Heavy-metal sludges as smelter feedstock. (Asarco Inc.) 

How Asarco Cleans Mine Discharge Water and Scores Economic Benefits As Well

Many industries produce a waste-water stream high in heavy metals. Disposal of sludge from these waste-water treatment plants has become increasingly difficult and expensive in the United States due to passage of the Resource Conservation  and Recovery Act's "land disposal ban" for hazardous wastes.

Innovative methods can be found for dealing with such wastes. For example, in  performing an Environmental Protection Agency (EPA) mandated clean-up under the  Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA o Superfund), Asarco's California Gulch water-treatment plant in Colorado meets 
CERCLA clean-up goals while using a waste water treatment sludge as a smelter feedstock, recovering incidental saleable metals, and producing non-hazardous products. In this plant, Asarco treats acidic mine-drainage water having high metal concentrations and uses the waste sludge generated as a lime replacement in lead smelting operations at its East Helena smelter in Montana.

Site History

The Yak Tunnel dates from 1895 and provides drainage for several mines, adits, shafts, tunnels, and winzes in the California Gulch near Leadville, Colo. In 1971, the Yak Tunnel/California Gulch drainage was identified as a significant source of deterioration in water quality in the upper Arkansas river. In 1982 and 1983, the EPA began investigating the site for possible inclusion on the National Priorities List (NPL) and thus eligibility for Superfund appropriations. At the same time, the EPA began to search for potentially responsible parties (PRPs). Thirteen PRPs were eventually named, including Asarco.

In 1983, the site was placed on the NPL. In 1984, the remedial investigation  began and was released in 1987. The record of decision (ROD) was released in 1988, modified (MROD) in March 1989, and an explanation of significant differences (ESD) issued in April 1989, and October 1991.

MRODs are issued when fundamental changes are made to the remedy selected in the ROD. ESDs are published when significant but not fundamental changes are made to the ROD.

The focus of the ROD was reduction in levels of cadmium, copper, lead, and zinc The selected remedy included plugging the tunnel, construction of surge ponds and their use for interim water treatment using lime softening. The 1989 MROD changed the tunnel plug to a flow-through design, converted the temporary water-treatment facility to a permanent one, and changed the process from lime 
softening to high-density sludge treatment (HDS). The ESDs fine-tuned this selected remedy. The HDS plant was placed on line Feb. 28, 1991, and conducted pilot operations until Sept. 1, 1992, when it was required to meet effluent restrictions.

Plant Feed Characteristics

The process by which metals are mobilized in abandoned mines is complex. In the California Gulch, as in other mining areas, oxygenated rain water with a near-neutral pH percolates through pyrite-containing formations. As the water does so, it oxidizes pyrite, decreasing the pH, and increasing the solubility of metals.

In the past year, monthly average flow rates have ranged from 290K to 1,180K gal/d. Over the same period, pH varied from 3.6 to 7.0. The tunnel discharge had high concentrations ([is greater than]5 ppm) of Fe, Mn, and Zn; high concentrations ([is greater than]1 ppm) of Al and Cu; and lower concentrations of As, Cd, Cr, Pb, Se, and Ag. Ca, Mg, and Na are non-regulated cations also present in significant quantities. The predominant anions are sulphate and 
chloride.

Seasonal flows are largely related to snow melt. Higher metal concentrations in summer are probably due to flushing of precipitates caused by the higher summer flow rates rather than differences in chemistry.

Concentrations of Fe, Mn, and Al are much higher than an average surface water, but these elements do not present health concerns. Accordingly, the EPA issues "secondary maximum contaminant levels" (SMCL), essentially non-enforceable, recommended levels set for non-health reasons, for these metals. For those metals that do present health concerns, the EPA issues "maximum contaminant 
levels" (MCL), which are enforceable. As, Cr, and Se in the tunnel discharge generally meet MCLs even without treatment.

The EPA's focus is on the reduction in levels of Cd, Cu, Pb, and Zn, metals for which health concerns exist, and for which the tunnel discharge requires treatment to meet MCLs. Cd is toxic to fish at the ppb level and is a chronic hazard to humans. While not toxic to humans, Cu is extremely toxic to fish at the ppb level. Chronic exposure to Pb causes a variety of health problems in both aquatic and human life. The EPA is somewhat less specific on the hazards o 
Zn but states that its "synergistic/antagonistic interaction with other metals may cause problems."

Table 1 shows a weighted average of cations present in plant feed; 
concentrations are seasonally variable. Cations not subject to discharge limitations are omitted.


High-Density Sludge Treatment


High-density sludge (HDS) treatment, specified in the MROD as the treatment technology to be used, is not the only method that could be used in this situation, Conventional water treatment with pH adjustment would be equally effective. Electrowinning and ion exchange are also workable alternatives. Conventional water treatment, however, would produce voluminous amounts of 
high-water sludge and have higher chemical costs than HDS treatment. HDS has the significant advantage of minimizing the amount of sludge generated. Electrowinning and ion exchange would be effective but more expensive than HDS.

HDS treatment is similar to conventional water treatment but produces a lower water sludge, consumes fewer reagents, and the plants are simple to build and operate. Several manufacturers have patented HDS processes. Asarco's plant uses the Tetra process, which can be simplified to three major operations: neutralization, thickening, and filtration.


Neutralization is the critical stage. The feed water, with a pH of roughly 4.5, enters the first-stage reactor where it is mixed with high-pH recycle sludge from the thickeners. Mixing increases the pH and sludge is added until the pH reaches 7.3 whereupon the mixture is transferred to the first-stage neutralization tank. The first- and second-stage neutralization tanks are in series. A mixture of lime and additional recycled sludge from the thickeners is added to the two reactors to bring the effluent pH to 10.3, at which point the 
metals have largely precipitated. A polymer flocculant is added and the mixture transferred to two thickeners in parallel.


The thickener overflow, relatively clear and free of contaminants, is passed through sand filters to remove the remaining suspended solids, then neutralized with sulphuric acid in a pH-adjustment tank and discharged to California Gulch.

The thickener underflow is recycled to the first-stage reactor and lime-mixing 
tank. That sludge that cannot be reused is sent to a filter press where it is 
dewatered to 20-30% solids and prepared for shipment to the East Helena smelter

Since the process can not accumulate mass, the amount of sludge generated is equal to the sum of the mass of lime added, the mass of the contaminants remove from the water, and the mass of added polymer. These quantities are all variable. The mass of lime added is dependent upon the feed pH and varies from 100-500 mg/l. The metal contents are variable with water flow rate and time of year. These factors, combined with the variability of the base flow rate, can mean swings in sludge output from 50-300 st/mo.

The HDS process is able to attain a low water to solids ratio because of the sludge recycle. In typical water treatment, the sludge is fluffy and gelatinous due to the small size of the solid particles. However, when sludge is recycled it provides a surface for the metals to precipitate on. In essence, the solid particles grow each time they are recycled, attaining a size up to five times larger than those in conventional sludges. As the volume fraction of the solids in the sludge increases, the water content decreases. At Asarco, solids range from 20-40% of the sludge, although Tetra advertises that 50% solids sludge is attainable. Another advantage of larger particle sizes is more rapid settling.

Effluent Characteristics

Normally, a water-treatment plant would be required to meet discharge limitations set by an NPDES permit. Since this plant is part of CERCLA clean-up it is exempt from the direct requirements of the Clean Water Act (CWA) but must meet all "Applicable or Relevant and Appropriate Requirements" (ARAR). Essentially, this means that the effluent must meet NPDES discharge limitations even though the discharge will be regulated not by an NPDES permit but by a "Discharge Control Mechanism" (DCM). DCM limits are set by the same process as NPDES limitations: human health, aquatic health, and receiving body of water classification. The final draft of the DCM was released by the EPA on April 25, 1994.

Furthermore, for the first two years of operation the plant was required only to achieve performance standards attainable by the Best Available Technology. But from April 1, 1994, the plant is required to meet the lower of past historical performance or water-quality based standards. These new standards are summarize 
in Table 2.

Control

In addition to the above standards, the effluent must undergo an acute toxicity protocol in which fish and micro invertebrates are exposed to varying dilutions of the effluent.

According to Table 2, the plant is obviously meeting the effluent standards but the performance data are based on a weighted, annual average. Summer discharge concentrations are higher than winter. Also, the DCM has limitations on daily peak and average discharge concentrations.

Sludge Characteristics

Sludge composition varies as a result of metal loadings. However, one can complete a reasonable mass balance using weighted averages. The average production is 131 st/mo (June 1993 to February 1994). During this period the average moisture was a nominal 65%, and the average lime added was 270 mg/l. Average flow rate was 680K gal/d. Using these values and the weighted average, influent-effluent concentrations gives the sludge analysis shown in Table 3.

The sum of the masses shown in Table 3 is nearly 99%, leaving 1% for polymer added and trace constituents. While the sludge is variable, the order of the elements listed is not likely to change.

When addressing the issue of disposal, a determination must be made as to whether or not the sludge is hazardous. The following steps may be used in this determination:

1. Is the material a solid waste under RCRA section 1004 (27)?

2. Is the material a hazardous solid waste under RCRA Subtitle C?

a) Is the waste an exempt waste?

b) Is the waste a listed waste?

c) Is the waste a characteristic waste?

d) Is the waste mixed with or derived from a hazardous listed or characteristic waste?

e) Will the waste be legitimately and beneficially recycled?

3. Is the waste an intermediate step in an overall CERCLA clean-up? 

Except in certain cases of recycling, water treatment sludges are solid wastes. The question is then whether or not a sludge is hazardous. Unless exempted from Subtitle C, a solid waste is hazardous if it is specifically listed by the EPA as a hazardous waste, or if it displays a hazardous characteristic. There are four hazardous characteristics: ignitability, corrosivity, reactivity, and toxicity.

The Yak Tunnel Water Treatment Plant sludge falls under the RCRA definiton of solid waste. The sludge is exempt, however, from regulation as a hazardous waste by the Bevill Amendment. In this amendment, the EPA determined that wastes resulting from the mining and beneficiation of ores are not hazardous. As an 
intermediate step in a CERCLA cleanup, the sludge is also exempt. Additionally, since the sludge is currently being used/reused as a substitute for a commerical product, it is not even classified as a solid waste. Clearly, then, the Yak Tunnel Water Treatment Plant sludge is not a hazardous waste.

Other industries generate metal-containing sludges that may be classified as hazardous. While direct comparisons are not possible because of differences between these sludges and those generated by Asarco at the Yak Tunnel Water Treatment Plant, other industries may do well to study Asarco's waste minimization and recycling practices. Listed sludges that may have high metal 
concentrations include those from electroplating facilities, inorganic pigment processes, and zinc-processing facilities. A larger number of industries product a sludge failing the Toxicity Characteristic Leaching Procedure (TCLP) for one of the eight metals tested: As, Ba, Cd, Cr, Pb, Hg, Ag, and Se.

East Helena Smelter

The sludge is railed from the Yak Tunnel plant to Asarco's East Helena smelter in Montana, which produces 72.7K st/yr Pb. The smelter uses sintering, a blast furnace, dross furnace, and a reverberatory furnace. A key raw material in the process is lime which serves as a flux. At East Helena, locally quarried limestone is added to the feedstock prior to sintering, which drives off C[O.sub.2] to form lime. Sludge is substituted pound for pound of dry lime 
equivalent. The sludge is not dried prior to sintering. Last year the smelter used 2,397 st/mo lime (as CaC[O.sub.3]) and 131 st/mo sludge. After accounting for water and metal content, sludge replaces about 1.5 % of lime used (as CaC[O.sub.3]).


Other metals in the sludge are normally encountered in the smelting press. The Pb reports to the bullion, the Cu to the matte and speiss, Cd to the bag-house dust, and Zn, Fe, Al, and other trace metals to the slag. While the primary benefit of sludge addition is the lime content, incidental Pb and Cu units recovered have value as well.

Bag-house dust is shipped to Asarco's Encycle facility in Corpus Christi, Texas for processing and Cd recovery. The Cu matte is shipped to the El Paso Cu smelter, and the speiss to the Hayden smelter. Slag is landfilled on site as it is not a hazardous waste.

Cost of Disposal Options

This analysis focuses on the cost of various sludge disposal options, and compares these to the cost/benefit of recycling the sludge as a substitute for commercial products. Only disposal costs are addressed; other considerations such as resource conservation and environmental impacts are not examined.

Although the sludge is not a hazardous waste, costs of disposal in 
hazardous-waste landfills are also compared. A very important consideration in disposal is the potential for future costs. Table 4 summarizes the costs of the three disposal options compared to recycling as an alternative to disposal.

On-Site Disposal. Disposal on-site is what the EPA foresaw when writing the ROD Were Asarco to choose this option, it would likely have to meet as stringent construction requirements as for a hazardous-waste landfill. These costs are difficult to estimate as the land and construction costs would be internal to Asarco, and the permitting requirements for a RCRA landfill, normally a significant cost, would not apply. Additionally, the risk of future costs is high due to the fact that the CERCLA requires that "reviews of the remedial 
action will be conducted no less often than each five years after the initiation of the remedial action to assure that human health and environment are being protected by the remedial action being implemented."

Solid-Waste Landfill. Because the sludge is not legally a hazardous waste, disposal in a solid-waste landfill may be an option. The range of $30-$80/st is based on disposal in CSI's Bennett, Colo., landfill. The base cost is $30/[yd.sup.3], but disposal will cost $80/[yd.sup.3] if stabilization is required. RCRA regulations bar the landfilling of liquid waste. If the sludge shows any liquid-solid separation at all upon arrival, it will be treated as a liquid waste and require stabilization with a pozzolanic material, generally fl ash, or cement- kiln dust.

Hazardous-Waste Landfill. Although the sludge is not a hazardous waste, it does fail the TCLP for cadmium. In practical terms, this may necessitate disposal in a hazardous-waste landfill, regardless of the waste classification. The prices given are for disposal in USPCI's Grassy Mountain, Utah, facility. The book disposal price is $120/st, though quantity discounts to $80-90/st are common. For hazardous wastes, treatment prior to landfilling is required. The general method is stabilization using pozzolonics, which can be done either at the 
treatment plant or at the landfill. USPCI charges $40-60/st to treat at the landfill. Additional fees ranging up to $28/st are levied for out-of-state waste, local tipping fees, etc.

Disposal in either a solid- or hazardous-waste landfill is relatively little 
risk for future liability. Current landfill restrictions are much more 
stringent, making a future clean-up less likely than in the past. Since the waste maintains it's identity, a CERCLA-type clean- up in the future is at least possible.

Recycling in the East Helena Smelter. Not a disposal option, recycling allows recovery of resources in the sludge, has no hazardous byproducts, has no future liability risk, and costs nothing.

Transportation costs have been ignored in the analysis of all these options. Transportation may prove to be the most expensive factor in disposal in a solid-waste landfill and will be a major factor in disposal in a hazardous-waste landfill. Trucking costs are higher than rail costs and short-haul trucking higher than long haul. Specific dollar amounts are not calculated because possible conditions are so variable, but, in this case, the waste must be transported regardless of where it goes. Rail cost from Leadville to East Helena, a distance of approximately 800 mi, is $34.50/st.

Recycling has many benefits for Asarco. In addition to savings in lime costs, disposal costs are eliminated, future liability is minimized, benefits are realized in resource conservation, and the environmental impact of land disposa is not incurred. For similar wastes in other locations, without the unique situation of having a smelter with the need for lime, transportation costs may well be a major factor in selecting recycling over disposal options.

Applications to Other Industries

Using water-treatment sludge with a high lime content as a smelter flux has applications outside the case discussed here. Many industries, especially large metal platers and processors operate their own water- treatment plants. Some produce a waste sludge that is a listed waste, and many produce a sludge that is hazardous by failing a characteristic. Depending on distance to the nearest 
smelter, transportation costs, quantities generated, and other contaminants present, other industries may be able to use this process as an alternative to current disposal methods. Other industries have proven that hazardous wastes can be used as feedstocks. The cement industry use of waste solvents as fuel in 
kilns is a perfect example.

In conclusion, Asarco is meeting clean-up goals at California Gulch, while minimizing the amount of waste sludge produced. As an alternative to costly disposal, recycling recovers valuable lime units that would otherwise be lost.

COPYRIGHT 1994 Intertec Publishing Corporation 
Mosher, John, Heavy-metal sludges as smelter feedstock. (Asarco Inc.)., Vol.  195, Engineering & Mining Journal, 09-01-1994, pp 25(6). 


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