewatering Stack

Ratio, If Building 

New 200 Acre 

Gypsum 

Dewatering Stack

Maximum Gypsum 

Dewatering Stack

Area Allowed by 

Ratio Limit of 80, 

Assuming No 

Change in Capacity

(acres)

Existing Gypsum 

Dewatering 

Stack Area 

Required To Be 

Closed To Meet 

Ratio Limit 

(acres)

HF 

Emissions 

Reductions 

(tons/yr)

Mosaic New Wales 18 2,070 -- --

Mississippi Phosphates 54 409 -- --

Mosaic Uncle Sam 30 1,054 -- --

Mosaic Bartow 30 1,191 -- --

PCS Aurora 30 1,437 -- --

Mosaic Riverview 42 1,191 -- --

CFI Plant City 46 1,031 -- --

Agrium Nu-West 76 448 -- --

Simplot Rock Springs 71 704 -- --

PCS Geismar 101 355 91 18

PCS White Springs 100 1,120 285 57

Simplot Don-Pocatello 136 377 263 52

4
3.0 CONTROL MEASURES

The proposed revisions to subpart AA specify that phosphoric acid manufacturing

facilities must use control measures identified at § 63.602(e) for their gypsum dewatering stack

and cooling pond management plan. Each of these control measures were selected because they 

are already practiced by facilities in the industry2,3 and would be practical to implement. A brief 

discussion for each of the six control measures is presented.

3.1 Submerge the discharge pipe along with any necessary siphon breaks to a level 

below the surface of the cooling pond or the surface of the pond associated with the 

active gypsum dewatering stack

When water is discharged into ponds, significant agitation occurs. This area of agitation 

results in the highest level of emissions from the pond, when compared to the majority of the 

pond area. Submerging the discharge pipe below the surface of the pond will result in less 

agitation, which in turn will suppress emissions from the area surrounding the pipe.

3.2 Wet the active gypsum dewatering stack during hot or dry periods

This control measure may consider the rim of the gypsum dewatering stack, but would 

have a greater impact for those gypsum dewatering stacks which have smaller ponds on top (i.e., 

more exposed surface area as compared to gypsum dewatering stacks with large ponds). Wetting 

these exposed areas will help control fugitive emissions. 

3.3 Minimize the surface area of the active gypsum dewatering stack by using a rim 

ditch (cell) building technique or other building technique

The rim ditch building technique allows for larger cells and minimizes the surface 

disturbance. Larger lifts can also be built, which can help limit the amount of time spent on

construction activities. This technique would reduce emissions by limiting the amount of fresh 

gypsum which is exposed.

3.4 Apply slaked lime to the active gypsum dewatering stack surfaces

As gypsum dewatering stacks are raised, portions of the gypsum dewatering stack surface 

will be temporarily inactive when gypsum slurry is discharged in other sections of the gypsum

dewatering stack. During these inactive periods, the application of slaked lime to the surface of 

the gypsum dewatering stack would reduce emissions. This practice would neutralize the HF at 

5
the surface and dramatically reduce emissions of HF because most gypsum dewatering stack

emissions, particularly over short time periods, evolve from near-surface gypsum.

3.5 Apply soil caps and vegetation to all side slopes of the active gypsum dewatering 

stack up to 50 feet below the stack top and/or Close the active gypsum dewatering 

stack such that it meets the definition of a closed gypsum dewatering stack specified 

in § 63.601

The most permanent technique for reducing HF emissions from gypsum dewatering 

stacks is to cap or close them. This is implemented when the gypsum dewatering stack will no 

longer receive gypsum and virtually eliminates any emissions because the stack will be covered 

with soil and vegetation. Florida (Rule 62-673, F.A.C.) and Louisiana (LAC 33:VII) currently 

require facilities to close their gypsum dewatering stacks per specific guidelines at the end of 

their life. Prior to closure, facilities can implement an intermediate step of capping the sides of 

gypsum dewatering stacks. Due to the height of gypsum dewatering stacks, the sides of gypsum 

dewatering stacks cover a significant area and reduced emissions can be realized as the stack is 

built up.

4.0 REFERENCES

1. Eastern Research Group, Inc. “Emissions Data Used in Residual Risk Modeling: 

Phosphoric Acid and Phosphate Fertilizer Production Source Categories.” Docket ID: 

EPA-HQ-OAR-2012-0522. August 2014.

2. Wyoming Department of Environmental Quality, Air Quality Division. Operating Permit 

for Simplot Rock Springs (Permit Number 3-2-125), August 17, 2011.

3. Jacobs Engineering, Inc. Agrium Redwater Operations Fluoride Reduction Study, 

Agrium Fugitive Emissions Study. Appendix D. December 12, 2005.

6