Iron Mountain, Redding CA - Bench Testing / Site Evaluation


Iron Mountain Mine had the distinction of being the largest generator of metal contaminants into a drinking water supply in the United States. The varying volume of AMD flowing from Iron Mountain Mine ranges between 150 gpm in the summer to over 1200 gpm during periods of heavy precipitation.


On this site, acid water draining from the underground workings within the massive sulfide ore bodies and mine waste dump are creating a monumental problem. Iron Mountain Mine contains many miles of tunnels and stopes containing millions of gallons of AMD. In the lower levels the AMD is under several hundred pounds of pressure and heat is being generated, assisting the normal chemical processes of breaking down the sulfide ores. This is creating a very concentrated AMD. One AMD source is generating an effluent that measures a pH of ‑3.6.


This site has been an EPA Super Fund Site for many years. A decision was made to treat the AMD at Iron Mountain with lime; first, a single lime and, second, a dual lime system. By EPA estimates the amount of metal leaving the discharge stream from the site has been reduced by 95%.


The volume of toxic sludge created by this double liming process is almost unimaginable. HPT has repeatedly attempted to obtain actual AMD treatment, sludge handling and storage costs at this site but neither EPA nor the Principal Responsible Party has been forthcoming with any financial information. The only information available at this point is the amount of sludge being generated and the cost of handling the volume of sludge are considerably higher than had been anticipated.

Major Treatment Problems Associated With Iron Mountain Mine AMD

The plant mix at Iron Mountain Mine is unusually high in copper (over 185,000 PPB).


The copper present is in both of its naturally occurring ionic states: Cu+1 and Cu+2 with most of it in the Cu+1 state. Copper in the +2 state can be easily removed by addition of a lime Ca (OH) 2. The addition of CA (OH)2 starts a two‑phase reaction: first, neutralization of the acid (H+) by it's association with the hydroxides (+OH‑) =>H2O creating water, and second, the copper +2 is hydrolyzed to form insoluble copper hydroxide Cu (OH)2 that will precipitate. The calcium in the lime reacts with the sulfates (SO4) and forms CaSO4 (calcium sulfate) in large quantities. The treated AMD is now free of copper +2. However, some copper +1 is still in solution as Cu OH, or precipitated as an unstable hydroxide. When the pH is raised above 7.5, the copper +1 hydroxide will re-dissolve, re‑contaminating the treated AMD with copper that may/will exceed discharge standards.


Tests done at Lawrence Livermore National Labs (LLNL) showed the Iron Mountain Mine plant mix had an arsenic level of >17.5 PPM, a manganese level of >4.7 PPM and a cadmium level of >13 PPM.


Because of the high pH necessary to remove these metals it was necessary to change the initial single lime treatment to a much more expensive double liming system.


The use of both slack lime Ca(OH)2 and quicklime (CaO) make the process very labor intensive. The process has to be monitored extensively because mine run lime varies considerably in solubility and the amount of inert materials in each batch.


For AMD sites that require the pH to be raised only to 7.0 or slightly above, this can be easily accomplished with common slack lime Ca(OH)2. If the AMD contains metals that require a higher pH the more active quick lime (CaO) must be used.


The use of this double liming process has serious financial consequences. The amount of sludge generated can be double or triple what a single lime process generates. The second problem is that quick lime is very corrosive. Employees must use both breathing protection equipment as well as protective clothing. Quick lime is also very hard to store and has a tendency to absorb moisture from the air and this moisture oxidizes the quick lime into slack lime and usually turns it into a concrete like material making it hard to dispense.


Lastly, any treated AMD that has a final pH above 9.0 will have to use some sort of pH control chemical to reduce the pH below the 9.0 discharge specification usually weak hydrochloric acid.

Use of the Proprietary AMD Systems on the Iron Mountain Plant Mix AMD

This AMD also contained high levels of arsenic, cadmium and manganese. By using the ISM pretreatment system these metals were oxidized or reduced so that they could all be precipitated at a pH below 9.0.


The testing by LLNL proved our AMD process had successfully reduced the arsenic from 2600 parts per billion to 11 parts per billion.


The cadmium and manganese were also significantly reduced. Starting cadmium levels were 4670 parts per billion and after treatment it was non‑detectable or below 10 parts per billion. The manganese was reduced from 13,000 parts per billion to less than 90 parts per billion.


The treated AMD was demetalized but was still very high residual sulfate level (over 60,000 parts per million). The sulfates were reduced with lime, creating a metal free calcium sulfate. During the LLNL tests, an agricultural grade of calcium hydroxide and some unknown calcium material was used to reduce the sulfate levels. This material successfully reduced the sulfates to 147 PPM from 35,700 PPM. We have never been able to duplicate this level of sulfate reduction by this method.