FEATURE
Waste controls at base metal mines Canadian company shows how it handles large volumes in an economically and environmentaiy safe manner Alan V. Bell Montreal Engineering Company Limited Fredericton, New Brunswick E36 1E8 Mining and milling of copper, lead, zinc, and nickel in Canada involves a nationwide accumulation of at least a half million tons of waste material each day and requires approximately 250 million gallons of process water per day. The waste management considerations with which the industry must be concerned in order to handle such large volumes in an economic and environmentally acceptable manner are far-ranging and complex. In Canada the past three to four years have represented a period of particular environmental awareness to the industry and have led to the application of several innovative programs and the development of new regulatory requirements in many jurisdictions.
lethal level defined as the 96-h median lethal concentration (96 h LC50) for rainbow trout, which are used as a standard test species. The toxic unit (TU) is now a commonly used receivingstream quality parameter, and in recent months standard procedures have been developed to enable the toxicity of treated mine effluents to be directly evaluated and expressed. The sublethal effects of heavy metals are very much of concern, particularly where anadromous fish are involved. Levels of heavy metals lower than the lethal levels cause an avoidance reaction in many species and can therefore affect fish migration.
Potential environmental effects
Waste management
The form and extent of effects on specific developments varies (see box material) as does the number of environmental parameters that must be considered. One parameter given particular attention in Canada in recent years is the toxicity of mining wastes. This is partially the result of a general concern with effluent toxicity that is being addressed in all recent federal industrial effluent requirements; it is also the result of the sensitivity of fish such as the Atlantic salmon and speckled trout to dissolved heavy metals. This concern was recently brought into focus in the northeastern part of New Brunswick, where there has been a rapid development of base metal mining activity in a region that also represents the core habitat of the Atlantic salmon on the North American Eastern Seaboard. In 1970, a major multidisciplinary program was established. It determined that the valuable fishery resources of the region could be maintained in the face of existing and future base metal mining developments as long as stringent but practical waste management measures were applied by the mines of the region. This program, termed the Northeastern New Brunswick Mine Water Quality Program resulted in recommended interim effluent requirements.
In the great majority of cases the previously listed potential environmental effects of base metal mining can be controlled to acceptable levels by the application of established waste management practices. These can be summarized in three main categories: minimization, collection and treatment of mine drainage, mill process water and contaminated surface drainage handling, storage and ultimate disposal of tailings and waste rock rehabilitation of the site so as to minimize long-term environmental effects once active mining has ceased.
Toxicity Heavy metals such as copper, lead, zinc, and nickel are toxic to salmon and other fish at extremely low concentrations. The reactions to threshold concentrations have been well defined by biologists. Figure 1 reproduces the relationships for rainbow trout (Salmo gairdneri) to various heavy metals. The lethal threshold concentration of each metal is raised by increasing water hardness and the corresponding value in Table 1. Such values are now used in Canada to evaluate incipient lethal levels of copper, zinc, and lead. Similar relationships for nickel have yet to be formulated. The toxic unit concept, the ratio of actual concentration to incipient lethal concentration is widely used with the incipient 130
Environmental Science & Technology
Aqueous wastes-mine drainage The principle sources of mine drainage are: groundwater seepage into the mine water used for machine operations including drilling, dust suppression, cooling and air conditioning sanitation and drinking water drainage from hydraulic backfill operations, and in the case of open pits, direct rainfall. Volumes vary widely depending on mining methods and the hydrogeological characteristics of the region. In Canada average mine water volumes pumped to the surface vary from 12 gpm/1000 tpd operating capacity to 700 gpm/1000 tpd for underground operations, and from 3-400 gpm/ 1000 tpd capacity for open pit operations. Mine drainage can vary in pH from basic to very acid depending on the nature of the ore and its host rock. It may contain high levels of dissolved metals, suspended solids, some oils and often ammonia, resulting from the use of ammonium-nitrate-based blasting compounds. Some mining operations are able to recycle relatively small amounts of water for drilling and dust suppression by using simple sumps to clarify the water, but in most cases the total volume is pumped to the surface for treatment with
Number ol Copper, lead, zinc & niekel mines:
Capacity 1000 tpd or less tpd < capacity < 50 tpd < capacity < 10000 tpd < capacity < 2000 ity > 20000 tpd
Copper Lead
Zinc
the other aqueous drainage components. Seven operations in Canada use all or part of their mine drainage as mill process makeup, thus minimizing overall water requirements and effluent volumes. Surface drainage The influence of such drainage on waste management at metal mines may be the largest aqueous waste component with highly toxic properties or a relatively easily controlled component of minor significance. The factor that most influences the characteristics and implications of surface drainage as a waste component is the acid generation potential of the minerals exposed in the course of mining activities. Acid production is normally associated with the oxidation of sulfide minerals and particularly pyrite (FeS2)and pyrrhotite (Fe,-,S,) according to the following simplified overall reaction (for pyrite): 2 FeS2
+ 2H20 -k 702
-
2FeS04 4- 2H2S04
This reaction results in a toxic drainage of low pH containing high concentrations of leached metals that must be segregated, collected, and treated. Many base metal mines in Canada encounter iron sulfide concentrations in excess of 50% and the control of surface drainage at these mines is a major concern and expense. A 3000 tpd operation in eastern Canada was recently re-
32 43 16 7
6
quired to install drainage controls costing $2.7 million in order to reduce the discharge of acid metal-bearing drainage into a salmon stream. The improvement in the water quality in the stream because of the reduced metal toxicity is dramatically shown in Figure 2. In areas where there is no tendency for acid generation, such as the porphyry deposits in western Canada, surface drainage problems are considerably fewer and limited almost entirely to physical contamination such as the erosion of embankments, waste dumps, and stockpiles together with particulate contamination from ore and concentrate handling areas. These problems are relatively easily controlled at source. A first consideration in the design of surface drainage facilities is to determine whether any of the rock types that will be exposed or disturbed in the course of operations have an acid generation potential. A standard test procedure to determine this potential from small samples, such as exploration drilling cores, has recently been developed under contract for the Canada Department of the Environment and is coming into general use. Process wastes Mill process wastes are usually alkaline and, in addition to their high slurried solids level, commonly contain dissolved metal ions, residuals of process reagents added in the mill, Volume 10, Number 2, February 1976
131
and a high level of dissolved solids. In most cases, these wastes are discharged to a tailings impoundment area where the solids are retained in perpetuity, and the supernatant is treated prior to discharge or recycle to the mill. The important waste management consideration of these wastes is the treatability of the effluent and the degree of which the final effluent can be reduced by recycling back to the mill. In recent years, the degree of recycle practiced by Canadian base metal mills has increased dramatically. The use of recycled water to provide 60% of the process requirements or more is not uncommon. The volume of process water discharged from Canadian base metal mills averages 4 15 gpml1000 tpd capacity with a range of 150 to 1040 gpm/1000 tpd. These values represent the volume discharged to the tailings pond before recycle. Constituents of mill process effluents presently receiving attention in Canada are the partially oxidized components of sulphur, such as thiosulphates and polythionates. These are sometimes formed during the grinding and flotation of sulfide ores under alkaline conditions although the precise mechanism of their formation is not yet fully understood. Unless these compounds are stabilized in the treatment system, they are oxidized biochemically in the receiving stream, thus generating acid sulfate conditions many miles from the point of discharge. pH depression in the range of 2.5-3.5 was produced in one receiving stream 13 miles distant from the point of mine effluent discharge. The effluent had a pH of about 9 at the point of discharge but contained thiosalt concentrations in the order of 1000 mg/l. Cyanide is used as a flotation reagent in at least 16 base metal mines in Canada, not including cyanidation circuits for gold extraction. Where feasible, its use is generally discouraged in favor of alternate reagents because of the toxic properties of residuals and complexes that may not be biochemically stabilized in the treatment system under winter conditions. Similarly, the use of ammonia as an alkaline reagent is generally discouraged because of the environmental implications of residuals in the effluent which, again, may not be broken down biochemically under winter conditions. Aqueous waste treatment By far the most common means of treating base metal mining wastes in Canada, as elsewhere, is to discharge them into a tailings pond in which the pH is controlled. In this way, the heavy metal ions in the waste are precipitated as hydroxides, and are settled out with other suspended solids to be re132
Environmental Science & Technology
tained in perpetuity. Consistently high effluent qualities can be achieved in many instances by this relatively simple method of treatment i f a high degree of control is exercised over pH that must normally be maintained in the range of 9.5-10.5 to precipitate copper, lead, zinc, and nickel. Good sedimentation conditions must also be provided for the removal of these precipitates and other suspended solids from the decant. A minimum retention of five days and a pond of 10-25 acres for each 1000 tons of tailings solids discharged per day is recommended by the Canadian Mines Branch (Technical Bulletin TB145, 1972), but pond geometry and factors such as surface skimming have a greater influence over sedimentation performance than theoretical retention. The previously mentioned Northeastern New Brunswick Mine Water Quality Program determined that well-controlled tailings pond systems could consistently achieve the following effluent metal levels: copper 0.03 mg/l (extractable); zinc 0.15 mg/l (extractable); lead 0.1 mg/l (extractable); total iron 1 .O m,g/l (extractable). In some instances it is either necessary or desirable to treat acid metal-bearing drainage separately from the mill tailings treatment system in situations such as exploration workings in sulphide ores or operational mines remote from the mill and its tailings system. In recent years, several Canadian mines have installed, or are presently in the process of installing, mechanized systems to provide treatment under such circumstances. A project termed the “Mine Wastewater Pilot Treatment Project” evaluated the effectiveness of this method of treatment on a 5-15 (US) gpm plant. The flowsheet for this plant is shown in Figure 3. These systems have the advantage of enabling sludge recycle to be used that produces underflow sludge densities in the order of 20-35 wt % relative to the low density 1-2 wt % thixotropic sludge otherwise resulting from lime neutralization of most acid mine drainages. The emphasis of these systems in Canada is very much on the removal of heavy metals to levels consistent with the national minimum requirements (see Table 2). The pilot treatment project concluded that the following levels were attainable and could be further reduced by polishing methods such as quiescent settling: copper, 0.05 mg/l (extractable) lead, 0.25 mg/l (extractable); zinc, 0.37 mg/l (extractable); and iron, 0.28 mg/l (extractable). It was previously mentioned that thiosalt-bearing wastes are presently an area of concern in Canada. Only one mine is known to provide a specific treatment system for the reduction of thiosalts in its effluent. In this case a biostabilization lagoon of 30 days nominal retention is provided. Under summer conditions greater than 80 % thiosulphate oxidation to sulfate is achieved, but in winter, ice cover and the slower rate of biooxidation results in only minor removals. As an integral part of the pilot treatment project rotary biological disc reactors were demonstrated as being technically feasible for the stabilization of thiosalt-bearing wastes as was chemical oxidation with ozone. However, the economic impact of employing such methods under full-scale conditions has yet to be evaluated.
Solid wastes Large volumes of solid wastes and particularly of tailings are an integral part of any base metal mining operation. Tailings are dealt with by one of the following means: impoundment on land disposal in marine waters disposal in deep waters use as backfill in mining operations. The first method is by far the largest category in Canada and causes few problems where the tailings are inert; where proper engineering design and construction methods are used for the retaining structures; and where provision is made to rehabilitate the area as it falls from active use. Some of the more recent mines in western Canada involve
New Brunswick stream la Mine dewatertrg p+ c
u1 C
1
1
14 Closed-minie
Y
0
m a
IO
1961
1963
196'1
1967
19b9
1971
1973
=dr
capacities in excess of 20 000 tpd and thus accumulate huge volumes of tailings. At the Brenda mine (a copper molybdenum property in British Columbia), for example, storage is required for 24 000 tons of tailings per day over a 20-yr period. At this property the tailings are cycloned into sand and slime fractions, and the sand is used to construct a free draining dam between upstream and downstream toe dams. The final structure will ultimately reach a height of almost 400 feet. Seepage in the dam is controlled by a series of finger drains and filter blankets and is collected and recycled from a downstream impoundment. The need to apply accepted engineering design and construction principles to structures of the magnitude used at Brenda is apparent. Considerable emphasis is now being placed on the use of geotechnical specialists to design and supervise the construction of retaining structures of all sizes at the majority of new mines. Similarly, the need to engineer tailings disposal facilities to provide the best feasible treatment and waste control regime is also being increasingly
recognized. The structural aspects of tailings dam design are reviewed by Klohn 1973 and the waste management aspects by Bell 1974. The use of impoundments for tailings disposal becomes less attractive, although often the only alternative, when the tailings contain significant concentrations of pyrite or pyrrhotite that lead to acid generation and leaching of metal values. This results in a long-term contaminated seepage problem and renders the large surface area of tailings all but impossible to revegetate unless a complete soil cover is placed over the area and precautions taken against the upward migration of acid into this layer: an expensive approach. It has been suggested that in some instances it may be advantageous to float off pyrite in tailings so as to have a smaller volume of acid-generating material separate from the bulk of inert tailings. It has not been found economically feasible in Canada to process tailings for secondary values as is the case in some European and Japanese situations: but in the long term this Volume 10, Number 2, February 1976
133
approach may UrrVlllr , , m e attractive because of the combined effect of rising by-product values and the long-term implications of environmental controis. Future markets for iron and sulphur will largely influence such development. nenabilitation The term rehabilitation is collectively usea to describe the rumwing activities as a mine ceases active operation: revegetation of tailings areas continued control of contamination from mine pits and underground workings continued control of contaminated surface runoff prevention or control of mining subsidence improvement to the general esthetics and redevelopment of the area. In many instances the emphasis is placed on the first of these categories because of problems associated with windblown particulate material and contaminated seepage from tailings disposal areas. At mines where the tailings contain significant quantities of pyrite or pyrrhotite difficulty has been experienced in maintaining "soil" pH values conducive to plant growth or the maintenance of a proper mineral balance
Table 1
Incipient lethal levels for Zn, Cu, and Pb t o salmonid fish Total
hardness. hardness, mgll as CaCO,
5.0 7.5 i 10.0 I 11.0 12.0 13.0 i 14.0 15.0 16.0 17.0 I 18.0 i 19.0 i 20.0 21.0 I 22.0 23.0 b 24.0 I 25.0 ' 26.0 27.0 , 28.0 29.0 b 30.0 31.0 32.0 ; 33.0 34.0 i 35.0 36.0 37.0 ' 38.0 39.0 40.0 41.0 42.0 43.0 44.0 45.0 : 46.0 , 47.0 ~
' ~
'
Incipient l n c l p m t lethal level
Total
Ph pb
hardness.
zn zn
382 439 488 508 528 548 568 587 607 627 646 666 686 706 725 745 756 764 804 824 844 863 883 903 923 942 962 982 1001 1021 1041 1061 1081 1101 1120 1140 1160 1180 1199 1209
cu cu
18 23 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70 72 74 76 78 80 82 84 86 88 90 92 94 96 98 100 102
575 719 764 782 800 818 836 854 87 1 889 907 925 943 961 979 997 1015 1033 1050 1068 1076 1086 1096 1106 1116 1127 1137 1147 1157 1167 1178 1188 1198 1208 1218 1229 1239 1249 1259 1269
mgll as CaCO,
48 49 50 51 52 53 54 55 60 65 70 75 80 85 90 95 100 105 110 115 120 125 130 135 140 145 150 160 170 180 190 200 225 250 275 300
Table 2
Mining regulations in Canada
British Columbia, new mines
effluent
50
2500
6.5-8.5
Yukon Territory
effluent
30
-
6.5-9.0
Ontario
effluent
15
as low as pos.
5.5-10.6
receiving stream
-
-
New Brunswick (recommended interim levels for salmon areas)
effluent
30
Canada EPSa Draft. min. requirements
effluent
25
-
6.0 (min.)
U.S. EPA
effluent
20
-
6.0-9.0
8.0
h w , hard water; I W . soft water. a Environmental Protection Service of Environment Canada
Incipient lethal level lnclplent
7" zn
1220 1230 1240 1250 1260 1270 1281 1291 1342 1393 1444 1495 1546 1597 1648 1699 1750 1801 1852 1904 1955 2006 2056 2107 2158 2209 2260 2311 2464 2566 2668 2770 3025 3280 3535 3790
r,, cu
104 106 108 110 112 114 116 118 126 134 141 149 157 165 172 180 188 196 204 212 220 228 236 244 251 258 265 281 297 313 329 345 383 422 461 500
Ph Pb
1280 1290 1300 1310 1320 1331 1341 1351 1402 1453 1504 1555 1606 1657 1708 1759 1810 1861 1912 1963 2014 2065 2116 2167 2218 2269 2320 2422 2524 2626 2728 2830 3085 3340 3595 3850
even with massive doses of lime or limestone, up to 40 tons/ acre in some cases. Furthermore, the uptake of toxic constituents in the vegetation is a source of concern where the vegetation might be harvested or used as forage by wildlife. Solutions to these problems are being actively sought by several mining companies and government research groups in the Sudbury and Eiiiot Lake areas of Ontario, in Noranda, Quebec, and elsewhere. Similarly the problem of stabilizing the drainage accumulating in open pit mines or seeping from underground workings once they are abandoned is the cause of concern at several properties in Canada. From the regulatory standpoint several provinces now require the posting of a rehabilitation bond as a condition of obtaining a permit either to mine or to withdraw and discharge water. The responsibility for rehabilitating a site or of main-
Treatment plant. INCO's GO-mgd plant at Copper Cliff Creek handles wastewaters for metals removal
Parameter, a l l concentrations as mgll CN
NH,
0.10
0.50
0.05
-
As
Ca
Cr
cu
Fe
Pb
Hg
Zn
Mi
Toxicity requirement
0.05
0.001
0.30
0.50
Min. 10%survival in 100% effluent for 96 h exposure
0.05 0.02 0.05 5.0 0.20 metals expressed as extractable except As
0.005
0.8
0.2
Effluents shall be non-toxic
0.05 0.005 0.05 0.05 0.30 metals expressed as dissolved fractions
-
Total concentration of any individual heavy metal not t o exceed 1 mg/l and the cumulative concn of Cu, Pb, Zn, & Ni not to exceed 1 mg/l Cd & Hg not t o exceed stream background levels 0.01 1.5 0.05 0.5 0.07hw 0.3 O.lhw l.Ohw 0.2hw 0.03s~ 0.05s~ 0.4sw 0.02s~ metals expressed as dissolved fraction
-
-
-
-
0.05 0.04 0.03 1.0 metals expressed as extractable
0.10
-
-
-
0.5
-
0.2
-
-
0.3
-
0.5
0.15
Receiving stream 0.1 TU
0.5
Min. 50% survival in 100% effluent for 96 h exposure
metals expressed as extractable, arithmetic mean monthly concentration
-
-
-
-
-
0.05
-
0.20
taining treatment once active operations have ceased is therefore clearly made that of the developer and the costs of such measures are therefore internalized into the cost of production. Jurisdiction Basically, the British North America Act of 1867 delegated control and development of natural resources to the provinces that are therefore responsible for the regulation of their respective mineral industries. In addition, all provinces have a ministry or department responsible for environmental control and many of these provincial bodies have recently developed, or are in the process of developing environmental regulations for mining operations in their jurisdiction. The specific requirements and even the approach to regulation differ somewhat from province to province, depending on regional needs and philosophy. Some presently developed requirements applicable to the industry in Canada are summarized in Table 2. In order to maintain some uniformity across the country, and to provide requirements for mines in areas under Federal jurisdiction, the Environmental Protection Service, a branch of Environment Canada, has recently developed minimum base-line effluent requirements for all mining developments in Canada. More stringent requirements can be applied in an environmentally sensitive area as necessary by either the Federal or provincial agency in question. The Federal Government is jurisdictionally able to apply such requirements under the auspices of the Canada Fisheries Act, which gives it the authority to control deleterious substances discharged into waters supporting fish. The Federal Government also has the jurisdiction over international boundary waters and waters of the Northern Territories. The federal requirement shown in Table 2 are presently of draft status but will soon be issued as regulations for new mines and as guidelines for existing mines. It is interesting to note that the philosophy of the Fisheries Act is one of minimizing the discharge of wastes, and that emphasis is placed on effluent concentrations attainable by the use of "best practicable technology." The total or unit quantity of metal discharged is not directly regulated because of the difficulty of expressing unit waste volumes for the base
-
-
0.20 mill 0.50 mine
metal mining industry. However, the assimilative capacity of the receiving stream is not taken into account and the "optimization approach" of utilizing the stream for waste assimilation is not endorsed by the spirit of the Act. The discharge requirements imposed on most new mining developments are relatively stringent and dictate the use of well planned and operated waste management measures. In the case of most new mines, the cost of such requirements is significant but not inhibitive, as the opportunity exists to minimize cost at the planning stage. However, for existing operations that were developed before the formulation of recent requirements, the cost implications are far more severe, although in most cases allowance is made by the application of guidelines rather than regulations and the negotiation of compliance schedules by most regulatory agencies. Additional reading Bell, A. V., and Nancarrow, D. R., "Salmon and Mining in Northeastern New Brunswick," CIM Bulletin Nov. 1974. Bell, A. V., Phinney, K. D., and Behie, S. W., "Some Present Experiences in the Treatment of Acidic Metal Bearing Mine Drainages" CIM Bulletin Dec. 1975. Brown, V. M., "The Calculation of the Acute Toxicity of Mixtures of Poisons to Rainbow Trout", Water Res., 2, 723-733 (1968). Joe, E. G., and Pickett, D. E., "Water Reuse in Canadian Ore Concentration Plants" Proc. Minerals and the Environment Symposium IMM, London, 1974. Schmidt, J. W., and Conn, K., "Abatement of Pollution from Mine Waste Waters" Can. Mining J., p 54, June 1969. This feature is based in part on a presentation at an International Symposium on Minerals and the Environment (London, 1974), which was sponsored by the Institution of Mining and Metallurgy. Alan V. Bell is assistant manager of the Water Resources and Environmental Division of Montreal Engineering Company Ltd. In this capacity, he has been involved in the evaluation, development, and design of base metal mining waste systems across Canada in recent years in addition to other industrial waste management and environmental projects. Volume 10, Number 2, February 1976 135
