Abstract:
An improved method that requires no electricity or moving parts to increase lime utilization where pebble quicklime (CaO) and/or hydrated lime (Ca(OH) 2 ) is used for mine drainage treatment. Lime utilization at such facilities has been historically poor due to the low solubility, high density, and large particle size of pebble quicklime. This invention takes two passive technologies, a diversion well-inspired MixWell system followed by a TROMPE-driven, air lift mixer for enhancing lime dissolution. It showed an estimated 40 to 57 percent reduction in lime usage.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a perfection of U.S. Provisional Ser. No. 61/609,670, filed on Mar. 12, 2012, the disclosure of which is fully incorporated by reference herein. 
     FIELD OF THE INVENTION 
     This invention relates to water treatment facilities and, specifically, to mine drainage systems. Particularly, it relates to systems that use pebble quicklime (CaO) and/or hydrated lime (Ca(OH) 2 ) as feedstock. This invention improves the overall efficiencies of such systems by providing a passive air source that can function without the need to supply any outside electricity. 
     BACKGROUND OF THE INVENTION 
     Pebble Quicklime 
     Pebble Quicklime is derived from the heating of limestone to convert the calcium or magnesium carbonate into the respective oxide. Depending on the limestone source there are varying ratios of Calcium and Magnesium oxide. In addition to the Calcium/Magnesium oxide, there is an inert component that does not contribute to acid neutralization. Pure Pebble quicklime has a neutralization equivalent of 0.56 tons per ton of acidity compared to hydrated lime with a neutralization equivalent of 0.74 tons per ton of acidity. In addition, its higher density 55 to 60 pounds per cubic foot vs. 30 to 40 pounds per cubic foot allows for more alkalinity to be delivered on a per truck basis and it also allows for a smaller product silo. 
     Water powered mixers or “dosers” have been deployed to deliver pebble quicklime at sites where electricity is not readily available. These water-powered systems are sometimes referred to as semi-active. The solubility of pebble quicklime is somewhat limited, 1.33 g/L while hydrated lime is somewhat more soluble at 1.76 g/L. Before it can dissolve in water, pebble quicklime has to be converted from Calcium oxide to calcium hydroxide, a process called slaking. 
     The slaking equation is:
 
CaO+H 2 O→Ca(OH) 2   (1)
 
It is an exothermic reaction commonly confined to a slaker where elevated temperatures promote the slaking process. Limited quantities of water are used to keep temperatures elevated. If excessive amounts of water are used, the lime is said to be “drowned” and the slaking reaction is inhibited with a coating of calcium hydroxide on the particle surface to restrict water penetration. It also restricts the reaction. This is the typical state of reaction at most semi-active treatment sites. To overcome this limitation, pebble quicklime is allowed to deposit in long channels where dissolution can proceed slowly. This can lead to low lime utilization at the treatment site due to un-dissolved lime buried in the channel or lime that has absorbed carbon dioxide from the air and converted back to calcite.
 
Manor
 
     The Manor mine is located in North Central Pennsylvania 12 miles northeast of the town of Clearfield. The mining took place in the Lower Kittanning seam in an up-dip direction. The mine discharge emanates from a wet seal installed about 2004. Under a consent agreement, Pennsylvania Department of Environmental Protection (PADEP) has been managing the operations a treatment plant at Manor since 2004. Prior to the recent modification, treatment consisted of calcium oxide addition using a water wheel (AquaFix) to regulate dosing. A mixing channel 300 feet in length with a vertical drop of about 20 feet was provided to allow for mixing and dissolution of the pebble quicklime. The treated water was allowed to cascade for aeration and then was allowed to settle in three baffled ponds before discharging. As a result of this process, significant quantities of lime were deposited in the mixing channel and in the first settling pond. This reduced the storage capacity of the settling pond and resulted in the disposal of large amounts of unused lime along with the cost of lime sludge removal. 
     SUMMARY OF THE INVENTION 
     Due to difficulties with the aforementioned plant operation, the inventors were asked to redesign the plant with a view to improving lime utilization and overall system operation. Two new concepts for lime mixing were designed and installed at the Manor facility. 
     The first device, called a MixWell, is an improvement on the known diversion well concept. Raw water is directed via a smaller diameter pipe into a larger diameter vertical pipe where it is discharged at the bottom. Pebble quicklime is directed into the MixWell where the dense calcium oxide particles descend to the bottom. There they are agitated by incoming raw water. Either abrasion, exposure to low pH water or a combination of both enhances the breakdown of the large particles for improving lime utilization efficiency. Unlike a diversion well, there is not a thick bed of material to be suspended with the present invention. Furthermore, the lime is added to the MixWell on a continuous basis rather than using the typical batch delivery for known diversion wells. 
     The second device, called an “A-Mixer”, is derived from an airlift mixer. It consists of a large tank to provide residence time. In the center of that tank is a vertical pipe suspended from the tank bottom and rising to just below the normal water level in that tank. An air pipe, with an air distributor (diffuser), is suspended in the middle the vertical pipe and is connected to a source of compressed air. In this case, a Trompe located below the treatment plant discharge was installed to provide the compressed air. Air is bubbled up through the vertical pipe inducing water flow through the pipe. This causes a convective-like circulation in the tank that keeps small lime particles suspended and available for dissolution. In addition, a perforated pipe located at the bottom of the tank and plumbed to the bottom of the airlift mixer provides water circulation through any settled lime particles. 
     This invention addressed the effectiveness of these two devices in terms of individual unit operation and the combined effect on lime utilization. 
    
    
     
       BRIEF SUMMARY OF THE DRAWINGS 
       Further features, objectives and advantages of this invention will become clearer when reviewing the following Description of Preferred Embodiments, made with reference to the accompanying drawings in which: 
         FIG. 1  is a schematic diagram of a whole system according to one embodiment of this invention; 
         FIG. 2  is a close up schematic of the MixWell component from  FIG. 1 ; 
         FIG. 3  is a close up schematic of the A-Mixer component from  FIG. 1 ; 
         FIG. 3A  is a close up schematic of an alternate embodiment of A-Mixer component according to this invention; 
         FIG. 3B  is a close up of the circled region in  FIG. 3A ; 
         FIG. 4  is a close up schematic of the condensate trap component from  FIG. 1 ; 
         FIG. 5  is a graph showing the pebble quicklime particle size distribution observed with one example of the present invention; 
         FIG. 6  is a graph showing the MixWell discharge particle size distribution observed; 
         FIG. 7  is a graph showing the A-Mixer discharge particle size distribution observed; 
         FIG. 8  is a graph comparing pH levels through stages of the invention during various trials at one test location (i.e. the Manor site); 
         FIG. 9  is a graph comparing Dissolved Oxygen levels through stages during various trials at the Manor site; 
         FIG. 10  is a graph comparing field alkalinity levels through stages during various trials at the Manor site; 
         FIG. 11  is a graph comparing Dissolved Iron levels through stages during various trials at the Manor site; 
         FIG. 12  is a graph comparing Dissolved Calcium and Sulfate levels through stages during various trials at the Manor site; 
         FIG. 13  is a graph comparing pH to Dissolved Iron levels through stages during various trials at the Manor site; 
         FIG. 14  is a graph comparing Lime Utilization percentages based on initial acidity and stoichiometric calcium levels; and 
         FIG. 15  is a graph comparing Lime Utilization levels before and after implementation of the invention at the Manor site. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     It should be noted that common features in the different views of this invention are shown with the same reference numeral(s). For alternate embodiments of the same component, there is consistent numbering though in the next thousand series. And when referring to any numerical ranges herein, it should be noted that all numbers within the range, including every fraction or decimal between its stated minimum and maximum, are considered to be designated and disclosed by this description. 
     There is shown in accompanying  FIG. 1 , a schematic showing a MixWell system in combination with an A-Mixer from which an air line is lead from a Trompe chamber according to one embodiment of this invention. Subcomponents of that arrangement are then focused upon for close up views in  FIG. 2  (the MixWell);  FIG. 3  (the A-Mixer) and  FIG. 4  (the condensate trap). The A-Mixer component herein represents an improvement over U.S. Pat. No. 4,630,931. It improves over the known art primarily by adding a perforated pipe across its tank bottom for greater lime dissolution. The purpose of prior art systems was to suspend solids. This invention, in addition, uses water pressure to cause a flow through of settled lime particles to occur ultimately leading to a better, fuller dissolution of such particles. The airlift of this invention creates a low pressure zone, preferably in a central pipe at its base from which a perforated pipe extends in both directions. Alternatively, the central pipe can be removed and replaced with a long extension from one end of the perforated pipe for exiting the tank at a point below its normal water level per  FIG. 3A . 
     The Trompe device used in conjunction with the same is an improvement over U.S. Pat. No. 892,772. And the freeze-proof hydrant used in preferred embodiments of this invention is as shown in U.S. Pat. No. 4,483,361. 
     Referring now to  FIG. 1 , moving from left to right, there is shown the MixWell component  100  starting with a reagent feed system  102 . One such system is the AquaFix water-powered reagent auger system as disclosed in U.S. Pat. No. 5,167,800. It is preferably situated adjacent or near a reagent silo or hopper  104  or other comparable container. Reagent drops into the top  106  of MixWell from that auger (or other comparable device) as needed, i.e. a sluice. 
     The finer reagent particles leave the MixWell with dissolved reagent in mine drainage water. Ideally, discharge D from the MixWell flows directly into the A-Mixer, generally  200 , via channel C or another conveyance device. The other (sufficiently larger) reagent particles flow down annulus  108  of the MixWell  100 , in the direction of arrow A, toward the base  110  of annulus  108 , where is situated a nozzle  112  and opening  114  to flush drain  116  (for serving as a clean out pipe). Alternately, a gravity drain pipe  136  may be used where feasible. A plug or cap  118  to the top of flush drain  116  allows access for mechanical cleaning, pumping, etc. A source of flush water S (i.e., mine drainage or other water) may be passed through valve V for assisting with the clean out of flush drain  116 . 
     The main constituent to MixWell component  100  is the holding well itself. As schematically shown, holding well  120  consists of a large pipe set vertically though it is to be understood that a tank or other similar structure may be substituted therefore. This configuration is designed to allow for larger reagent particles to remain near the base  110  of holding well  120  where said particles can; (a) be ground smaller by collision with other reagent particles; and (b) continue to dissolve. 
     Toward the top of holding well  120  is a well outlet  122  through which the finer reagent particles leave the MixWell component  100  with dissolved reagent in mine drainage/water. Ideally, such MixWell “discharge” flows directly (preferably by gravitational flow) into the A-Mixer  200  via channel C or another intra-component conveyance device. 
     The MixWell component  100  of preferred embodiments (as schematically shown) further includes an incoming pipe  130  through which mine drainage or other water is conveyed; and an overflow pipe  132  (positioned above incoming pipe  130 ) used for conveying excess flow away from the MixWell. 
     As for the A-Mixer component  200 , the focus of  FIG. 3 , it starts with a tank inlet  202  immediately adjacent channel C for receiving the MixWell discharge and holding same in its main holding tank  204 . As discharge flows down to the bottom  206  of holding tank  204 , it surrounds a preferably centrally located pipe stand  208 . As shown, pipe stand  208  consists of a plurality of L-brackets having legs  209  that rest or are mounted to bottom  206  of tank  204 . At an intermediate point along the body of each stand  208 , there exists an angled ledge  211  on which will rest the lower end  213  to vertical tube, channel or pipe  212 . An air diffuser  214  is lowered and allowed to discharge air into pipe  212 . 
     With the foregoing, there is formed an aperture or gap  210  around most of the lower end  213  of pipe  212  with legs  209  to stand  208  resting at least as low as the circulation line for sludge leachate within tank  204 . Water can circulate into that gap  210  as indicated by the water flow arrows on both sides to gap  210 . It is anticipated that four, evenly spaced legs  209  are sufficient for supporting and suspending stand  208  upwardly from bottom  206  to tank  204 . But fewer or greater amounts of legs  209 , or thicker variations of stand supports (with a plurality of apertures/gaps there beneath) may be used as well. 
     In this embodiment of the invention, air diffuser  214  from an external air line  216  that passes through union U and back to a condensate trap  300  with pipe  302 , the focus of  FIG. 4 . As shown, air diffuser consists of several fork-like prongs extending downwardly from a common air line exit point, said prongs serving to increase the distribution area of air exposure from air line  216 . 
     Holding tank  204  has its own fill line F such that when liquid levels exceed that line, they will pass through tank outlet  220  where it is conveyed to a settling pond SP or other settling basin/tank. Optionally, in order to help retain fine particulates inside of holding tank  204 , there is included a top baffle  222  behind or otherwise adjacent tank outlet  220 . Also, for better circulation and tank cleaning, there may be added an optional drainpipe  224  from the bottom  206  of holding tank  204 , said drainpipe  224  being fitted with its own valve DV. 
     On the bottom of holding tank  204 , a perforated pipe  226  is placed horizontally. The perforations in pipe  226  are sufficiently small to prevent settled particles from entering the pipe. This pipe is directed, but not connected to the bottom of pipe  212 . A centrally located, vertical extension  227  to pipe  226  rises to near the ends to the various prongs of air diffuser  214 . Low pressure created by the air lift in pipe  212  induces water flow through vertical extension  227  and hence through the settled particles, into the perforated pipe and hence into the flow in pipe  212  thus promoting dissolution of the lime particles. 
     In an alternative embodiment, shown in  FIG. 3A , the equivalent to central vertical extension  227  has been replaced by a vertical pipe extension  1227  from one end  1226 E of perforated pipe  1226 . That vertical extension rises along or adjacent to one side  1205  of tank  1204  before exiting the tank at a preferred point P below outlet  1220  of tank  1204 . There, this alternate exit  1228  to perforated pipe  1226  will rejoin/reconnect to outlet  1220  external to tank  1204 . With this configuration, the water pressure represented by the difference between fill line F and exit point P causes flow to occur through the settled lime particles promoting dissolution of the lime. 
     For greater clarity of components,  FIG. 3B  provides an exploded view of the lower end elements to the alternative embodiment specifically showing stand  1208 , its legs  1209  resting on tank bottom  1206  and stand ledge  1211  upon which rests the lower edge  1213  to pipe  1212 , all of said components working together to create/gap water flow gap  1210 . For illustration purposes, the horizontally extending perforated pipe has been deleted. It is to be understood that a similar arrangement exists near the bottom to tank  204  in  FIG. 3  with the addition of vertical extension  227  thereto. 
     As best seen in  FIG. 4 , one preferred embodiment of condensate trap, generally  300 , consists of a condensate reservoir  304  through the top  306  of which runs an air line AL. At the bottom  308  of said reservoir  304 , there is a connection to a freeze-proof valve  310  that, itself, rises above ground level before terminating in its own yard hydrant  312  or other valve-bleeding device/means. 
     Also in preferred embodiments of this invention, the discharge from settling pond SP, outside of holding tank  204 , can be connected to a Trompe air chamber unit, generally  400 . The fluid inlet  402  to that Trompe chamber sits near the top to settling pond SP with at least some portion of same extending outwardly above the fluid levels in said settling pond at all times. That Trompe inlet  402  connects to a Trompe air inlet  404  that channels downwardly, over and back up to its own Trompe outlet  406 . At an intermediate point along cross channel  408  to the Trompe chamber  400 , a Trompe air chamber  410  extends upwardly. An air pipe line  412  connected to that air chamber  410  passes through Trompe valve TV before connecting to the aforementioned condensate trap  300 . That connecting air line accepts compressed air from Trompe chamber  400  for use to power the air diffuser unit  214  in A-Mixer  200 . 
     Example 
     Raw Water 
     Water discharging from the Manor mine ranges in flow from 78 to over 500 gallons per minute. The raw mine water contains the following constituents: 
                                                                                                       TABLE 1                   Raw Water Quality            Parameter   Date                Date   Jan. 6, 2012   Jan. 16, 2012   Jan. 23, 2012   Jan. 31, 2012   Feb. 7, 2012   Feb. 21, 2012   Units                    pH Field   3.48   3.43   3.53   3.38   3.45   3.69   S.U.       Acidity   466   468   458   445   422   417   mg/L       Iron   233   201   231   183   210   203   mg/L       Aluminum   19.3   22.3   15.6   12.9   11.4   16.8   mg/L       Manganese   3.62   2.99   3.13   3.23   3.43   3.40   mg/L       Calcium   166   140   160   138   145   141   mg/L       Magnesium   43.3   49.7   51.7   54.1   44.9   45.7   mg/L                    
Field Investigation
 
     Water from the modified treatment plant was sampled at four locations: Raw water; MixWell outflow; A-Mixer inflow; and A-Mixer outflow. Note, due to site conditions, the A-Mixer is located down gradient of the MixWell. The influent of the A-Mixer travels approximately 150 feet along the existing mixing channel prior to entering the A-Mixer. With the exception of the raw water, these samples were quite complex in that they contained calcium oxide/hydroxide particles in suspension along with a suspension of ferrous hydroxide flock which is actively trying to oxidize to ferric hydroxide. Special procedures had to be taken to get a representative sample from these three locations. Field filtered sample were desired so that the dissolved lime could be separated from the lime that was still in particulate form. A 0.45 micron filter was used for this purpose. The presence of the ferrous hydroxide flock greatly inhibited this filtration. To help avoid this problem these samples were allowed to sit for 10 minutes before filtering so that the suspend particles could settle. 
     Particle Size Analysis 
     Pebble Quicklime samples were taken from the AquaFix feeder at the Manor site. These samples were sieved through a 10, 60, 80, 200, and 325 mesh screens. Effluent from the MixWell and the A-Mixer were also wet sieved through the 10, 60, and 80 screens; finer screens were clogged, plugged, occluded or “blinded” by the ferrous hydroxide flock.  FIG. 5  shows the particle size distribution of the raw lime product,  FIG. 6  shows the particle size contained in the effluent water from the MixWell, and  FIG. 7  shows the particle size contained in the effluent from the A-Mixer. It was not possible to sieve the entire flow from the MixWell, consequently, a mass balance based on particle weight is not possible. 
     The graphs show continued degradation of the particle size as it moves through the system. Over 60 percent if the raw lime is retained on the number 10 sieve. Effluent from the MixWell has no material on the number 10 sieve and 80 percent of the material retained on the number 60 sieve. Effluent from the A-Mixer has between 40 to 55 percent retained on the number 60 sieve with a similar range being retained on the number 80 sieve. Note that all of the raw lime that was retained on the number 10 screen has been reduced to minus 10 mesh by the MixWell. 
     pH 
     pH was measured, in the field, at four locations: raw water; effluent from the MixWell; influent into the A-Mixer; and effluent from the A-Mixer.  FIG. 8  is a graph of these data measured on four different dates. The pH is substantially increased in the MixWell. Two of the plots show an increase in pH as the water crosses the lime bed and two of the plots show a pH decrease as the was traverses the lime bed. Three of the plots show a slight increase in pH in the A-Mixer and one of the plots shows no change in pH. 
     The chemistry driving the pH rise in the MixWell is straightforward. The pebble quicklime dissolves raising the pH and some of the dissolved ferrous iron precipitates as ferrous hydroxide flock. Between the MixWell discharge and the A-Mixer inlet is about 150 feet of the existing mixing channel. In the mixing channel; minor amounts of additional raw/other water is added, lime is dissolved, and some ferrous iron is oxidized. As a consequence the pH is variable depending on lime dosing. The pH in the A-Mixer is also complicated, a steady to slightly rising pH is observed but this must be viewed in relationship to the dissolved iron data. 
     Dissolved Oxygen 
     A plot of the DO data is shown in  FIG. 9 . As soon as the pebble quicklime is added to the raw water nearly all available oxygen is consumed. As the water flows over the lime channel oxygen is reintroduced, but it is again consumed in the A-Mixer even though air is also being added. 
     These drops in DO concentration are attributed to ferrous iron oxidation to ferric iron (equation 1) or alternatively, ferrous hydroxide oxidation to ferric hydroxide (equation 4). The ferrous oxidation reaction consumes oxygen and generates acidity by the following two equations:
 
Fe +2 +¼O 2 +H + →Fe +3 +½H 2 O  (2)
 
     When the ferric iron precipitates three moles of acidity are generated for each mole of iron. This equation will be useful when we consider iron precipitation in the A-Mixer.
 
Fe +3 +3H 2 O→Fe(OH) 3 +3H +   (3)
 
     The net reaction is that two moles of acidity are created for each mole of iron oxidized and precipitated. This oxidation reaction is pH limited and is only expected to be significant above a pH of 7. 
     A second reaction may also be occurring. Ferrous hydroxide also known as green rust has been observed in all parts of the semi-active treatment system. This ferrous hydroxide can be converted to Ferric hydroxide without the generation of acidity.
 
Fe(OH) 2 +½H 2 O+¼O 2 →Fe(OH) 3   (4)
 
     In this case the addition of oxygen is the only requirement as all of the other reactants are present. At pH lower than 7 this is expected to be the dominant reaction in the A-Mixer. 
     Alkalinity 
       FIG. 10  shows the alkalinity measured in the field for the treatment system. As expected alkalinity rises as the pebble quicklime is added in the MixWell. The alkalinity then decreases in the lime channel as oxygen reacts to convert ferrous iron into ferric hydroxide. Alkalinity is again given a slight boost in the A-Mixer despite the introduction of air in the system. 
     Dissolved Iron 
     Samples for dissolved iron were taken at each of the four sampling locations. These samples were allowed to sit for 10 minutes while the flock settled then the supernatant was field filtered using a 0.45 micron filter.  FIG. 11  shows a significant decrease in dissolved iron as the water traverses the treatment system. 
     Dissolved iron concentrations within the A-Mixer dropped on two dates, Jan. 16, 2012 and Jan. 24, 2012, but were stable on the other two dates when there was no change in dissolved iron concentration. This variation is correlated with the pH of the A-Mixer inlet. The pH was between 6.8 and 7.0 on the days that showed a decrease in dissolved iron. The pH was below 6.8 on the days that dissolved iron remained unchanged. 
     Sulfate &amp; Dissolved Calcium 
     Sulfate concentrations in the raw water ranged from 1,095 to 1,192 mg/L and the dissolved calcium values ranged from 134 to 159 mg/L.  FIG. 12  is a plot of the Sulfate and Dissolved Calcium data across the treatment system. Note that the dissolved calcium increase resulting from the MixWell corresponds to a Sulfate decrease at the same time. This suggests gypsum precipitation in the MixWell. There is little consistent change in these dissolved constituents throughout the rest of the treatment system. 
     It is clear from  FIGS. 8, 10, and 12  that the MixWell is doing the bulk of the pebble quicklime dissolution, and that there is gypsum precipitation occurring in the MixWell. On January 6, the MixWell contributed 75 percent of the dissolved calcium added during the treatment process. 
     Efforts to use dissolved calcium to measure system performance of the A-Mixer were frustrated by spatial, temporal, and possibly gypsum and calcite variations across the site. As a consequence on one sampling trip the dissolved calcium increased in the A-Mixer and on three occasions the dissolved calcium level decreased. This decrease occurred even though alkalinity increased. 
     From the raw lime and other materials taken from the bottom of the MixWell (MixWell Sludge), the inventors noted the percentage of “dark matter” or grit concentration relative to white calcium oxide therein. While heavier grit particles can help with the mechanical degradation of the pebble quicklime, at some point the grit must be drained from the system when it becomes excessive. The inventors also noted the rounded surfaces to some of the pebble quicklime extracted. Such rounding can be from abrasion or chemical dissolution of the particle surface. Still other pebbles, including some gypsum pebbles, showed the effects of abrasion. 
     In two effluent samples, black particles were observed. They are believed to be the inert components of pebble quicklime product. Manufacturer-provided data indicated that the pebble quicklime product used had a calcium oxide content of 94.4 percent and a lime index of 92.3 percent. About six percent of that product was grit. As the lime dissolved, the percentage of grit in the remaining particles increased. 
     The MixWell and A-Mixer discharges were wet sieved. This limited the particle size that could be captured to screens that were not blinded by the ferrous flock (80 mesh screen). In addition, the 12-inch sieves were not large enough to capture the entire flow consequently; a mass flow rate was not possible. Because of that, the relative pile sizes observed should not be quantitatively compared.  FIG. 7  showed a particle size reduction between the MixWell discharge and the A-Mixer discharge. It is suspected that more lime will be dissolved in the A-Mixer when the inlet pH is greater than 7 as the acidity from the dissolved ferrous iron is released. This suspicion is based on the drop in dissolved iron that occurs when the pH is greater than 7 combined with an increase in pH at the same time.  FIG. 13  shows this relationship on the January 16 sampling date, the dissolved iron dropped from 88 to 22 mg/L while the pH rose from 7.1 to 7.38. Water with lower pH does not benefit from the dissolved iron reduction. At the higher pH oxygen reacts more quickly with the ferrous iron converting it to ferric iron. This process releases acidity that is neutralized by the remaining undissolved quicklime particles being held in suspension by the A-Mixer. The net result is that when the inlet pH is maintained at or above 7 iron oxidation in the A-Mixer occurs rapidly releasing acidity into the water this acidity is immediately neutralized by the suspended calcium oxide particles leading to a higher outflow pH and a higher alkalinity. 
     The proof of the technology is based on the lime utilization rate. Three approaches were taken to establish this rate. First, the acidity of the mine water was used to calculate the amount of pebble quicklime required to neutralize that acidity. This calculated value was then compared with the actual amount of pebble quicklime added. On three occasions, pebble quicklime was captured from the lime auger over a one minute period and these samples were weighed and used to compute the lime efficiency. On Feb. 21, 2012, the lime feed was not captured. Instead, the lime delivered per revolution on the prior sampling was used to calculate the lime added for this date. Consequently, it is not known if the lime dosage on this date is an accurate reflection of the actual lime dosage. The lime dose on this date was further complicated by the system being down for several days prior to the sampling event. It is believed that the operator was overdosing in an effort to raise the pH in the settling ponds. The lime efficiency in excess of 100 percent may be due to higher quality pebble quicklime being delivered than was reported on the lime analysis, or it could represent some amount of under treatment in the system. These data are shown in  FIG. 14  which also shows a plot of the lime utilization rate based on the stoichiometric amount of calcium needed to treat the mine water. This approach is confounded by gypsum and possibly calcite formation within the system. The formation of gypsum or calcium, if taken into account, would increase the reported lime utilization rate. 
     The ultimate test of lime utilization is a comparison of the lime usage before and after the MixWell and A-Mixer retrofit. Plant personnel have adjusted the lime delivery of the plant so that their discharge criteria are maintained both before and after the plant retrofit. Using the plant log book the flow rate and the Aquafix revolutions per minute (RPM) were recorded. The RPM data were divided by the flow data to yield a metric of RPM/gal. This metric was then plotted against time and graphed in  FIG. 15 . The before and after data show a definitive improvement in the lime required. The average RPM/gal before the retrofit was 0.1255 after the retrofit the RPM/gal was 0.0719. This indicates that the new plant is operating on 43 percent of the lime that was required prior to the rebuild. Before the retrofit annual lime cost was $30,000. The indicated savings in lime cost is $17,100 per year. This does not include the cost of dredging and disposal of un- or under-utilized lime in the first settlement pond. 
     CONCLUSIONS 
     Passive mixing technology can have a very significant improvement in lime utilization where pebble lime is the source of the alkalinity. 
     The MixWell technology is very effective at dissolving pebble quicklime and reducing the particle size of its effluent. 
     The A-Mixer, if operated at pH 7 or above, can advance the oxidation of ferrous iron while maintaining pH across the system. 
     While certain illustrative embodiments have been shown in the drawings and described above in considerable detail it should be understood that there is no intention to limit the invention to the specific forms disclosed. On the contrary the intention is to cover all modifications, alternative constructions, equivalents and uses falling within the spirit and scope of the invention as expressed in the appended claims.