PT Freeport Indonesia Ltd

Papua Province, Indonesia

PT Freeport Indonesia Ltd

Papua Province, Indonesia
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Miller S.D.,P.A. College | Stewart W.S.,P.A. College | Rusdinar Y.,PT Freeport Indonesia Ltd | Schumann R.E.,University of South Australia | And 3 more authors.
Science of the Total Environment | Year: 2010

In the long-term phase of an acid rock drainage (ARD) evolution profile, after any short-term neutralisation capacity provided by carbonate minerals is exhausted, the net acid release is a product of a declining acid generation rate (AGR) and a slower, long-term acid neutralisation rate mainly provided by gangue silicate minerals. At some point, the AGR and the non-carbonate acid neutralisation rate (ANRnc) will be similar. Matching of the AGR and ANRnc near 10 mg H2SO4/kg/week is demonstrated in data from 10-year columns. This long-term neutralisation is not measured at present in any accepted assessment tests. Methods to estimate ANRnc, based on silicate mineralogy and solution assays from long-term column leach tests, are compared. Good agreement is demonstrated between rates measured from the solution assay data and those calculated from mineralogy using kinetic databases. More rigorous analysis of the leachate chemistry of selected long-term leach tests also suggests possible cover design criteria based on the maximum AGR that will maintain a pH > 4 in leachate from ARD materials. The data show a distinct break at an AGR of 3 mg H2SO4/kg/week, below which no leachate pH is less than 4. The results indicate that an AGR of 10 t H2SO4/ha/year is conservative and a suitable cover design target for ARD control that would be matched by ANRnc. © 2010 Elsevier B.V. All rights reserved.


Smart R.S.C.,University of South Australia | Miller S.D.,P.A. College | Stewart W.S.,P.A. College | Rusdinar Y.,PT Freeport Indonesia Ltd | And 3 more authors.
Science of the Total Environment | Year: 2010

The result of leaching of a 75% acid rock/25% limestone column with limestone-saturated solution has shown that the pH of the effluent recovered from 2.5, after apparent loss of acid neutralizing capacity after 4. years with water leaching, to pH 7 in less than 3. years. Bulk assay results, XRD and SEM/EDS analyses of samples from the column at 384. weeks (pH 3.6) and 522. weeks (pH 6.9) during this recovery have suggested that this is due to formation in situ of fine calcite. Calcite, initially blended to the column material at 25. wt.% was not found in the XRD of the 384. week sample but is clearly found in the 522. week XRD. This increased calcite content appears to be derived from the limestone-saturated water as finely divided solid precipitated in the drying cycles in the column. This result is confirmed by assessment of the 522. week sample as non-acid forming. Loss of some reactive aluminosilicate minerals, formation of secondary, precipitated, surface-attached gypsum and loss of fine secondary jarosite occurs across this pH range but fine, surface-attached jarosite is still found in the 522. week sample implying relatively slow dissolution kinetics. In comparison with the 384. week sample, armouring of highly reacted pyrite particles by surface layers of iron oxyhydroxides and aluminosilicates has become more extensive at 522. weeks after return of the pH to neutral values. This is consistent with results from Freeport field samples from limestone blended test pads where pyrite armouring was also substantially increased at higher pH. The results suggest that it may be possible to effectively maintain neutral pH and passivate pyrite, reducing oxidation rates by more than an order of magnitude, using limestone-saturated solution dump feed rather than bulk limestone blending or covers. © 2010 Elsevier B.V.

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