Section 6. MINING WASTES
ELECTROFLOTATION OF GROUP VI-A HEAVY METALS
FROM MINING TAILINGS WASTEWATER
Ernest R. Ramirez, Consulting Engineer
American Standard, Inc.
Wayne, New Jersey 07470
INTRODUCTION
Ten years ago, industry was especially interested in removing molybdenum from tailings
and processing wastewater primarily because it interfered in the recovery of other metals.
For example, molybdenum contents in the range of 10 mg/1 eventually poisoned ion exchange resins used for the extraction of uranium. It was also known that molybdenum contents in excess of 200 to 300 mg/1 in the processing waters often resulted in unacceptable
limits of molybdenum in the uranium yellow cake. To this end, many researchers have
studied methods and ways of removing molybdenum from mining processing waters.
Today it is no longer adequate to keep molybdenum values in the ranges of 2 to 10 mg/1.
Recent restrictions by the EPA on molybdenum discharge values (NPDES) for mining wastewater necessitates removal of this metal to less than 1 mg/1, and in some instances, as low as
0.5 mg/1. Actually, this metal changes from being a general nuisance to processing to a metal
which is restricted to the environment because of its toxic effects. As a result of this, new
research and development is needed to economically remove molybdenum values from
mining tailings wastewater. In this regard, George [ 1 ], Maugno [2] and Merrit [3] published
information showing that it is possible to remove molybdenum from mining processing
wastewaters by forming a ferric-molybdenum insoluble complex. The process is so effective
that molybdenum values as low as 0.2 mg/1 can be readily achieved. These results are especially promising since they fulfill the guidelines established by the EPA.
In 1977 Gott [4] presented a paper at the American Mining Congress in San Francisco,
CA, showing that electrocoagulation can be effectively used to remove heavy metal hydroxide precipitates from mining tailings wastewater. In the electrocoagulation process, it was
clearly shown that insoluble metal precipitates could be readily removed in a two-step operation. The first step having a retention time of about 1 minute, while the second step requires
approximately 25 minutes.
Based on these two scientific facts, it became evident that molybdenum could be separated from mining tailings wastewater by the application of the two principles described
above; namely, precipitation of the molybdenum with ferric ion, and subsequent flotation of
this precipitate with the electrocoagulation technology. Details of this new technology have
been adequately described elsewhere [5-8].
LABORATORY TESTS
Based on the concept that precipitation and removal of the molybdenum could be carried
out under proper conditions, laboratory tests were run to establish the validity of this new
concept. In the laboratory tests, simulated electrocoagulation and flotation cells were constructed to handle a one-liter sample. This operating hardware is packed in a suitcase and
tests are run at the mine site (Figure 1). Using the data available from George, a series of experiments were carried out to determine the degree of removal as a function of pH. For the
laboratory experiment runs in the field, both spiked and actual mining tailings wastewater
were used. Molybdenum concentrations ranging from 20 mg/1 to 1.5 mg/1 were evaluated.
An electrolytic energy input of 2 amp-min/1 were carried out in the electrocoagulation cell,
and 1 amp-min/1 in the flotation basin. The results of these tests are shown in Figures 2 and 3
as a function of pH. In these figures, the pH was lowered with sulfuric acid, followed by the
addition of ferric sulfate; lastly, sufficient lime was added to raise the pH 0.5 units.
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