Study On Passivation Of Metal–sulfide Tailings By Surface Coating And Passivation Mechanism

Large amounts of tailing were produced and stored in tailing ponds or in piles exposed toair around the world evevry year. Many toxic metals were released from tailings due to theoxidation by O2, which would contaminate the surrounding environment and do harm to thehealth of people through food line. In order to control the oxidation of tailings at sourse, thisstudy took three kinds of metal-sulfide tailing samples, weathered and fresh tailings, andtailing sediments which were obtained from the Dabaoshan sulfur-polymetallic mines in thenorth of Guangdong Province, China as research objects and employed a new kind ofmethods to coat the metal-sulfide tailings without pre-oxidation. The new method couldinhibit or slow down the release of heavy metals from tailing samples by suprresing theoxidation of sulfide minerals in microcosmic level. Three kinds of different coatings wereadopted as coating agents on taling samples at last after testing many chemical materials.The probable mechanisms of the passivation of tailing samples were proposed. We alsoinvestigated the mechanisms of the coatings protecting the coated tailings surface from O2and H2O attack. Stability researches were conducted to examine the duration of coatings inweathering conditions. The main experiments and conclusions are as follows:1. In order to get better understanding of tailing samples, the charactization of the physical,chemical and mineralogical properities of three tailing samples were conducted. The tailingsmainly contained the elements of Fe, Si and Al, while he content of S is very low relatively.The main mineral compositions for fresh tailing are quartz, goethite and hematite. Quartz,lead alum, iron alum and gibbsite are the main minerals contained in the weathered tailingssamples. While for tailing sediment samples, the main mineral compositions are quartz andgoethite. All the three tailings samples are rich in Cu, Zn and Pb，but the content of Cd isrelatively lower than other three heavy metals. The distribution and chemical forms of Cu,Pb, Zn, and Cd in three tailings samples were studied based on mineralogical and chemicalanalyses as well as sequential extraction. The potential migration ability of heavy metalswas also disscussed on the basis of the speciation of heavy metals. Cu, Zn, Pb and Cd infresh tailing samples were mainly existed in sufides and the mobility of four heavy metalsfollowed the order: Cu>Cd>Zn>Pb. The dominating chemical forms of Cu, Pb, Zn, and Cdwas residual silicates in weathered tailing samples and the mobility of the four metalsfollowed the order Cd>Zn>Cu>Pb. For the tailing sediments, Cu, Zn, Pb and Cd also existresidual in and the mobility of the four metals followed the order: Zn> Pb>Cd> Cu. Theacid producing potential of three tailing samples was studied using both NAG （net acid generation） and NAPP （net acid producing potential） test. The results indicated that all thethree tailing samples had high acid producing potential and low acid neutralization capacity.2. Research was conducted to investigate the optimum concentration of TETA. Theconclusions were as follows: when the concentration of TETA was1.5%, the effect of filmwas the best for fresh and weathered tailing samples; when the concentration of TETA was2%, the effect of film is the best for tailing sediment. The effect of TETA was not positiverelation with the concentration of TETA. When TETA concentration was more than5%, theeffect of passivation began to be weak. TETA was able to suppress the oxidation of all thethree tailing samples. After the coated tailing samples were oxidized24h by H2O2, themetals production were reduced more than75%for all of the three tailing, especially for Cuand Zn. However, the oxidation extent of the coated fresh tailing sample was less than thatof coated weathered tailing and coated tailing sediment. The main reason for this was thatthe concentration of Cu2+was relatively high at the beginning of the metal releasing from thecoated fresh tailing. So the Cu2+could more easier to form precipitate on the fresh tailingsurface than other two tailing samples. This could be seemed as the second passivating layer.As for the passivation mechamism of TETA, the first step was TETA coated on the tailingsurface. When the coated tailing samples were oxidized by H2O2, TETA could takeadvantage of amine groups to react with H2O2. Redox reaction could consume oxidant toreduce the oxidation of tailing. On the other hand, TETA would take acid-baseneutralization reaction with H+released from tailing sampes and form precipitate on thetailing surface to protect the tailing samples from O2attack.3. Both of the coating oleic acid and stearic acid could inhibit the oxidation of pyrite.The major mechanism of coating formation was the chelation. The carboxylic group of oleicacid and stearic acid could chelate with heavy metal on the mineral surface. And the chelateproduct covered with the mineral surface to inhibit the pyrite oxidation. However, theinhibition rate of oleic acid was much higher than that of stearic acid at the same experimentcondition. The main reason was the effect of the carbon-carbon double bond in oleic acidmolecule. The passivating layer had been formed on the surface of pyrite after theinteraction of oleic acid or stearic acid, and this layer was very steady in the solution. Ourstudies showed that the physical coating layer of oleic acid would form on the surface ofchemical passivation layer by carbon-carbon double bond. At the meantime, physicaladsorption would react between carbon-carbon double bond and the sulfur of pyrite, whichwould increase the surface area of the passivation layer on pyrite surface. The potassiumoleate also was used as a passivator to suppress the oxidation of mineral. Compared with oleic acid，the optimum concentration of potassium oleate was much greater. The reasonwas the poor chelate selectivity of potassium oleate with metals. For example, the potassiumoleate could chelate with Ca²⁺in the tailing samples. And this would result in the increaseddoasage of potassium oleate when used it as the coating for tailing samples.4. Sodium triethylenetetramine-bisdithiocarbamate （DTC-TETA） was synthesized of as acoating for pyrite and tailing surfaces to suppress ARD production at source. Leachingexperiments showed decreased Fe leaching by99.8%and98.5%upon pyrite exposure to pH6.0and3.0solutions, respectively. Column leaching also decreased by>90%for Cu, Zn, Cd,Pb, and Fe metals at pH6.0and3.0solutions for a period of30days in fresh and weatheredtailing samples. The probable mechanisms of the passivation of pyrite and tailing sampleswere also proposed. Unlike most of other coatings, DTC-TETA was covalently coordinatedto metals and formed a cross-linked hydrophobic passivating layer on the pyrite or tailingsurface to inhibit the release of metals in acidic solutions. Compared to other coating agents,DTC-TETA has the following advantages. First, in previous studies, dithiocarbamates havebeen used as chelating agents to bind heavy metals. DTC-TETA can directly coat thesurface of pyrite without using H2O2for initial surface oxidation. Second, the DTC-TETAligand is thought to utilize terminal sulfur groups to facilitate effective and covalentcoordination to iron atoms in pyrite so that the ligand can work well under very low pH.Third, DTC-TETA is composed mainly of straight chain carbons and poses no foreseeablethreat to the environment. Fourth, DTC-TETA is efficient for engineering applicationsbecause it easily dissolves in water and avoiding using other organic solvents to increase itscost. Fifth and last, most coating experiments have been conducted on pyrite or pyrrhotitesamples. The compatibility of these coatings with mineral wastes in tailing ponds is not yetclear. DTC-TETA has effect on inhibiting the oxidation of pyrite and sulfide tailing samples.