Evaluation of Acid/Base Accounting Techniques Used in Surface-Mine Reclamation

Abstract

Abstract. Surface mine reclamation laws in Texas permit the use of mixed overburden as a topsoil substitute. Overburden material must have a positive acid/base account (ABa) to remain within 4 feet of the reclaimed surface. Acid/base account is calculated by subtracting potential acidity (PA) plus exchangeable acidity (EA) from the inherent acid neutralization potential (NP) of the material. Material with a negative ABa may be limed to increase the AB a value. Two separate research studies were conducted to determine 1) the effects of siderite (FeC03), a frequently identified mineral in mixed overburden, on NP determination and 2) the long-term pH stability of potentially acidic mixed overburden limed to an AB a of zero. Siderite stoichiometrically weathers to produce acidity upon complete oxidation and hydrolysis of released ferrous iron. Siderite contributes to NP because of incomplete iron hydrolysis under the conditions used to determine NP in the laboratory. This "siderite error" can lead to a false positive AB a value when the current NP procedures are used on samples containing significant quantities of PA and siderite. The long-term pH stability of potentially acidic mixed overburden limed with CaC03 was evaluated using a simulated weathering study. The liming rates ranged from Oto 125% of the theoretical amount of lime needed to neutralize the measured AB a deficit. The data showed the addition of CaC0:3 significantly slowed the oxidation of FeS2-however, the dissolution of the applied lime proceeded at a steady rate with each percolating water front. The dissolution of applied lime was measured to be significantly faster than the release of PA in all treatments, thus the ABa values of all the treatments were found to decrease with time. Treatments which received less than a 50% lime rate became highly acidic within one year. The results indicate that all treatments would eventually become highly acidic.

Neutralization Potential Determination of Siderite (FeCO3) Using Selected Oxidants

“…Neutralization potential measures easily weatherable minerals (i.e., olivine and micas), including carbonates, present in an overburden (Doolittle et al, 1992). The NP of an overburden is determined using USEPA Method 600/2‐78‐054 3.2.3 (Sobek et al, 1978).…”

“…Siderite affects NP because Fe 2+ released during acid digestion does not undergo complete oxidation and hydrolysis (Eq. [4] and [5]) during NP evaluation (Doolittle et al, 1992; O'Shay et al, 1990).…”

“…If the ABA in negative, the overburden may produce acid mine soils and/or acid drainage and should be buried more than 1.2 m (4 feet) below the surface. Neutralization potential measures easily weatherable minerals (i.e., olivine and micas), including carbonates, present in an overburden (Doolittle et al, 1992). The NP of an overburden is determined using USEPA Method 600/2-78-054 3.2.3 (Sobek et al, 1978).…”

Environmental isues of mining activities in Tay Nguyen

“…Đối với các mỏ khoáng sản mà trong thành phần quặng hoặc đất đá vây quanh có chứa các khoáng vật sulfid (ví dụ pyrit), chẳng hạn như vàng, Pb-Zn và một số loại khoáng sản khác, khả năng tạo dòng thải axit mỏ là rất lớn (Doolittle, et al, 1992; Global Acid Rock Drainage Guide). Đơn cử như trường hợp mỏ vàng Đắk Ripen ở Kon Tum.…”

Modified Np Testing for Lignite Overburden Samples in Texas

“…However, continued weathering of FeC0 3 produces a neutral to slightly acidic solution. Doolittle et al (1992) concluded that FeC0 3 affected NP because the Fe 2 + did not undergo complete oxidation and hydrolysis.…”