Acid Drainage

Background

Drainage flowing from sulphide ores are typically highly acidic with elevated levels of dissolved metals.
Drainage comes from areas where exposed rocks contain sulphides
Lowers pH and degrades quality of waterway
Leaches out toxic metals ( Zn, Cd, Hg etc) into soil
Affects aquatic plant and animal life and disrupts food chain
Corrodes man-made structure like pipes , dams

Study Site: University of Kentucky & Center for Applied Energy Research lab

Acid drainage

Acid production region

Potential Treatment of Acid Drainage

•At source pyrite stabilization by coating the pyrite surface with an inert hydrophobic film

•Spontaneous coating of pyrite surface with long chain collectors to form a uniform densely packed hydrophobic layer

•The 2 collectors that have been evaluated in this study are sodium oleate and xanthate

Contact Angle Studies

•Contact angle is a good indicator of the measure of hydrophobicity of any surface

•Sessile drop method was applied to measure contact angle

•Contact angle studies found both single and double coating of xanthate and oleate made the pyrite surface extremely hydrophobic

•Coal and ore pyrite coated with sodium oleate had a contact angle of 95 degree at neutral pH

•Contact angle decreased at acidic pH due to adsorption of hydrophillic oleic acid

•Contact angle decreased at alkaline pH due to preferential adsorption of OH- ions

Zeta Potential Studies

•Unoxidized pyrite normally have an point of zero charge (PZC) at pH < 2

•PZC of oxudized pyrite is nearly 9

•The high PZCvalue is due to the formation of positively charged ferric hydroxy complexes on the pyrite surface during polishing and grinding

•Pyrite was coated with two different concentrations of sodium oleate

•PZC of pyrite decreased to pH 3.5

•Charge reversal at acidic pH indicates strong adsorption of hydrophobic ferric oleate on the pyrite surface

Leaching Studies

1 gm of pyrite was subjected to aqueous oxidation for 30 days

Objective was to study the oxidation and dissolution rate of coated and uncoated pyrite sample.

Aim was to find a suitable coating which would reduce leachibility of pyrite to zero.

•Based on H+ ion production of uncoated pyrite was 3 times more reactive than pyrite coated with sodium oleate

•Leaching behavior improved significantly when pyrite was coated with a double coating of xanthate and oleate

•Recoating the pyrite surface after 14 days significantly mitigated pyrite oxidation

Weathering Studies

•Objective was to study the oxidation of pyrite as a function of weight change

•Two gram of pyrite were kept in each of the six leaching column under moist condition and exposed to air

•Pyrite coated with sodium oleate retained 90% of it’s weight compared to uncoated pyrite surface .

Cyclic Voltammogram Studies

Objective was to obtain mechanistic and kinetic information on the surface reaction of pyrite oxidation

Before each experiment, the electrolytic solution was purged with oxygen for an hour to simulate mill chemistry

Difference in CV between coal pyrite and ore pyrite can be attributed to difference in morphology, carbon inclusion and heterogenity

No oxidation current was observed from 0 to 0.5 V vs SCE

Onset of anodic oxidation increased by 50 mV

During the cathodic scan,the cathodic current was smaller

Fe (II) oxidation peak got reduced

Similar to Oleate, the oxidation kinetics of pyrite coated with Xanthate get’s reduced

Electrochemical Impedance Spectroscopic studies

Objective was to measure the stability of Xanthate and Oleate coating on pyrite surface

Impedance measurements were carried out using Z-Plot.

Frequency range varied from 2 MHz to 1 mHz and amplitude of the sine wave was kept at 10 mV

All tests were conducted in 0.05 M sodium borate solution at a pH of 9.2.

•Low frequency region corresponds to pyrite-collector interface

•Impedance value of coated pyrite sample remains high

•Impedance doesn’t change even when pyrite surface is scratched with a swipe blade

•Impedance studies found the coating to be highly stable

Fourier Transform Infrared Spectroscopic Studies (FTIR)

Photograph of the Nicolet 870 FTIR

FTIR may be used to directly determine the oxidation products formed on the ore pyrite surface when coated with oleate or xanthate

•Objective was to identify the adsorption mechanism of xanthate and oleate on pyrite surface

•Diffuse reflectance infrared Fourier Transform spectroscopy (DRIFTS) technology was used to identify the adsorption mechanism

Adsorption Mechanism of Sodium Oleate on Coal Pyrite

FTIR studies found oleate to chemisorb on the pyrite surface

Dixanthogen was found to adsorb on the pyrite surface through ferric xanthate contact sites

Characteristic doublet at around 1600 and 1530 cm-1 and a strong adsorption band at 1460 cm-1 indicates the existence of both metal dioleate and chemisorbed oleate on the pyrite surface

Summary

Contact angle studies found both single and double coating of xanthate and oleate made the pyrite surface extremely hydrophobic
Though coating degraded from pyrite surface after 7 days in leaching studies, in real life coating is expected to last for a longer period of time as oxidation occurs at a much slower rate
Leaching behavior improved significantly when pyrite was coated with a double coating of xanthate and oleate
•Recoating the pyrite surface after 14 days significantly mitigated pyrite oxidation
•Impedance studies found the coating to be highly stable
•FTIR studies found oleate to chemisorb on the pyrite surface
•XPS or FTIR may be used to directly determine the oxidation products formed on the ore pyrite surface when coated with oleate or xanthate

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