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Technical Assessment of Environmental Performance and Emission Reduction Options for the Base Metals Smelters Sector - Final Report

Emissions

2.1 Overview
2.2 Emissions Projections
2.3 Sulphur Dioxide (SO2) Emission Sources
2.4 Total Particulate Matter (PMT) Emission Sources
2.5 Mercury Emission Sources
2.6 Measuring, Monitoring and Uncertainty in Emission Estimates
2.7 Emission Reductions Required to Meet Gazette I Targets


2.1 Overview

Emissions of sulphur dioxide (SO2), total particulate matter (PMT) and mercury from the 6 base metal smelters have substantially declined over the last two decades. Between 1988 and 2004, SO2 and PMT emissions dropped by approximately 60%. Mercury emissions declined by approximately 90% in the same period, with most of the reductions achieved at the Hudson Bay Mining and Smelting (HBMS) facility in Flin Flon, Manitoba. However, HBMS continues to account for approximately 80-85% of total mercury emissions reported by the 6 smelters. Approximately 78% of the reported dioxins and furans emissions are from the Falconbridge, Sudbury, ON operations.

Table 1: Emission Trends for the Six Base Metal Smelters
PollutantUnits1988199520002004
Sulphur Dioxide (SO2)kilotonnes1708822205680
Particulate Matter (PMT)tonnes18516583587177694
Mercury and its compoundskilograms20807218516841793
Dioxins and Furansgrams
I-TEQ
   2.31

Source: Sums from base metals industry sources. Draft numbers were prepared from Environment Canada's Strategic Options Report for the Management of Toxic Substances from the Base Metals Smelting Sector, 1997 (for 1988 data) and National Pollutant Release Inventory (for 1993-2003) data. Base metal companies reviewed and replaced some of these estimates with their own updated numbers.

A variety of technologies and operating practices have been implemented to reduce emissions at the 6 base metal smelters. Some of the major reductions in SO2 emissions have come about as a result of installation of sulphuric acid plants. Sulphuric acid plants are operating at 4 of the 6 smelter sites. Gas streams with relatively high SO2 concentration are captured and directed to these acid plants where the SO2 is converted into sulphuric acid for sale in North American markets. Smelters with sulphuric acid plants also tend to have lower emissions of mercury, other metals, and PMT. The principal reason is that gases routed though the acid production process need to be cleaned from these pollutants so that the acid plants operate efficiently and the acid meets market quality specifications. Mercury and other metal emissions may also be low due to the content of ores, and the concentrates processed.

In addition to the installation and operation of acid plants, there are other technologies and operating practices that have contributed to lowering potential emissions over time. Examples of these are listed below. However, not all of technologies listed have been adopted by all facilities, nor to the maximum degree possible. These and additional technical feasible options can be employed to further reduce emissions from the base metal facilities for them to achieve the Gazette I targets.

Table 2: Examples of Technologies and Operating Practices Employed by the Base Metal Facilities
Technology / Operating PracticeComments
Reduced production. Production delays. Shifting stages of production to more favourable times.Temporary reductions in production and production delays are employed at some facilities to achieve local ambient air quality objectives and standards (usually for SO2). At these facilities the environmental management process features ambient air quality monitoring, modelling, with "instantaneous" feedback mechanisms.
Selection of ores and concentratesConcentrates selected have a lower sulphur and/or mercury levels.

Pyrrhotite (iron sulphide) rejection

 

Pyrrhotite rejection is a mill/concentrator process that is used to increase the nickel content and reduce the amount of concentrate fed to the smelter. It lowers energy use in the smelter; and reduces the quantity of silica flux required for slag formation. The practice also has the desirable effect of removing a significant portion of the sulphur in the ore prior to smelting, thereby reducing SO2 emissions. The degree to which pyrrhotite rejection can be applied is limited in that high rejection levels can result in economic penalties due to increased loss of valuable metals and sulphur that has heating value.
Sulphuric acid plantsAcid plants are installed at 4 of 6 the base metal smelters studied. Mostly they capture and treat gases from the roasters and furnaces. They reduce SO2, mercury, and a portion of the potential PMT process stack emissions.
Double absorption sulphur acid plant process and tail gas scrubbing

This technology reduces the amount of SO2 that is not made into sulphuric acid (increases the conversion rate of SO2 versus single absorption).

 

Noranda ReactorThis technology is installed at one facility (Falconbridge Horne). The Noranda Reactor replaced the roaster and reverberatory furnace. It provides a more continuous, steady SO2 gas stream for treatment in the acid plant.
Continuous converterA continuous converter is installed at one of the six facilities studied (Falconbridge Horne). It increases SO2 capture and concentration of converter gases for treatment in the acid plant.
Mercury towers in acid plant gas cleaning system

This technology captures and reduces mercury emissions at acid plants.

 

Electrostatic precipitators (ESP)ESPs are installed at all facilities as part of the main control system for process PMT emissions. Efficiencies of the ESPs vary by facility.
Baghouses (fabric filters)Baghouses are installed at some facilities for selected streams for higher levels of PM control (versus ESPs only). Efficiencies vary.
Variety of dust control optionsMost facilities employ dust control practices to reduce PMT emissions from tailings, roads, materials, etc.
Increasing temperatures in furnace freeboard

Testing is being conducted as a means to reduce formation of dioxins and furans at one facility.

 

The sulphur fixation rate is the ratio of sulphur being emitted to air (as SO2) to the total amount of sulphur (expressed as SO2) entering the smelter.4 The six base metal smelting facilities analyzed in this report differ with respect to their sulphur fixation rate. Facilities that capture the potential SO2 emissions and make sulphuric acid or liquid sulphur dioxide have achieved higher fixation rates. The two facilities in Manitoba do not operate sulphuric acid plants or other systems to control potential SO2 emissions. Therefore, fixation rates for these facilities are low, with only some sulphur being captured in the slag, residue, and final products. In 2004, the average fixation rate for the 6 smelters was 67%, ranging from 16% to 91%. The average fixation rate implied by the Gazette I targets for the 6 smelters is close to 95%, based the amount of sulphur entering the smelters, estimated on assumed sulphur to product ratio.

Table 3: Sulphur Fixation Rates, SO2 Emissions and 2015 Gazette I Targets
CompanyFacility
Location
Estimated 2004 Total Sulphur Entering Facility2004 Sulphur Fixation Rate
(% of S entering Facility)
Total 2004 SO2EmissionsSO2 2015
Target in Canada Gazette, Part I
  (ktonnes
-SO2e)*
 (ktonnes)(ktonnes)
FalconbridgeSudbury, ON272#

89%

 

3011
FalconbridgeBelledune, NB11091%106

Falconbridge

 

Rouyn-Noranda, QC36385%5522
IncoSudbury, ON76573%20938
IncoThompson, MB228**

16%**

 

19212
Hudson Bay Mining & SmeltingFlin Flon, MB334**45%**18416
Total / Overall 207267%680105

* SO2e means sulphur expressed as sulphur dioxide equivalent.
# Cheminfo Services estimate for 2004 based on 2003 data.
** Source: Environment Canada, Email correspondence to Cheminfo Services, February 28, 2006

2.2 Emissions Projections

It is useful for the purpose of estimating costs to achieve the emission targets contained in Gazette I, to develop a projection of emissions to the year 2015. One such scenario, of many possible, is provided in Table 4. It shows that further SO2, PMT and mercury emission reductions are being planned by industry as part of the "business-as-usual" scenario. These future estimates take into account emission reduction technologies and practices currently underway. However, these reductions will not be sufficient to achieve the emissions targets contained in Gazette I.

Table 4: "Business-as-Usual" Emission Projections for the Six Base Metal Facilities
PollutantUnits200420082015
Sulphur dioxide (SO2)kilotonnes680609481
Particulate Matter (PMT)tonnes769444743785
Mercury and its compoundskilograms179315091423

Notes: Excludes major reductions for achieving Ontario MOE Reg. 194/05 emission trading cap in the case of Inco's Sudbury smelter. Includes reductions for which capital costs are already being incurred to achieve some reductions.

2.3 Sulphur Dioxide (SO2) Emission Sources

Smelting facilities are designed to remove sulphur, iron, and other metal impurities from concentrates to produce a form of metal that is suitable for markets or for further refining to achieve purity levels acceptable to customers. In examining the sources of SO2 emissions from base metal smelters it is useful, for the purpose of this analysis, to distinguish between the initial and later stages of the smelting processes. Generally, the initial smelting stages involve partial oxidation of the metal sulphides in the concentrate, while silica fluxing agents are often added for iron removal. Initial stages can involve use of roasters, flash furnaces, electric furnaces, unique reactor designs, as well as other equipment. These account for the majority of the SO2 generated at smelters (much of it controlled at most smelters), since the sulphur concentration in the material being processed through these stages is high. Stages later in the smelting process can also involve converters and finishing vessels. The amount of SO2 generated from these sources is typically lower. However, at smelters where SO2 emissions are already being controlled from the initial smelting process equipment sources, the smelting equipment involved with the latter stages of the process can represent the majority of the SO2 emissions.

Table 5: Typical Contribution of SO2 Emission Sources
Emission SourcesFacilities with Acid PlantsFacilities without Acid PlantsNature of Emission Stream

Roasters, flash furnaces and sintering (sintering only at one smelter)

 

1-25%

 

30-60%

 

High SO2 concentrations 5-15% by volume.

Relatively continuous stream.
Relatively low flow rate. Routed through stacks

Smelting process furnaces

 

30-60%

 

5-15%

 

Medium SO2concentrations.
Relatively continuous stream.
Medium flow rate. Routed through stacks.

Converters,
finishing vessels

 

20-65%

 

30-50%

 

Dynamic SO2 concentrations, varying over converter blowing cycle. Intermittent flow rate due to batch switching.

Relatively high flow rate. Majority of SO2 routed through stacks.

Acid plant tail gases

 

1-15%

 

0%

 

Very low SO2 concentration (200-1,000 ppm)

Continuous flow rate.
Relatively low flow rate. Routed through stacks.

Other sources
(e.g., fugitives)

 

0-5%

 

0-5%

 

Low SO2 concentration.
Unknown flow rates.
Fugitives emitted from equipment without stacks.


It is often economically more attractive to capture and treat SO2 (e.g., in a sulphuric acid plant) from the initial smelting stages versus more dilute sources from later stages of the process. Several of the smelters5 use conventional technologies of roasters followed by furnaces. The process off-gases from these sources are generally more continuous (or semi-continuous) and higher in SO2 concentration versus batch converters and other sources found later in the smelting process. Inco Sudbury uses direct flash smelting furnaces and Falconbridge Horne uses its proprietary Noranda Reactor and Noranda Continuous Converter. At Falconbridge's Brunswick lead smelter, the majority of the SO2 is generated from the sinter machine in the smelting process.

Converting is usually a finishing step at most smelters where it is used to remove residual levels of sulphur, iron and other metal impurities from the furnace mattes in order to meet product specifications. Oxidation "converts" the remaining sulphur to SO2 and the iron to iron oxide slag. Converting is usually a batch process. However, Falconbridge employs continuous converting at the Horne smelter to achieve a high level of sulphur oxidation in their process. Horne then relies on a series of finishing vessels (operated in batch mode) to further reduce sulphur and iron content, in order to achieve the necessary final product quality specifications. The SO2 generated from batch converting processes is usually less concentrated than the SO2 from initial smelting stages (e.g., roasting and furnaces) because the furnace matte contains relatively low residual sulphur levels. In batch converters, the SO2 concentration declines over time. It is highest when the oxygen is first blown into the matte, and decreases over the blowing period. In some cases, smelters find batch converter off-gases too dilute to be treated in an acid plant. For facilities that operate acid plants, the majority of the remaining SO2 emissions usually comes from multiple converters or finishing vessels operated in batch mode.

Improved capture systems can enhance the containment of batch converter gases, reducing air infiltration, minimizing dilution of these weak SO2 streams, and lowering their flow rate. This makes these streams more suitable as feeds for acid plants or other treatment systems (i.e., alkali scrubbing, liquid SO2 plants). Water-cooled hoods capture the process off-gases coming from the openings of the converters. The percentage of SO2 captured depends on the tightness of hoods that can be placed over the openings (mouths) of the converters. Secondary capture hoods can achieve higher capture rates. A technical issue is the variability of the gas flow rate and concentration over the blowing cycle.

In addition to other uncaptured sources, fugitive emissions6 from converters can often lead to high ground-level concentration (GLC) around the facility. The reason is that the converters have openings, which are required to handle charging (or pouring) of molten matte from large ladles. These openings also present sources of emissions. Fugitive sources of SO2 are usually a minor portion of total facility emissions.

2.4 Total Particulate Matter (PMT) Emission Sources

Particulate matter emission sources can be grouped into two types, namely: process and area sources. The major process sources include roasters, flash furnaces, sintering units, electric furnaces, and converters. Emissions from these sources can be from stacks or fugitive. Fugitive emissions are usually leaks from a variety of equipment, and are not routed through existing stacks. They may exit the buildings that house the processing equipment through roof vents, wall vents, doors, windows, loading bays, and other openings. PMT area sources include, but are not limited to: dust from unpaved roads; outdoor raw material piles; tailings erosion; material handling operations; mobile equipment and vehicles; and construction activities. There are many pieces of equipment or small areas from which PM emissions can result at each smelter.

Some of the 6 facilities analyzed in this study have yet to develop estimates of PM area sources. They have only estimated and reported process sources from stacks. Therefore, the current and historical emission data reported to Environment Canada's National Pollutant Release Inventory (NPRI) are not on a consistent basis for all facilities. Furthermore, there has been inconsistent reporting of PMT emissions to NPRI over time.

All of the 6 base metal facilities analyzed in this study operate electrostatic precipitators (ESPs) to address process emissions. Some facilities also operate fabric filters (baghouses) to address PMT emissions from selected sources. The degree of PM emissions control may be high (i.e., over 90%), but cannot easily be precisely determined since potential PMT emission quantities (before passing through controls) are not readily available for all sources.

The table below provides an example of the contribution of PMT emissions from major sources or source groups for one facility, where the process and area sources have both been estimated. The PMT emissions profile for other facilities will likely be different. In general, area sources can contribute as much as controlled process PMT sources. For facilities that have yet to estimate area sources, the majority of the PMT emissions from ESPs and/or baghouses are routed to the main stacks, which represent the major source of reported PMT emissions.

Table 6: PMT Emissions by Source (Example for One Facility)
(Where process and area sources have been estimated)
Source% of Total Facility PMT Emissions
Main stack (Roaster, furnace, converters sources after controls)29 %
Erosion and material handling64 %
Converter aisle fugitives2 %
Other smelter process fugitives2 %
Paved and unpaved roads2 %
Landfill erosion, all other1 %
Total100 %

Base smelter facilities are undertaking operating practices and applying control technologies to address area sources. Even companies that have yet to estimate area source emissions are undertaking measures to minimize emissions. Some of the options that have been being implemented include:

  • vegetation (grasses) planting on barren surfaces;
  • paving of roads;
  • the use of water cannons and sprays to reduce dust;
  • covering exposed tailings using dust control agents;
  • flooding exposed tailings where possible; and
  • creation of barrier zones to minimize dust travel.

2.5 Mercury Emission Sources

Mercury (and its compounds) as well as other metals may be contained in the ores and subsequently the concentrates fed to smelters. The 6 smelters vary substantially with respect to their mercury emissions due to the amount of mercury in concentrates received and the controls used that reduce emissions.

Mercury contained in the concentrate feed may be volatilized in the high temperature roasters, flash furnaces, electric furnaces, and converters, and carried in the off-gases. It may be combined with particulate matter (PM), which is collected by PM control devices or is emitted. A portion of the potential mercury emissions is controlled through the use of ESPs and baghouses that address PM emissions. However, at high temperatures some metals like mercury may be gaseous and pass through ESPs and other PM control systems. For facilities that have sulphuric acid plants, off-gases are passed through dedicated treatment towers. This removes a high percentage of the mercury, so that only a minor amount is emitted to air. A small portion is contained in the acid product.

Controlling the amount of mercury that enters the smelter is one method of minimizing emissions. HBMS accounts for approximately 80-85% of the total mercury emissions reported to Environment Canada's NPRI by the 6 smelters. At HBMS efforts continue to reduce mercury content in the materials entering the Flin Flon smelter. For example, purchased concentrates processed in 2004 had an average mercury concentration of less than 3.78 ppm. This is considerably lower than some concentrates previously processed, which contained up to 40 ppm mercury. The focus of emission reductions at HBMS is now the reverberatory furnaces, which are believed to account for nearly 80% of the origin of the emissions (routed with other sources through the main stack).

2.6 Measuring, Monitoring and Uncertainty in Emission Estimates

Emissions are estimated by base metal facilities using different methods for each pollutant. Sulphur dioxide emissions are typically calculated using mass balance approaches. In this case, total sulphur entering the smelter is measured (through sulphur content of concentrates). Total sulphur contained in products (e.g., metals, mattes) and by-products (e.g., sulphuric acid) or waste leaving the plant is also measured. The calculated difference is SO2 emitted. Periodically, process stacks are measured for pollutant concentration and flow rate, which are then used to calculate SO2 emission rates. Continuous emission monitors (CEMs) are on some stacks. Particulate matter, mercury, other metals, and dioxins and furans emissions are usually estimated using periodic (e.g., annual, quarterly or more frequent) testing of concentrations in specific stacks. In some cases, emission factors obtained from literature sources (e.g., US EPA AP-42) are applied.

Not all facilities estimate annual fugitive and area sources of PM emissions. These sources can include, but are not limited to: emissions from processes directly to air - not through a stack; road dust from heavy mobile equipment and vehicle traffic; dust from raw material piles; dust from tailings; and loading and unloading activities (e.g., conveyors, transferring operations). All facilities include emission estimates from dedicated process stacks. This presents inconsistencies between facilities with respect to total PM emissions reported.

Table 7: Summary of Emission Estimation Methodologies and Range of Uncertainty
PollutantTypical Methods for Estimating Annual EmissionsLower Range of Uncertainty in Estimates IdentifiedUpper Range of Uncertainty in Estimates Identified
SO2Mass balance.
Periodic stack testing.
Continuous emissions monitoring on some stacks.
+/- 5%+/- 10%
PMTPeriodic stack testing.
Application of emission factors
+/- 15%+/- 50%
MercuryPeriodic stack testing.+/- 35%+/- 50%
Dioxins and FuransEngineering estimates plus periodic stack testing.+/- 15%+/- 100%

Source: Ranges based on industry input.

In general, the uncertainty in estimated SO2 emissions is relatively low in comparison to lower-volume pollutants, such as PMT, mercury, and dioxins and furans. In some cases, high levels of uncertainty may present challenges in demonstrating compliance with specified emission target limit levels.

2.7 Emission Reductions Required to Meet Gazette I Targets

The 6 base metal facilities analyzed in this study, their 2004 SO2 and PMT emission levels, and the related Canada Gazette I Proposed Notice targets are contained in the table that follows. The overall reduction by 2015 implied by the Gazette I target for SO2 and PM is approximately 85% versus 2004 emission levels. All facilities would need to achieve some reductions in SO2 to achieve their targets. The mercury emission target contained in Gazette I for all facilities is 2.0 grams-Hg per tonne of metals produced, as per the requirement of the Canada-wide Standards for Mercury Emissions. The Gazette I Notice contains a specific mercury emission target for Hudson Bay Mining and Smelting, which is 373 kg-Hg/year for 2008. Falconbridge's Sudbury smelter had a specific 2008 dioxins and furans target of 0.5 grams/year grams (I-TEQ).7

Table 8: Summary of SO2 and PMT Emissions and Gazette I Targets, by Facility
CompanyFacility
Location
2004
SO2
Emiss-ions
2008
SO2 Target
2015
SO2 Target
2004
PMT
Emiss-ions
2008
PMT Target
2015 PMT
Target
  kilo-tonneskilo-tonneskilo-tonnestonnestonnestonnes
FalconbridgeSudbury, ON305711106542447
FalconbridgeBelledune, NB1012.7658104104
FalconbridgeRouyn-Noranda, QC554522445500500
IncoSudbury, ON2091763833961430275
IncoThompson, MB19217412156573592
HBMSFlin Flon, MB184166161165930100
Total 680631105769441231118
Reduction vs. 2004Quantity 49575 35716576
Reduction vs. 2004% vs. 20040 %7 %85 %0 %46 %85 %

* Mercury emission intensity target for all 6 base metal smelters: 2.0 grams per tonne metals produced, by 2008.

* Hudson Bay Mining and Smelting, Flin Flon, MB mercury target: 373 kg-Hg/year by 2008

* Falconbridge Sudbury smelter specific dioxins and furans target: 0.5 grams/year grams (I-TEQ), by 2008


4 Operating practices (e.g., feed selection) and technologies (e.g., pyrrhotite rejection) that reduce the amount of sulphur coming into the smelter are not accounted for in the fixation rate calculations.

5 This sequence is used at Hudson Bay Mining & Smelting, Inco Thompson, and Falconbridge Sudbury.

6 Fugitive emissions are released through windows, doors, roof vents, wall vents, etc. (not from stacks).

7 International Toxicity Quotient.

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