Molten salt

Summary

Molten salt is salt which is solid at standard temperature and pressure but has become liquid due to elevated temperature. Regular table salt has a melting point of 801 °C (1474 °F) and a heat of fusion of 520 J/g. A salt that is normally liquid even at standard temperature and pressure is usually called a room-temperature ionic liquid, and molten salts are technically a class of ionic liquids. Uses Molten salts have a variety of uses. Molten chloride salt mixtures are commonly used as quenching baths for various alloy heat treatments, such as annealing and martempering of steel. Cyanide and chloride salt mixtures are used for surface modification of alloys such as carburizing and nitrocarburizing of steel. Cryolite (a fluoride salt) is used as a solvent for aluminium oxide in the production of aluminium in the Hall-Héroult process. Fluoride, chloride, and hydroxide salts can be used as solvents in pyroprocessing of nuclear fuel. Molten salts (fluoride, chlorid

Official source

https://en.wikipedia.org/wiki/Molten_salt

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Aluminum is produced by the electrolytic reduction of alumina dissolved in molten cryolite. This process requires carbon anodes which are consumed during the electrolysis. The anodes contribute roughly with 15% to the total aluminum production cost. If the anode quality is poor the share can as well reach 25 %. It is known that the anode behavior during aluminum electrolysis is significantly influenced by the anode baking process. However, there is a lack of knowledge to predict the anode behavior through given baking parameters. In addition, process control of industrial furnaces is still to a large degree empirical. Often the emission of industrial furnaces causes problems as deposits can accumulate in the exhaust system which leads to fire incidents. This investigation has three major goals: The prediction of the anode behavior during electrolysis for a given heat treatment. The understanding of the process control of industrial scale open-top anode baking furnaces. The emission control of the open-top anode baking furnace. The baking parameters are defined in this thesis as the anode heat-up rate and the baking level. Through pilot baking it is shown that the baking level is basically determined by the final baking temperature although the soaking duration has some influence too. The anode baking level can be estimated with a calculation for any given baking curve. Therefore the impact of the temperature and soaking duration on the real density anode property needs to be determined through a pilot study. The optimum baking parameters regarding the anode behavior during electrolysis depend on the material. Some anode materials react more sensitive to the heat treatment than others do and are therfore more difficult to handle in the industrial baking process. Through pilot scale anode baking it is shown how to predict the anode behavior during electrolysis. Process control of the anode baking in industrial open-top furnaces is complex since the anode baking is indirectly controlled by the combustion process inside the flues. Through measurements the combustion process is analyzed in this thesis. The results show that a furnace operated at its physical limits is difficult to control regarding to emissions. However, the gained knowledge allows to optimize existing furnace process control systems. The furnace emissions are mainly caused by insufficient combustion of the pitch fumes formed during the pyrolysis of the anode binder pitch. It is shown that emission problems cannot always be solved by a well chosen control system setup. In such a situation combustion control is required in addition to temperature control. A technology has been developed to measure online the combustion situation of each flue. For this purpose 2-color pyrometers, together with an intelligent signal processing, are required. This technology is integrated since 10 months in an industrial baking furnace control system. The results are promising. The developed technology is an important step for furnace operation at maximum productivity and minimum emission. A patent has been applied for the combination of online combustion analysis concurrent with temperature measurement (swiss apply No. 01598/02 by Mauriz Lustenberger and Felix Keller, R&D Carbon Ltd. Sierre).

EPFL2004

Room-temperature molten salt electrolytes for photoelectrochemical solar cells

Ulf Anders Hagfeldt

In 1991, Gratzel and co-workers published promising results from dye-sensitized, nanocryst., photoelectrochem. solar cells (DNSCs), with huge internal surface areas. A Gratzel cell consists of a dye-sensitized TiO2-electrode, a platinized counter electrode and an electrolyte contg. a redox pair of iodide/triiodide ions. Room-temp. molten salts have qualities, such as high cond., ionic mobility, thermal stability and negligibly volatility, which make them suitable as electrolytes for solar cells. Two systems of potential electrolytes have been successfully synthesized and analyzed, based on the redox pair I-/I3- using different cations: 1. trialkylsulfonium iodides, (R2R'S)I 2. trialkylsulfonium iodides with addn. of metal iodides The capability to act as electrolytes in DNSCs was investigated by measuring the overall light-to-elec. energy conversion efficiency in 100-1000 W/m2 illumination (AM 1.5) using a solar simulator, and also the incident monochromatic photon-to-current conversion efficiency (IPCE). Results showed that liq., iodine-doped trialkylsulfonium iodide-salts work very well as electrolytes in DNSCs.

American Chemical Society2004

Model and study of a solar tower power plant using molten salt and Rankine cycle

Thomas Wicht

In the present paper, a model of a solar tower power plant (STPP) is presented. This is the result of a Master semester project led at the Laboratory for Industrial Energetic Systems (LENI) at the Swiss Federal Institute in Lausanne (EPFL). The modelized plant uses molten salt as heat transfer fluid (HTF), within a circuit including a direct storage system. The hot salt is used to provide power to a conventional Rankine cycle based on steam. For this first step a Rankine cycle without any improvement is modelled, in order to provide a simple running model being subject to improvements later. The model is realized using the program Vali, allowing to submit it to a multicriteria optimization algorithm, aiming the best feasible solution seen from a thermo-economic point of view. This report is exposing the work done on the model. The modelling of certain details is carefully described, then the operating parameters used for the model validation are presented. The results are then exposed in details, including several different strategies for the usage of the stored mass. A discussion of the presented results is then led, the results being compared to those of an unknown solar power plant (Solar Two). Finally, the major points that are subject to improvements are listed and the further work to be done is detailed.

2009

EPFL2004