The prevalence of acid mine drainage (AMD) with coal mining has been nicely documented. Much less documentation is on the market of its affiliation with different varieties of mining, e.g., copper, gold, zinc, and sulfur. Yet there are various places in the western states where acid mine drainage is as nice a pollution drawback as in Appalachia and in some instances, as a result of of the excessive copper, zinc, and arsenic concentrations, they present an much more troublesome drawback.
Till a number of years ago, it was usually believed that the neutralization of acid mine drainage was uneconomical thus, AMD was allowed to discharge freely into our streams. Nevertheless, the demand for good water finally dictated that the mining business be included underneath air pollution legal guidelines. In 1964 the Commonwealth of Pennsylvania passed a regulation requiring that each one lively mine discharges meet a discharge normal of pH 6-9, iron lower than 7 mg/l, and the water should have a internet alkalinity. A number of different states now require lively mines to treat AMD.
In all however a couple of instances, lime neutralization, typically at the side of aeration, has been the remedy technique used. The high value of lime as compared to limestone and the poor quality sludge, (sluggish settling, giant volumes, and low solids content material) has stimulated work within the utilization of limestone. This paper stories on the current state-of-the-art of limestone remedy of AMD.
The neutralization process supplies the next advantages:
- Removes the acidity and provides alkalinity.
- Will increase pH.
- Removes heavy metals. The solubility of heavy metals depends on pH; that is, up to some extent, the upper the pH, the decrease the solubility, (see Determine 1).
- Ferrous iron, which is usually related to AMD, oxidizes at a quicker price at greater pH’s (Figure 2).
- Sulfate may be removed if enough calcium ion is added to exceed the solubility of calcium sulfate, nevertheless, solely in extremely acidic AMD does this happen.
Some shortcomings of the neutralization process are:
- Hardness just isn’t lowered and could also be increased.
- Sulfate just isn’t decreased to a low degree.
- The iron focus cannot be lowered to lower than Three-7 mg/l. A waste sludge is produced that have to be disposed of.
Chemical Properties of Limestone
The term limestone is a basic one embracing carbonate rocks or fossils, composed primarily of calcium carbonate or mixtures of calcium and magnesium carbonate with varying quantities of impurities, the most typical of which are silica and alumina. Since limestone does not have a continuing chemical composition, it is very important know what characteristics are needed for a great neutralizing agent.
Most limestones are rated by the producer as to their calcium carbonate or calcium carbonate equivalent content material. The higher the CaCO3 content material, the larger the alkalinity obtainable and the less the impurities. In evaluating lime and limestone, it ought to be famous that a pound of lime has 1.35 occasions the alkalinity of a pound of limestone.
Several investigators have reported that limestones that include magnesium carbonate in considerable portions react very slowly. Hoak, et al reported that dolomitic limestone’s price of response was roughly inversely proportional to the amount of magnesium carbonate it contained (above about two %). Ford carried out studies with 14 limestones of numerous compositions by treating both synthetic and actual mine drainage and located that normally the neutralizing efficiency of a stone elevated with larger percentages of CaO and decrease percentages of MgO, thus, the calcites, CaCO3 have been simpler than dolomites or magnesites.
The silica (SiO2) found in most limestones is mainly an impurity and thus, its presence in giant portions signifies that much less alkalinity per pound is on the market.
Bodily Properties of Limestone
Along with the chemical properties of the limestone, the geological history of the stone and its crystal construction in all probability plans some position in its neutralization capability. Crystal construction has some bearing on the surface space of the stone particle. Ford has shown that a development exists between surface space (BET) and neutralization price. Nevertheless, the correlation does not have a excessive confidence degree. Half of the shortage of correlation may be because of the use of BET surface space measurements, which measures each the outer and internal particle surface areas. Because the CaCO3 should dissolve and then diffuse away from the particle, the diffusion inside the small interval channels can be insignificant to that of the outer surface area. Thus, a correlation of simply the outer surface space to neutralization price may be more valid. Several investigators have proven that the reaction fee is a perform of the fineness of the stone. Ford obtained the info introduced in Determine Three. The limit on the fineness of the stone is an economic one. Value of grinding increases at an accelerating fee because the particle measurement decreases. The most cost effective small particle measurement material in mining areas is “rock dust” of which 60 to 70 % passes a 200 mesh. To obtain a smaller measurement, a price in extra of that paid for lime could also be required.
As discussed above, the next elements must be thought-about within the selection of a limestone:
- High calcium carbonate content.
- Low magnesium content.
- Low quantity of impurities.
- Giant floor measurement.
- Smallest particle measurement inside financial bounds.
Stone evaluation alone has not been proven to be totally reliable for choosing a limestone. After a preliminary screening of the proposed stones by their chemical analysis, a simple laboratory check is really helpful. Twice the stoichiometric quantity of limestone of the dimensions to be used is added to a pattern of AMD. The sample is combined by introducing air. The pH is recorded with time for five hours. A pH – time plot is used to guage the limestone. In Determine four, plots of knowledge acquired by Ford on this manner are proven. Ford means that an efficient limestone will probably be situated in Area A.
In addition to the response price, the traits of the sludge also needs to be thought-about. Three traits of the sludge are essential, i.e., settling fee, sludge quantity, and sludge solids content. To perform these exams a pattern of the unsettled neutralized AMD is positioned in a 1,000 ml graduated cylinder and the depth of the sludge blanket decided periodically for two to 12 hours. These knowledge are then plotted as in Figure 5. The final reading is considered the sludge quantity, often expressed as a % of the whole pattern. The supernatent ought to then be drained off. The sludge is then dried and the % solids is calculated.
A great limestone should have a excessive neutralizing fee, fast settling sludge, volume of sludge, and a sludge with a excessive solids content material.
Limestone Treatment Techniques
The sort of limestone remedy system used depends upon the quantity and ionic state of the iron in the AMD. Three teams of AMD are thought-about here (1) low iron AMD, (2) ferric iron AMD, and (Three) ferrous iron AMD.
Low Iron AMD
Low iron AMD is the simplest to treat. Limestone beds (Determine 8) and pulverized limestone (Determine 9) may be effectively used. This sort of AMD often has a low acidity and subsequently, coating the stone with calcium sulfate just isn’t an issue. Both upflow and downflow stationary limestone beds may be utilized. The stone ought to be as small as attainable, however not so small that the bed compacts and seals itself. Jacobs reported that a limestone bed 5 ft deep and a move price of zero.5 to 1 gpm/sq. ft. successfully treated a 50,000 mg/l acidity answer. For extra dilute options, larger circulate charges and shallower beds can be utilized. Laboratory measurement check limestone beds for determining the optimum circulate price and mattress depth for any specific AMD and limestone are advisable.
Fluidized beds are superior to stationary beds in several methods. A better stream fee may be maintained, the agitation between particles removes deposits which may build up, and carbon dioxide that typically binds to the stone floor is flushed from the mattress. Reidl had good success with an expanded mattress 18-24 inches deep and a movement fee of 40 to 50 gpm per sq. ft. Gehm carried out some moderately detailed research on expanded limestone beds and reported the next:
- Comparable results have been obtained for calcite and amorphorus stone.
- A relationship exists between bed depth and price of software to acquire comparable effluent high quality. For a waste having an acidity of four,240 ppm, the bed might have an software fee of 40 gpm per sq. ft. per foot of depth. A two foot deep mattress is the minimum prompt.
- A linear relationship exists between price of software and acidity of waste (see Determine 6).
- The higher limit for acidity of water is 5,000 ppm, and
- stone measurement must be between 10 and 20 mesh.
Jarrett and Kountz and Yen have shown on the laboratory scale that the development of a limestone barrier within the AMD stream can effectively treat low iron AMD. Ten developed design standards. The Pennsylvania Division of Mines and Mineral Industries plans to assemble a full scale set up of this sort.
Low iron AMD might be handled by the direct software of pulverized limestone both as a strong or a slurry. Limestone tumblers may also be used. Each of these methods is mentioned later.
Apart from instream limestone limitations, the limestone techniques are composed of the response mattress, tank, or tumbler, followed by a sedimentation tank where the sludge is separated from the handled water.
Ferric Iron AMD
Stationary limestone beds don’t look like applicable for ferric iron AMD. As a result of of the sluggish software fee, coating of the stone with precipitated iron is certain to occur. Braley and others have reported of this prevalence.
Expanded beds might have some purposes, notably if the system is made to include an attrition gadget to take away the iron coatings. Glover found that passing the positive limestone bed by means of a pump resulted in the impeller removing the iron coat. Later he patented a mechanical attrition system in the type of a rapidly rotating impeller that operated inside the expanded mattress.
Limestone beds placed in a stream have also been found to shortly coat with iron, leaving them ineffective. Nevertheless for short-term remedy, comparable to throughout a surface mining operation and draining of an AMD lake, this technique may be relevant.
Tumblers have been proven to be efficient on ferric iron AMD. Calhoun reported of a full scale plant by which a rotating drum, 30 ft lengthy and 3 ft in diameter, containing 5,000 pounds of three by one inch limestone, and rotating at 15 rpm, was used to treat an AMD having 100 to 500 mg/l acidity, 20 to 120 mg/l iron (90 % ferric) and a stream of 50 to 300 gpm. The water flowed from the drum to a settling basin. Extra limestone within the amount of 1.42 occasions the stoichimetric was required.
The appliance of pulverised limestone both within the dry or slurry type has been shown to be effective. Wilmoth and Scott have carried out a quantity of pilot plant studies on the remedy of ferric iron AMD. In Table I, a typical set of knowledge is introduced.
It is of interest to notice that by sustaining a pH four.3, all of the acidity is removed and the iron and aluminum concentration is close to minimal. Treatment to larger pH by limestone serves little or no function. To boost the pH above this degree is troublesome as a result of no acidity is out there to assault the limestone and the CO2 being released buffers the reaction. Thus, giant excesses of limestone, which are principally misplaced to the sludge, have to be utilized. Wilmoth and Scott discovered once they raised the pH to six.6 that larger than 3 times the stoichiometric limestone was required. Later research of theirs showed that it required 1.5 occasions as a lot limestone to deal with to pH 6.1 and two occasions as a lot limestone to deal with to pH 7.zero as to pH 5.1, with no vital improve in effluent water quality. The corresponding limestone charges have been 2.55 occasions stoichemetric at pH 7.zero, 2.25 occasions at pH 6.1, and 1.52 at pH 5.1. By growing the reaction time from 7 to 20 minutes the limestone necessities have been lowered to 1.29 stoichemetric. At this requirement, the fabric value of limestone remedy was 0.0033, cents per mg/l acidity per 1,000 gallons or 23 % less than lime.
A minimum response time of 20-30 minutes is advisable with vigorous mixing. Sludge return with and without attrition can also be a risk for decreased limestone utilization.
When the iron is within the ferric state, speedy settling ff the sludge may be expected (Figure 5) and a comparatively small quantity of sludge (3-5 %) with high solids content (9.5 %) is produced. The treating of AMD to a lower pH additionally helps the sludge settling price. Nevertheless, effluent turbidity has been shown to increase at decrease pH’s.
Because of the slowness of the limestone response and carbon dioxide buffering, pH management of the limestone feed may be troublesome. The pH within the reactor is as a lot as a unit lower than that of the settling tank effluent. Nevertheless, over remedy is just not a problem as it is with lime.
Ferrous Iron AMD
Probably the most troublesome AMD to treat with limestone is that with high ferrous iron. A number of investigators have reported that the mineral acidity and ferric iron in ferrous iron AMD could be easily removed, nevertheless, the ferrous iron and the acid released upon its oxidation and/or hydrolysis are troublesome to take away.
When limestone is reacted with ferrous iron AMD, the mineral acidity is neutralized, the pH increased (usually not larger than 6.5) and the ferric iron made insoluble. Nevertheless, the oxidation of ferrous iron at low pH’s could be very sluggish (Determine 2) and ferrous iron cannot be precipitated at low pH’s, thus, little ferrous iron is removed. If the neutralization step is adopted by aeration to oxidize and hydrolyze the ferrous iron, a lower in pH happens because of the hydrogen liberated. Thus, an excess of limestone have to be added to the AMD. Holland, et al, reported that the larger the excess the quicker the ferrous iron oxidation, Figure 7 (in all probability partially because of the larger pH attained with larger quantities of limestone).
In view of the excess limestone that have to be applied, the use of stationary, expanded, and in-stream beds for top ferrous iron AMD doesn’t appear practical. A glass and direct feed of pulverized dry or slurried limestone look like the one applicable techniques. Mihok, et al, demonstrates with comparatively low ferrous iron content AMD (20-140 mg/l that a tumbler could possibly be used to supply a limestone slurry with very small particle measurement (90 % less than 400 mesh) and successfully added to the AMD. The combined AMD and limestone slurry have been partially reacted in an aeration basin. Holland, et al, has mentioned the problems of pulverized limestone remedy of high ferrous iron (400-500 mg/l).
Preoxidation of the ferrous iron earlier than limestone neutralization is one technique of fixing the problem. Working towards this technique is the sluggish oxidation of ferrous iron at low pH’s. Nevertheless, at these low pH’s, the iron oxidizing micro organism are in an optimum setting they usually speed up the reaction. In fact micro organism metabolism is temperature dependent and tremendously decreased at low temperatures. A number of investigators have shown the feasibility of biological oxidation of ferrous iron. Others have reported on the ferrous iron discount when the raw AMD was held in a holding pond for several hours previous to neutralization. The ferrous iron may be chemically oxidized using such agents as ozone and chlorine.
In abstract, it can be concluded that the limestone remedy of high ferrous iron AMD is just not practical. The upper limit of the ferrous iron being in the range of 50-100 mg/l, as a result of of the extreme pre or submit aeration time required at greater concentrations.
Limestone – Lime Treatment
The cut up remedy of AMD with limestone and lime might supply some benefits in value and improved sludge traits. It may additionally be used on ferrous iron AMD. A two-step process is required. First, the AMD is handled with limestone to a pH of four.zero to four.5 to take benefit of the pH vary when limestone is only. The water then passes to a second reactor the place lime is applied to boost the pH range to the desired degree. This process might have a price benefit over lime alone and the desired sludge traits of the limestone course of. Holland, et al, discovered that with the right mixture of limestone and lime that a good effluent might be obtained even with ferrous AMD.
- All of the limestone processes described are applicable to the low iron acid mine drainage state of affairs.
- Expanded beds, tumblers and pulverized limestone techniques are relevant to ferric iron acid mine drainage conditions.
- Above a ferrous iron concentration- of 50-100 mg/l, limestone remedy does not appear applicable. Under this vary, tumblers and pulverized methods with aeration are feasible.
- A limestone with a high CaCO3 content material and with low magnesium and different impurities is greatest fitted to treating AMD.
- The choice of a limestone ought to be based mostly on the chemical composition, value, and a laboratory check between the actual limestone and acid mine drainage.
- For pulverized limestone methods, the smaller the limestone particle measurement, the quicker the reaction and the higher the utilization of the limestone.
- An excess of 1.four to 3 occasions the stoichiometric quantity of limestone can be required.
- A limestone – lime system may be most applicable in some situations.