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 Stack s 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 manufac turing 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
requirement s for existing and new gypsum dewatering stack s and cooling ponds to minimize
fugitive emissions

. Gypsum dewatering stack s and cooling ponds are the largest source of HF
emissions for the industry, accounting for approximately 96 percent of the HF emission s for
phosphoric acid manufacturing facilities

. 1 The w ork 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 phosph oric 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 t he Phosphoric Acid Manufacturing NESHAP include a work
practice that would require phosphoric acid manufacturing facilities to limit the total area of the ir

active gypsum dewatering stack s in proportion to the phosphoric acid manufacturing capacity
upon development of new gypsum dewatering stack s. This work practice w ould limit the total

1area that phosphoric acid facilities devote to gypsum dewatering stack s 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 stack s and provided the total area of each , in acres. P hosphoric acid
manufacturing capacity data was available in Title V permits, and corresponds to the maximum
input ( equivalent P 2 O 5 feed ) for each phosphoric acid process line. T he gypsum dewatering stack

area and manufacturing capacity data fo r 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 facilit y

.

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 P 2 O 5

feed per year) Gypsum Dewatering Stack Area to Capacity

Ratio (acres / 100,000 ton s P 2 O 5

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

2d ewatering 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 c oncluded 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 P 2 O 5 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. The se two facilities with the highest ratios we re considered outliers

. A
value of 80 acres per 100,000 ton s o f phosphoric acid manufacturing capacity (equivalent P 2 O 5 feed basis) was selected as the limit, s hown 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 stack s. Gypsum Dewatering Stack Ratio Limit 80 acres per 100,000 tons of equivalent P 2 O 5 feed manufacturing capacity per year

Gypsum dewatering stack s are active for an extended period of time, and the EPA has no
knowledge that there are any new gypsum dewatering stack s in the planning stages. Therefore,
the EPA does not anticipate that there will be an economic impact on phosphoric acid
manufacturing facilities w ithin the next five years

. However, an impact model for future

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

, we assu med 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 ton s

. Three facilities would exceed 80 acres per 100,000 ton s , 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 stack s. An em ission 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 t he
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 t he existing gypsum dewatering stack area that would be closed and the

correspo nding 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 , A ssuming N o C hange in C apacity (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

43.0 CONTROL MEASURES
The proposed revisions to s ubpart 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 industry 2,3 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 t he 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 all ows 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 expo sed.

3. 4 Apply slaked lime to the active gypsum dewatering stack surfaces

As gypsum dewatering stack s are raised, portions of the gypsum dewatering stack surface
will be temporarily inactive when gypsum slurry is discharge d 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 neutraliz e the HF at

5the surface and dramatically reduce emissions of HF because m ost 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 ac tive 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

stack s 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 so il and vegetation. Florida ( Rule 62

- 673, F.A.C. ) and Louisiana ( LAC 33:VII ) currently
require facilities to close the ir gypsum dewatering stack s 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 stack s. Due to the height of gypsum dewatering stack s, the sides of gyps um dewatering stack s 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