Browsing by Author "Paul, Restituta"

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    Modelling the leaching behaviour of selected heavy metals in solidified/stabilised gold mine tailings
    (University of Dar es Salaam, 2020) Paul, Restituta
    This study aimed at modelling the leaching behaviour of selected heavy metals, namely Cu2+,Cd2+,Cr3+, Zn2+ and Pb2+ in stabilized/solidified (S/S) gold mine tailings intended for utilization in construction works. The modelling focused on factors such as Ph, tailings particle size, cements content and liquid to solid ratio (L/S ) interface conditions, which could potentially influence the leaching behaviour of the heavy metals in S/S tailings. The leaching behaviour of heavy metals in S/S tailings was investigated following standard leaching test protocols. Chemical speciation of the heavy metals in leachant was modelled using Visual MINTEQ 3.1 R Software. The major tailing constituents were found to be quartz (SiO2) and muscovite. The tailings had average concentrations of Cr3+ ,Cu2+ ,Pb2+,Zn2+ and Cd2+ ranging from (14.1 -79.9) ,(42.3 -45.9), (122.3 -135.9, (31.4 -43.9) and (21.1-37.4) mg/kg, respectively. The Ph –dependent leaching tests revealed a strong ph-dependent leaching behaviour of the selected metals. The lowest leaching rates were observed in alkaline conditions (i.e. –ph 10), regardless of initial concentrations. Using visual minteq 3.1 it was established that the immobilization of heavy metals at high ph was due to the precipitation as insoluble hydroxides in the stabilized matrix. Immobilization of Zn in the tailings was through substitution on cement ettringite and formation of insoluble complex compound (Zn6A12 (OH)16CO3. 4H20) AT PH Values 9.5 Pb exhibited amphoteric leaching behaviour with minimum leaching between Ph 10.5 -11.5. The immobilization of Cu and Cd at high Ph was due to its precipitation as Cu (OH)2 and Cd(OH)2 respectively, while the immobilization of Cr in alkaline conditions was associated with a reduction of the more soluble Cr +6 salts less soluble Cr (OH)3.results of speciation modelling with visual MINTEQ 3.1 indicate that the dissolution of alkaline metals and oxyanions (such as SO42-, HCO32-) raised both the PH and EC of leachant which in-turn affected leaching behaviour of heavy metals. It was revealed that under acidic conditions, the leaching solution was dominated by metal chlorides. Under alkaline conditions; mostly metal hydroxides were leached into the solution. Adsorption simulation reveals that only 1% of each metal is immobilized on either HFO or Gibbsite with a PH –dependent sorption process.it has further been observed that the major leaching mechanism for the soluble fraction of the metals in S/S was surface washout.Multi-linear regression models were developed for Cu2+,Cd2+ ,Cr3+ ,Zn2+ and Pb 2+ .The models and laboratory experiment showed that metal leaching was controlled by PH of leachant and cement content . It is concluded that S/S is an effective method for reducing leaching of heavy metals in the tailings. However, since risk levels are dependent on the composition of the tailings, it is recommended that a risk assessment must precede S/S tailing utilization in construction Chemical technology.
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    Modelling the leaching behaviour of selected Heavy Metals in Solidified/Stabilised Gold Mine Tailings
    (University of Dar es salaam, 2019) Paul, Restituta
    This study aimed at modelling the leaching behaviour of selected heavy metals, namely Cu2+, , Cd2+ , Cr3+, Zn2+, and Pb2+ in stabilized/ solidified (S/S) gold mine tailings intended for utilization works. The modelling focused on factors such as pH, tailings particle size, cements content and liquid to solid ratio (L/S) interface conditions, which could potentially influence the leaching behaviour of the heavy metals in S/S tailings.t the leaching behaviour test protocols. Chemical speciation of the heavy metals in leachant was modelled using visual MINTEQ3.1 (R) software. The major tailing constituents were found to be quartz (SiO2) and muscovite. The tailings had average concentrations of Cr3+, Cu2+,Pb2+, Zn2+ and Cd2+ ranging from (14.1-79.9),(42.3-45.9),(122.3-135.9). (31.4-43.9) and (21.1-37.4) mg/kg. respectively. The PH-dependent leaching tests revealed a strongly Ph dependent leaching behaviour of selected metals. The lowest leaching rates were observed in alkaline conditions(i.e>pH 10), regardless of initial concentrations. Using visual Minteq 3.1 (® ) it was established that the immobilization of heavy metals at high PH was due to their precipitation as insoluble hydroxides in the stabilized matrix. Immobilization of Zn in the tailings was through substitution on cement ettringite and formation of insoluble complex compound (Zn6Al2 (OH)16CO3-4H2O) at pH values> 9.5. Pb exhibited amphoteric leaching behaviour with minimum leaching between pH 10.5-11.5. the immobilization of Cu and Cd at high pH was due to its precipitation as Cu(OH) and Cd(OH)2 respectively, while the immobilization of Cr in alkaline conditions was associated with a reduction of the more soluble Cr+6 salts less soluble Cr(OH)3. Results of speciation modelling with MINTEQ3.1® indicate that the dissolution of alkaline metals and oxyanions (such as (SO42- ,HCO32- ) raised both the PH and EC of leachant which in –turn affected leaching behaviour of heavy metals. It was revealed that under acidic conditions, the leaching solution was dominated by metal chlorides. Under alkaline conditions, mostly metal hydrozxides were leached into the solution. Adsorption simulation reveals that only <1% of each metal is immobilized on either HFO or Gibbsite, with a pH-dependent sorption process. It has further been observed that the major leaching mechanism for the soluble fraction of the metals in S/S was surface washout. Multi-linear regression models were developed for Cu2+, Cd2+, Cr3+, Zn2+ and Pb2+ . the models and laboratory experiment showed that metal leaching was controlled by pH of leachant and cement content. It is concluded that S/S is an effective method for reducing leaching of heavy metals in the tailings. However, since risk levels are dependent on the composition of the tailings, it is recommended that a risk assessment must precede S/S tailing utilization in construction works.