Removal of Metals by Sulphide Precipitation Using Na2S and HS−-Solution

Abstract

Precipitation of metals as metal sulphides is a practical way to recover metals from mine water. Sulphide precipitation is useful since many metals are very sparingly soluble as sulphides. Precipitation is also pH dependent. This article investigates the precipitation of metals individually as sulphides and assesses which metals are precipitated as metal hydroxides by adjustment of the pH. The precipitation of different metals as sulphides was studied to determine the conditions under which the HS− solution from the sulphate reduction reaction could be used for precipitation. H2S gas and ionic HS− produced during anaerobic treatment could be recycled from the process to precipitate metals in acidic mine drainage (AMD) prior to anaerobic treatment (Biological sulphate reduction), thereby recovering several metals. Precipitation of metals with HS− was fast and produced fine precipitates. The pH of acid mine water is about 2–4, and it can be adjusted to pH 5.5 before sulphide precipitation, while the precipitation, on the other hand, requires a sulphide solution with pH at 8 and the sulphide in HS− form. This prevents H2S formation and mitigates the risk posed from the evaporation of toxic hydrogen sulphur gas. This is a lower increase than is required for hydroxide precipitation, in which pH is typically raised to approximately nine. After precipitation, metal concentrations ranged from 1 to 30 μg/L.

Passive removal of sulphate and heavy metals from acid mine drainage using sewage sludge and fly ash

“…Whilst sulphide precipitation according to equations ( 1) and ( 2) correlated with strong smell H2S releases, during experiment was only detected in M1 and M3 (Table 4). It confirmed that in M2 reactors IOP Publishing doi:10.1088/1755-1315/1201/1/012076 7 with pH reached 8.7, the sulphide precipitation was weak and hydroxide precipitation as the primary mechanism [21]. Ammonia was detected high all reactors water except water from M4 and M5.…”

Metal Extraction and Recovery from Mobile Phone PCBs by a Combination of Bioleaching and Precipitation Processes

“…The precipitation efficiency recorded for Cu, Zn and Ni sulfides was 99.9%, 99.4 % and 99.0%, respectively. An additional advantage of increasing the pH with NaOH before the sulfide precipitation was avoiding the generation of gaseous H 2 S, widely known as a flammable, corrosive and highly toxic compound [56].…”

Assessing the Capability of Chemical Ameliorants to Reduce the Bioavailability of Heavy Metals in Bulk Fly Ash Contaminated Soil

“…This noticeably immobilizes the bio-available fraction of HMs and reduced their accumulation in the system [39][40][41]. On the other side, the application of Na 2 S performs this job in two mechanistic ways: first, by forming sodium hydroxide (NaOH) and hydrogen sulphide (H 2 S) in the system which immobilizes HMs as metal sulphide ensuing the reduction with H 2 S followed by precipitation with NaOH [42], and second, by producing hydroxides during the dissolution process which in turn form metal hydroxide and increases pH. This is in the line of work of Lewis [43] who detected such a mechanism while working with cadmium.…”