Large scale industrial mining and metal extraction operations are few in number in any country where they operate, their feed materials and production configurations vary significantly, and they may be significant mercury release sources. Given these factors, it is highly recommended to use a point source approach in the inventory, and compile point source specific data from the operating companies themselves, if feasible, as well as from other relevant data sources with knowledge of the specific production facilities.
5.2.5.1Sub-category description
Lead is extracted from a sulphide ore, primarily galena (lead sulphide), which also contains some mercury (US EPA, 1997a). The levels of mercury in the ores vary, and in some cases can be elevated compared to other natural raw materials (COWI, 2002).
Like described for zinc (section 5.2.3), the waste rock and tailings may, just like the generated concentrates, contain trace amounts of mercury. This material is much more susceptible to weathering due to the reduced particle sizes and higher accessibility to air and precipitation. For sulphidic ores, which are important ore types for production of several base metals, this weathering liberates and oxidizes the contained sulphur and produces sulphuric acid. The acid renders mercury and other constituents more soluble and thus increases leaching of the metal to the environment many fold as compared to the untouched mineral deposit. This process is called "acid rock drainage" (or ARD) and is considered a serious environment issue (European Commission, 2003).
In the extraction of the lead from the ore/concentrate, processes are used which release this mercury from the rock material. This mercury may evaporate and follow the gaseous streams in the extraction processes (in most cases) or follow wet (liquid) process streams, depending on the extraction technology used. Unless the mercury is captured by process steps dedicated to this purpose, major parts of it may likely be released to the atmosphere, land and aquatic environments. Retained mercury may be sold in the form of "calomel" (Hg2Cl2), normally sold for off site extraction of metal mercury) or on-site processed metal mercury, or it may be stored or deposited as solid or sludgy residues (Environment Canada, 2002). Besides these output pathways, parts of the mercury input follows co-produced sulphuric acid at trace concentrations (European Commission, 2001).
The principal steps in lead extraction generally resemble the "pyrometallurgical" extraction process described for zinc (section 5.2.3), and include production of lead-rich concentrate from raw ore, roasting of the concentrate, and smelting/reduction of the metal oxides in a furnace, which both occur at high temperatures. In some production facilities, the concentrate is not sintered prior to the introduction in the furnace. In these cases, most of the mercury present in the concentrate is expected to evaporate and follow the gas streams of the downstream process steps. Like for zinc and copper, mercury present in the off gasses from sintering and smelting may be removed in a dedicated mercury removal step before the gasses are lead to the sulphuric acid recovery plant (if present; see the detailed process description in section 5.2.3). Lead is sometimes co-produced with zinc or other non-ferrous metals. For a thorough description of the processes of lead extraction see for example (European Commission, 2001).
Recycled lead scrap may be added to the fed material to the sintering or smelting steps, but is not considered a major input source of mercury to the process. Metallurgical coke (or gas fuel) is used in the reduction step in the furnace, but is not expected to be major mercury input sources to the processes, as (in the case of metallurgical coke) most of the mercury present in the coal used evaporates in the coke production process.
5.2.5.2Main factors determining mercury releases and mercury outputs
Table 5 83 Main releases and receiving media during the life-cycle of lead extraction and initial processing
Phase of life cycle
Air
Water
Land
Products
*2
General waste
Sector specific treatment/
disposal
Wastes from mining and production of concentrates
x
X
X
X
Extraction of lead from concentrate
X
X
X
X
X
Manufacture of refined lead and products *1
Use of lead
Disposal of lead
Notes: *1: Mercury releases could in principle happen due to fossil fuel usage, but the lead metal
is not expected to be a mercury input source to the refining and manufacturing steps;
*2: In sulphuric acid, mercury by-products, and perhaps other process-derived by-products;
X- Release pathway expected to be predominant for the sub-category;
x- Additional release pathways to be considered, depending on specific source and national situation.
The concentration of mercury in the ore and amount of ore mined are important factors determining mercury releases.
Extraction and primary processing of lead (also called “primary lead smelting”) may lead to releases of the mercury to the atmosphere, to aquatic and terrestrial environments, and to accumulation of substantial quantities of mercury-containing mineral waste which may in turn lead to additional releases. The extent of releases is very dependent on how carefully the waste deposits are managed (COWI, 2002). US EPA (1997a) describe that the sintering reactions occur at very high temperatures (about 1000 ºC) and controls devices used at most plants (in the USA) are expected to have minimal effectiveness at capturing the mercury. Therefore, most of the mercury in the ore was expected to vaporize and be emitted to air during this sintering process. Improvements in this regard may however have happened since then in the sector. Any residual mercury remaining in the roast from the sintering process is generally expected to be released during the reduction step (US EPA, 1997a).
As with other non-ferrous metals desctribed above, extraction and processing of lead is often equipped with a variety of release reduction devices, with the potential to reduce direct releases of mercury to the atmosphere as well as to aquatic and terrestrial environments. Such technologies can involve retention of particulate matter and gaseous releases from flue gas, waste water treatment and in some cases mercury specifc filters. Atmospheric release reduction technology present normally yields additional solid or fluid residues (COWI, 2002).
5.2.5.3Discussion of mercury inputs
Table 5 84 Overview of activity rate data and mercury input factor types needed to estimate releases from lead extraction and initial processing
g mercury/metric ton in reject material produced *1
Input to extraction of primary lead from concentrate
Metric tons of concentrate used
per year
g mercury/metric ton concentrate
Notes: *1 Such wastes may include lower grade material (lower lead concentrations), and the mercury concentrations may be similar to concentration in the input ore material. If no concentration data for reject materials are available, concentration data for the ore used may be applied for forming a rough estimate.
The two most important input factors needed to estimate emissions from a facility in this sub-category are: an estimate of the average concentration of mercury in the lead ore concentrate used at the facility; and the annual capacity of the plant (in units such as metric tons of lead ore concentrate processed per year).
The concentration of mercury in lead ores can vary considerably. Hylander and Herbert (2008) collected data for mercury concentrations in concentrates for zinc, copper and lead production for all mines globally, for which data were available through market studies published by BrookHunt and Associates Ltd. (2005, 2006a; 2006b). The individual data are proprietary, but data were aggregated in charts showing the distribution of mercury concentration in relevant concentrates; see Figure 5 -13 for data on lead concentrates. The authors note that no data from Chinese mines were available.
Figure 5 13 Distribution of mercury concentrations in lead concentrates globally (reprinted with permission from Hylander and Herbert, 2008. Copyright 2008 American Chemical Society).
UNEP/AMAP (2012) proposed the following default mercury input factors for lead extraction based on (Hylander and Herbert, 2008; Outotec, 2012) as well as other information: Minimum: 2; medium: 30, and maximum: 60 g mercury/metric ton of concentrate used. Converted to a basis of lead produced, the corresponding factors were respectively 2.8, 75 and 214.3 g mercury/metric ton lead produced, when using a concentrate used/Cu produced ratio of 1.39-3.57 (intermediate value 2.50).
Some other data on mercury concentrations in lead concentrates are presented in Table 5 -85.
Table 5 85 Examples of mercury concentrations in concentrates for lead production
Country
Location
Type
Average Hg
concentration,
g Hg/metric ton
Range of Hg concentration in samples,
g/metric ton
Data source
In concentrates
Canada
Brunswik Works
Lead concentrate
2.7
Klimenko and Kiazimov, 1987
USA
Missouri
Lead concentrate
0.2
US EPA, 1997a
Russian
Federation
Unknown
Concentrate of stratiformic lead-and-zinc type
2 - 290
Bobrova et al., 1990; Ozerova, 1986
Global
Global average
34
(median 10)
(see Figure 5 -13)
Hylander and Herbert (2008)
China
2 lead smelters
2.15 and 18.7
Zhang et al (2012)
Typical medium value
30
Outotec (2012)
5.2.5.4Examples of mercury in releases and wastes/residues
Zhang et al (2012) have reported detailed mass balances for six non-ferrous smelters (zinc, lead and copper) with relatively low atmospheric emissions in China. The study results are described in the section on zinc extraction above. The few data available do not indicate major differences in the mercury output distribution pattern between different base metals production.
Klimenko and Kiazimov (1987) report mercury concentrations in reject material at 0.69 g/metric ton from combined production of lead, zinc, copper and compound concentrates (with mercury concentration in the input ore at 2.1 g Hg/metric ton ore), indicating that mercury concentrations in reject material may be significant.
The US EPA estimated that 0.10 metric tons of mercury was emitted from lead smelters in the USA for year 1994. Assuming that all mercury in the ore is released to the air, this emissions estimate can be calculated by multiplying total capacity (370,000 metric ton) times the average mercury concentration in these ore concentrates (0.2 ppm). However, US EPA actually used a somewhat more complicated equation (which can be viewed in Appendix A of the US EPA, 1997a report).
5.2.5.5Input factors and output distribution factors
Based on the information compiled above on inputs and outputs and major factors determining releases, the following default input and distribution factors are suggested for use in cases where source specific data are not available. It is emphasized that these default factors are based on a limited data base, and as such, they should be considered subject to revisions.
The primary purpose of using these default factors is to get a first impression of whether the sub-category is a significant mercury release source in the country. Usually release estimates would have to be refined further (after calculation with default factors) before any far reaching action is taken based on the release estimates.
Due to lack of data, no default factors can be set for the mining and concentrating processes. Note that this implies that the mercury release estimates calculated from default factors may likely tend to underestimate the total releases from the sector.
a) Default mercury input factors
Actual data on mercury levels in the particular concentrate composition used will lead to the best estimates of releases.
If no information is available on the mercury concentration in the concentrates used in the extraction step, a first estimate can be formed by using the default input factors selected in Table 5 -108 below (based on the data sets presented in this section). Because concentrations vary so much, it is recommended to calculate and report intervals for the mercury inputs to this source category. The low end default factors has been set to indicate a low end estimate for the mercury input to the source category (but not the absolute minimum), and the high end factor will result in a high end estimate (but not the absolute maximum). The medium estimate is used in the default calculations in Inventory level 1 of the Toolkit. If it is chosen not to calculate as intervals, the use of the maximum value will give the safest indication of the possible importance of the source category for further investigation. Using a high end estimate does not automatically imply that actual releases are this high, only that it should perhaps be investigated further.
Table 5 86 Default input factors for mercury in lead concentrates used for extraction of lead
Feed material
Default input factors;
g mercury per metric ton of concentrate;
(low end - high end (intermediate)
Lead concentrate
2 – 60 (30)
If desired, these default factors can be converted to a basis of mercury inputs per lead produced, by the use of a concentrate used/Pb produced ratio of 1.39-3.57 (intermediate value 2.5 ton concentrate used per ton lead produced) as derived by UNEP/AMAP (2012). The corresponding factors are low end: 2.8, 75 and 214.3 g mercury/metric ton lead produced. Note that the default Toolkit spreadsheet calculations are based on mercury per concentrate.
b) Default mercury output distribution factors
Based on the data on mercury output distribution presented in this section, as well as in the section above on zinc, the following default factors are suggested.
Table 5 87 Default output distribution factors for mercury from extraction of lead from concentrates
Phase of life cycle
Air
Water
Land
*1
Product
*1, *2
General waste
Sector specific treatment/
disposal *1
Mining and concentrating
?
?
?
?
x
x
Production of lead from concentrate:
Smelter with no filters or only coarse, dry PM retention
Notes: *1 Deposition of residues will likely vary much between countries and perhaps even between individual facilities, and may be on land, in the mine, in impoundments, often on-site.
*2: Marketed by-products with mercury content include, among others, calomel, elemental mercury, sludge for off-site mercury recovery, low grade washing acids, sulphuric acid, liquid sulphur and filter cake or other residues sold or transferred to other metal production activities or other sectors.
c) Links to other mercury sources estimation
In case of combined smelters producing several non-ferrous metals from the same concentrate, it is suggested to assign the mercury releases to the metal produced in the largest amounts. In case of parallel processing of different concentrates in parallel production lines, assign the mercury releases separately to the major metal produced in each line.
5.2.5.6Source specific main data
The most important source specific data would in this case be:
Measured data or literature data on the mercury concentrations in the ores and concentrates extracted and processed at the source;
Amount of ore/concentrates extracted and processed; and
Measured data on the distribution of mercury outputs with (preferably all) output streams, including mercury percentages retained by emission reduction equipment applied on the source (or similar sources with very similar equipment and operating conditions).
The presence of a mercury removal unit at a specific extraction plant may indicate that a major share of the mercury outputs is not released to the atmosphere, but is instead marketed as by-product or stored on-site.
