Abstract: A solution to be evaporated is passed through one or several evaporation stages. In each evaporation stage, the solution is circulated between a heat exchanger and an evaporating vessel, and steam is withdrawn from the evaporating vessel. For removing crusts, the evaporation in the respective evaporation stage is interrupted, and a cleaning suspension containing at least 3 wt % granular solids is circulated through the heat exchanger and the evaporating vessel. The solids, which have an abrasive effect, remove the crusts. Preferably, the solution to be evaporated is circulated through the evaporation stage for removing the crusts, where the heat exchanger is heated and the cleaning solution is evaporated to such an extent that solids crystallize out and a cleaning suspension is formed.

Abstract: An aqueous ammonium alum solution which contains Fe.sup.3+ ions and has a pH not in excess of 5 is passed through at least one half of the Fe.sup.3+ ions in the solution to Fe.sup.2+ ions. The solution is cooled and ammonium alum is crystallized whereas the Fe.sup.2+ ions remain in the liquid phase.

Abstract: The process and apparatus described seeks to prevent NO.sub.x emissions after an emergency shutdown in a process for manufacturing nitric acid by catalytic combustion of ammonia, compression of the nitrous combustion gases and subsequent absorption. Immediately on shutdown the intake nitrous gas supply to the compressor is interrupted, the residual intake side nitrous gases are conveyed to the delivery side, the delivery side gas volume is shut off, the gas on the delivery side is expanded to the intake side and then exhausted into a vacuum system. Subsequently the acid charged with NO.sub.x in the absorption stages is drawn off, degassed acid is circulated through the absorption stages until equilibrium is established, the pressure of the shutoff section is released and cooled acid is admitted to the absorption stages.

Abstract: A process and a plant are disclosed whereby nitric acid is produced by the catalytic combustion of ammonia with air to form nitrous gases which are contacted with water in an absorption zone to form nitric acid. Gas flow into and through the plant is maintained by a compressor system powered by turbines that are propelled by the heat energy released during ammonia combustion. In accordance with the present invention, a portion of the ammonia combustion heat energy is stored using heat accumulating means positioned at various possible locations throughout the system. Such stored heat provides a thermal energy reserve available to power the compressor turbines for a short period of time in the event that ammonia combustion is discontinued.