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MEMORANDUM 

To: David Putney, U.S. Environmental Protection Agency (EPA), OAQPS 

From: David Green and Mark Bahner, RTI 

Date: November 13, 2014 

Subject: Revised Draft Technology Review for the Primary Aluminum Production Source 

Category 

1.0 BACKGROUND 

1.1 Requirements of Section 112(d)(6) of the CAA 

Section 112 of the CAA requires the EPA to establish technology-based standards for sources of 

hazardous air pollutants (HAP). These technology-based standards are often referred to as maximum 

achievable control technology (MACT), standards. Section 112 also contains provisions requiring the 

EPA to periodically revisit these standards. Specifically, paragraph 112(d)(6) states: 

(6) REVIEW AND REVISION. – The Administrator shall review, and revise as necessary 

(taking into account developments in practices, processes, and control technologies), emissions 

standards promulgated under this section no less often than every 8 years. 

1.2 Description of the Primary Aluminum Production Source Category and Requirements of 

the Current NESHAP 

The current National Emissions Standards for Hazardous Air Pollutants (NESHAP) for Primary 

Aluminum Reduction Plants was promulgated on October 7, 1997 (62 FR 52407) as 40 CFR part 63, 

subpart LL (subpart LL). The rule was amended at 69 FR 18803, April 9, 2004; 70 FR 66284, 

November 2, 2005; and 71 FR 20458, April 20, 2006. Subpart LL applies to affected sources of HAP 

emissions at primary aluminum reduction plants. A primary aluminum reduction plant is defined as any 

facility manufacturing aluminum by electrolytic reduction. This electrolytic reduction process takes 

place in reduction cells, or pots, that are electrically connected in series to form potlines. Typically, a 

potline will be comprised of 100 or more reduction cells. 

There are currently 14 facilities that are subject to subpart LL, four of which are presently idle. 

One of these subject facilities does not manufacture aluminum but does produce prebaked anodes for 

use at primary aluminum reduction plants that utilize the prebake process and contains an affected 

source. Affected sources include new pitch storage tanks and new and existing paste production plants,

anode bake furnaces and potlines. The current subpart LL includes emission limits for total fluoride 

(TF), as a surrogate for hydrogen fluoride (HF), from new and existing potlines and anode bake furnaces 

and for polycyclic organic matter (POM) from new and existing Soderberg potlines and anode bake 

furnaces. The current NESHAP also includes work practice requirements to limit POM emissions from 

new pitch storage tanks and new and existing paste production plants. 
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Control devices currently in use to reduce emissions from affected sources subject to subpart LL 

include dry alumina scrubbers (with baghouses) or wet scrubbers to control TF and POM emissions 

from potlines, dry alumina scrubbers (with baghouses) to control TF and POM emissions from anode 

bake furnaces, dry coke scrubbers (with baghouses) to control POM emissions from paste production 

plants and carbon adsorbers and condensers to control POM emissions from pitch storage tanks. One 

facility also operates wet scrubbers to control secondary emissions from potlines. 

1.3 Additional Requirements of Proposed NESHAP 

The proposed NESHAP amendments include emission limits for carbonyl sulfide (COS) from 

new and existing potlines; for POM from new and existing prebake potlines; for particulate matter (PM,

as a surrogate for metal HAP) from new and existing potlines, anode bake furnaces and paste production

plants; and for nickel and arsenic from new and existing Soderberg potlines and work practice 

requirements to limit POM emissions from existing pitch storage tanks and from potlines, anode bake 

furnaces and paste production plants during periods of startup. 

1.4 Current Practices 

1.4.1 Potrooms 

At the time of promulgation of subpart LL two main primary aluminum reduction processes (i.e., 

the Soderberg and prebake processes) were in use in the primary aluminum production source category 

and several different pot configurations were in use for each of these processes (i.e., three for Soderberg 

processes and four for prebake processes, for a total of seven pot configurations). The differences in

achievable emission rates for each of the seven different pot configurations were recognized during 

development of subpart LL and different emission limits were promulgated for each pot configuration. 

With the closure of approximately one half of the plants in the industry since promulgation of subpart

LL, two Soderberg pot configurations are no longer in use. Thus, only five of the original seven pot 

configurations presently exist in the U.S. No improved pot configurations have been developed in the 

U.S. or in other parts of the world since promulgation of subpart LL. 

The Soderberg process is an older, less efficient, higher emitting process in which the anode 

paste mixture is baked in place in the reduction cells to produce anodes. The prebake process is a newer, 

lower emitting process in which the anode paste mixture is baked in a separate anode bake furnace to 

produce anodes. The difference in the TF emissions between the different pot configurations is due 

almost entirely to the capture efficiency achievable with the different pot configurations. The same types 

of control devices are used and control efficiencies of the captured emissions are similar. The different 

variants of pot configurations in the prebake and the Soderberg processes have different TF emission 

limits, reflecting their different capture efficiencies. The current subpart LL rule includes more stringent 

TF limits for new potlines (without regard to pot configuration, or process type) recognizing that the

best controlled processes are those prebake processes with the best capture efficiency. 

The difference in POM emissions between the different Soderberg pot configurations are mainly 

due to differences in capture efficiency achievable between the different pot configurations whereas the 
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difference in POM emissions between Soderberg processes and prebake processes are due both to 

differences in capture efficiency achievable between the different pot configurations and because POM 

emissions resulting from baking anodes in the pots (as in the case of Soderberg processes) are higher 

than those resulting from baking anodes in bake furnaces (as in the case of prebake processes). These 

higher POM emissions from the Soderberg processes reflect both the lower capture efficiencies of the 

Soderberg cell configurations and that prebaked anode production POM emissions are more efficiently 

captured and controlled from anode bake furnaces. The different pot configurations of the Soderberg 

process have different POM emission limits for existing potlines in the current subpart LL rule, 

reflecting their different capture efficiencies, and more stringent POM limits for new Soderberg potlines, 

without regard to pot configuration, recognizing that the best controlled processes are those pot 

configurations with the best capture efficiency. 

Emissions of POM and TF from potlines consist of emissions from stacks of the potline control 

device (typically a dry alumina scrubber followed by a baghouse), referred to as primary emissions, and 

emissions from roof vents which extend over the length (typically, about 1,000 feet) of the potroom, 

referred to as secondary emissions. The secondary emission stream is much higher in volumetric air 

flow than the primary emission stream. The secondary emission stream is controlled by wet scrubbers at

one facility and is uncontrolled at each of the remaining 11 facilities that are equipped with potlines. 

COS emissions, which are not currently regulated under subpart LL, are produced as the anodes 

are consumed in the reduction cells and the carbon, sulfur (found as an impurity in the anodes) and 

alumina react. Approximately 5 to 10 percent of the sulfur present in the anodes is emitted as COS, with 

the balance being converted to sulfur dioxide (SO2). No add-on control devices are currently in use by 

the primary aluminum reduction industry to reduce COS emissions. Theoretically, COS emissions from 

primary aluminum reduction facilities can be decreased by using coke with a lower sulfur content for 

anode preparation. However, petroleum coke is the source of anode grade coke used in the primary 

aluminum industry and the availability of low sulfur content petroleum coke is limited and insufficient 

to meet the current needs of the primary aluminum industry. 

PM emissions, which are not currently regulated under subpart LL, consist of condensed 

emissions of HAP metals, metal oxides and fluoride salts volatilized from the pots, as well as suspended 

alumina. Capture systems and control devices presently in place at all potlines to control emissions of 

TF and POM are also effective for control of PM emissions. As with TF and POM, secondary emissions 

of PM make up the majority of potline PM emissions and are minimized by effective capture of 

emissions in the hooding and routing them through the primary emissions control device. 

1.4.2 Anode Bake Furnaces 

Most prebake facilities, as well as one facility that does not produce aluminum but produces 

prebaked anodes for use off-site, are equipped with, and operate, anode bake furnaces. Anode bake 

furnaces emit POM which is released when the anode paste (consisting of a mixture of about 78 percent 

coke and about 22 percent pitch) is baked to produce anodes and TF, which results from recycling of the 

butt ends of previously used anodes (which have been exposed to fluoride in the pots) in the anode 
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paste. Anode bake furnaces also emit PM which results from condensed organics volatilized from the 

green anodes, fluoride salts and dust associated with the placement and removal of packing coke (used 

as an insulator). These PM emissions are known to contain HAP metals. POM and TF emissions from 

all anode bake furnaces are controlled by dry alumina scrubbers followed by baghouses. These controls 

are also effective for control of PM. 

1.4.3 Paste Production Plants 

Most prebake and Soderberg facilities are equipped with, and operate, paste production plants. 

Paste production plants emit POM which is released during the mixing of heated pitch with coke to 

produce anode paste. Emissions at all paste production plants are controlled with dry coke scrubbers 

followed by baghouses. Paste production plants also emit PM generated from the mixing of petroleum 

coke and pitch, as well as condensed pitch vapors. These PM emissions are known to contain HAP 

metals. The systems used to control POM emissions from the paste production plants are also effective 

for control of PM emissions from these sources. 

1.4.4 Pitch Storage Tanks 

Most prebake and Soderberg facilities are equipped with, and operate, pitch storage tanks. Pitch 

storage tanks emit POM from the displacement of POM laden gases during filling and from working 

losses that result from temperature changes. Most of the existing pitch storage tanks at these facilities 

are uncontrolled. However, three facilities do have control devices for pitch storage tank emissions. The 

most effective control devices are condensers and activated carbon adsorbers. 

2.0 DEVELOPMENTS IN PRACTICES, PROCESSES AND CONTROL TECHNOLOGIES 

For the purposes of this technology review, a “development” was considered to be a (n): 

• Add-on control technology that was not identified during the development of the current 

NESHAP for this source category; 

• Improvement to an existing add-on control technology that could result in significant 

additional HAP emissions reductions; 

• Work practice or operational procedure that was not identified during development of the 

current NESHAP for this source category; or 

• Applicable process change or pollution prevention alternative that was not identified and 

considered during the development of the current NESHAP for this source category. 

We investigated developments in practices, processes and control technologies through a 

literature review and discussions with industry representatives, and included questions in an information 

request that was sent to the companies subject to subpart LL in 2011. 

2.1 Potline, Anode Bake Furnace and Paste Production Plant Startup Procedures 

The MACT control technology typically used for potlines and anode bake furnaces is a dry 

alumina scrubber followed by a baghouse. The MACT control technology typically used for paste 
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production plants is a dry coke scrubber followed by a baghouse. Facilities further decrease emissions

from these sources by implementation of certain practices during periods of startup. For example, for 

anode bake furnaces and paste production plants these practices include the development of startup 

schedules and operation of the associated control systems within normal parametric limits prior to 

startup. Some examples of practices used during startup of potlines include the development of startup

schedules, minimizing the time that hooding is removed during anode changes, reducing cell 

temperatures to as low as practicable, and resealing pot crusts that have been broken as often and as 

soon as practicable. These startup work practices would be required under the proposed rule. 

2.2 Substitution of Coal Based Coke in Anodes 

We investigated the feasibility of substituting metallurgical coke for the petroleum coke used by 

all facilities in the production of anodes. Metallurgical coke contains lower sulfur and would thus lead to 

lower emissions of COS. In addition, metallurgical coke has a lower content of some HAP metals, 

particularly nickel. Testing concluded that unprocessed coke produced from coal does not have 

properties suitable for the production of carbon anodes. Coal-based coke has a high ash content and this 

leads to unacceptable properties, including poor electrical conductivity, high carbon consumption and 

unacceptable aluminum purity.1

Coal or coal-based coke could be processed (for example by solvent extraction) to remove the 

impurities at a considerable cost. However, anodes produced with 100 percent processed coal-based 

coke have similar drawbacks, such as brittleness and high carbon consumption.2 While it may be 

technically feasible to produce acceptable anodes by blending petroleum based coke and processed coalbased coke, the need to blend with petroleum coke to produce usable or acceptable anodes defeats the 

initial benefits of removing the impurities. In any case, suitable processed coal-based coke is not 

commercially available today. 

2.3 Improved Potroom Capture Efficiency 

A process has been developed3 to improve capture of emissions from prebake pots by detecting 

the drop in temperature in the air space above the pot crust, but within the hooding, associated with the 

temporary hooding removal for anode changes and using a control system to alter the air flow on a potby-pot basis. When hoods are open, pressurized air is supplied to the inside of the hood and increased

suction is supplied through a second duct to direct emissions to the dry alumina scrubber. As described, 

this system should improve capture efficiency and decrease secondary emissions. However, emission 

standards are not being proposed based on the installation of this process because: 

1) It is not currently in use in any primary aluminum production facility in the U.S.; 

2) It is not known to be suitable for retrofit to existing potlines; 

3) No data are available regarding the actual emissions of TF and POM from a potroom 

equipped with this system; 

4) No information is available about the cost of retrofit or the incremental costs associated 

with implementing this system in a new plant; and 
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5) While the patent description is publically available, details necessary for successful 

implementation are proprietary. 

3.0 CONCLUSIONS 

Other than the startup procedures for potlines, anode bake furnaces and paste production plants, 

this review did not identify any developments in practices, processes or control technologies that have 

been implemented in this source category since promulgation of the current NESHAP that indicate any 

proposed amendments to NESHAP would be appropriate under the Section 112(d)(6) technology 

review. At the time that subpart LL was developed, the potline configurations in use differed 

considerably in their potential HAP emission rates, therefore seven separate subcategories of potlines

were established, each with different MACT floors. As the primary aluminum production industry has 

contracted from 23 facilities to 14 facilities (10 of which are presently operating) in the time since

promulgation of the current NESHAP, some of the higher emitting potline subcategories have ceased 

operation. Proposed new source MACT standards are consistent with emissions from the lower emitting 

potline subcategories. 

Although no new developments in practices, processes or control technologies have been 

implemented in this source category since promulgation of the current NESHAP (again, other than 

startup procedures), existing technologies could be adapted to achieve lower emissions. As an example,

POM and TF capture can be improved by increasing the flow through the dry alumina scrubber. 

However, such an increase in flow would require a redesign of the scrubber/baghouse system and the 

data needed to estimate the associated reduction in emissions are not available. Overall POM and TF 

emissions could also be reduced by retrofit of a control device to treat the high volume, low 

concentration secondary emissions from the potroom roof vents. However, it is not known whether the 

current buildings could accommodate such control devices (e.g., it is not known whether the necessary 

space is available or whether the building structure could support the additional weight). Therefore, the 

EPA is not proposing any amendments to the NESHAP based on these control measures. 

Similarly, conversion of the one remaining existing idle Soderberg facility (i.e., Columbia Falls) 

to prebake potlines would result in decreased emissions of POM, TF and PM. However, the costs 

associated with such a conversion would likely be prohibitive. Therefore, EPA is not proposing 

standards based on conversion to a prebake potline configuration. 

1 Wells, C. Personal communication to David Putney, May 30, 2014. 

2 Hamaguchi, M., et al. Prebaked Anode from Coal - Utilization of Coal Extract as a 

Coke Feedstock. Light Metals. 2010. p. 923-928. 

3 Girault, G, and P. Cantin. Patent PCT/IB2007/001612 (US200090159434 A1). System 

and Process for Collecting Effluents from an Electrolytic Cell. 2009.