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MEMORANDUM 

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

From: David Green and Matt Hakos, RTI International 

Date: September 3, 2015 

Subject: Final 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 11 facilities that are subject to subpart LL, two 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 Amended NESHAP 

The final NESHAP amendments include emission limits for carbonyl sulfide (COS) and POM 

from new and existing potlines; for particulate matter (PM, as a surrogate for non-mercury (Hg) metal 

HAP) from new and existing potlines, anode bake furnaces and paste production plants; for Hg from 

new and existing anode bake furnaces; and for nickel, arsenic and polychlorinated biphenyls (PCBs) 

from new and existing Soderberg potlines; equipment standards to limit POM emissions from existing 

pitch storage tanks; and work practice standards for potlines, anode bake furnaces and paste production 

plants during periods of startup and normal operation. 

1.4 Current Practices 

1.4.1 Potrooms 

At the time of the initial 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 were no longer in use at the time of the 

proposal (and the last Soderberg plant is now permanently shut down). The amended rule includes 

standards for only five of the original seven pot configurations. 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. 
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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 

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 had different POM emission limits for existing potlines in the initially promulgated 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, TF and PM 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 9 facilities that are equipped 

with potlines. 

COS emissions, which were not initially 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 control 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 were not regulated under the initially promulgated 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. 
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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 

petroleum 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 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 non-Hg HAP metals. Anode bake 

furnaces also emit Hg, which is present in the anode paste at low concentrations. This Hg is volatilized 

and driven off during the baking process. 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 emissions and provide some control of Hg emissions. 

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. The final rule 

amendments include MACT equipment standards (promulgated under section 112(d)(2) and (3) of the 

CAA) that will require that all pitch storage tanks to be controlled such that inlet POM emissions are 

reduced by a minimum of 95%. 

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; 
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 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 

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 are required by the final rule. 

2.2 Work Practices to Limit Emissions During Normal Operations 

Work practices that have been developed to improve the performance and efficiency of potlines 

are also effective at limiting HAP emissions. The final rule specifies the following work practices which 

are applicable to periods of normal potline operations: 

(1) Ensure the potline scrubbers and exhaust fans are operational at all times. 

(2) Ensure that the primary capture and control system is operating at all times. 

(3) Hood covers should be replaced as soon as possible after each potroom operation. 

(4) Inspect potlines daily and a) identify unstable pots as soon as practicable but in no case more 

than 12 hours from the time the pot became unstable, b) reduce cell temperatures to as low as 

practicable, and c) reseal pot crusts that have been broken as often and as soon as practicable. 

(5) Ensure that hood covers fit properly and are in good condition. 

(6) If the exhaust system is equipped with an adjustable damper system, the hood exhaust rate for 

individual pots must be increased whenever hood covers are removed from a pot, provided that the 

exhaust system will not be overloaded by placing too many pots on high exhaust. 

(7) Dust entrainment must be minimized during material handling operations and sweeping of 

the working aisles. 
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(8) Only tapping crucibles with functional aspirator air return systems (for returning gases under 

the collection hooding) can be used, unless the regulatory authority approves alternative tapping 

crucibles. 

In addition, the final rule requires either twice daily visual emissions observation, the use of bag 

leak detection systems, or the operation of a particulate matter continuous emissions monitoring system 

on the exhausts from all control devices. 

2.3 Substitution of Coal-Based Coke in Anodes 

We investigated the feasibility of substituting metallurgical coke (also known as coal-based 

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 The EPA concluded 

that it is not appropriate to require substitution of metallurgical coke for petroleum coke as a means of 

decreasing COS emissions due mainly to increased ash content and mechanical weakness in anodes 

produced from metallurgical coke. 

While it may be technically feasible to produce acceptable anodes by blending petroleum based 

coke and processed coal-based coke, the need to blend with petroleum coke to produce usable or 

acceptable anodes defeats the initial benefits of removing the impurities. Furthermore, suitable 

processed coal-based coke is not commercially available today. Therefore, the EPA concluded that it is 

not appropriate to require such use of coal-based coke or the blending of these cokes. 

2.4 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, the EPA is 

not promulgating emission standards 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; 
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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 

5) While the patent description is publically available, details necessary for successful 

implementation are proprietary. 

3.0 CONCLUSIONS 

Other than the startup procedures and work practices for potlines, anode bake furnaces and paste 

production plants, this review did not identify any other developments in practices, processes or control 

technologies that have been implemented in this source category since promulgation of the current 

NESHAP that indicate any additional 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 11 facilities (9 of which are presently operating) in the time 

since promulgation of the initial NESHAP, some of the higher emitting potline subcategories have 

ceased operation. The new MACT standards that EPA has promulgated 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 and work practices), existing technologies could be adapted to achieve lower 

emissions. As an example, POM, PM 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, PM 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 has not promulgated any amendments to the NESHAP based on 

these control measures. 

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.