Chemistry of Stibnite, Orpiment and Other Sulfide Minerals Deposited from GeOthermal Brine

Abstract

Geothermal brines usually contain little arsenic and even less antimony, but deposits containing Sb or As are sometimes encountered. The chemical processes that produce these deposits are fundamentally different from processes that produce other, more common deposits, and they pose threats to plant personnel and the environment due to their toxic nature. Arsenic sulfides rarely form deposits because they are more soluble than stibnite (Sb2S3). However, As resembles Sb, and knowledge of the processes that produce deposits of stibnite and options to control them apply to deposits of As-Sulfides as well. Geothermal brine often contains dissolved Sb in chemical equilibrium with stibnite or another Sb-Sulfide mineral present in the reservoir rock. If the brine is cooled without separating steam, stibnite, which is less soluble at low temperature, will deposit in the heat exchanger. Separating steam from the brine removes most of the H2S; Sb remains in the brine as the hydroxide Sb(OH)3(aq). Further cooling of the brine does not cause precipitation of stibnite, a process that involves reaction of Sb(OH)3(aq)with H2S. Binary cycle powerplants that receive 2-phase geothermal fluid typically condense the steam in a Vaporizer unit, and add the condensate produced to the separated brine entering the Preheater unit. The condensate contains a substantial amount of dissolved H2S, and adding it to the brine causes stibnite to precipitate in the preheater. Deposits of arsenic sulfide are sometimes produced in the same manner at the same location. Results of modeling the chemistry and formation of Sb- and As-Sulfide deposits in a representative powerplant of this common configuration will be presented for a range of brine compositions and operating conditions. Suitable configuration of the power cycle and choice of operating conditions can prevent deposits containing Sb or As in a predictable manner; in particular, the configuration and/or operating parameters of the Vaporizer can be modified to decrease the amount of H2S that dissolves in the condensate. These deposits can also be controlled using familiar scale control chemicals in cases where an existing power cycle cannot be modified.