AU2010340924A1

AU2010340924A1 - Mixtures of alkali polysulfides

Info

Publication number: AU2010340924A1
Application number: AU2010340924A
Authority: AU
Inventors: Martin Gartner; Gunther Huber; Michael Lutz; Otto Machhammer; Felix Major; Stephan Maurer; Kerstin Schierle-Arndt; Fabian Seeler; Hans-Josef Sterzel; Jurgen Wortmann
Original assignee: BASF SE
Current assignee: BASF SE
Priority date: 2010-01-05
Filing date: 2010-12-23
Publication date: 2012-08-30
Also published as: CA2785174A1; CL2012001795A1; WO2011083054A1; TN2012000337A1; CN102712474A; IL220543A0; JP2013516531A; KR20120104423A; BR112012016656A2; MX2012007392A; EP2521692A1; MA33949B1

Abstract

The present invention relates to mixtures of alkali polysulfides and mixtures of alkali polysulfides and alkali thiocyanates, to a method for the production of same, to the use thereof as heat transfer media or heat storage media, and to heat transfer media or heat storage media comprising the mixtures of alkali polysulfides or the mixtures of alkali polysulfides and alkali thiocyanates.

Description

Mixtures of alkali polysulfides Description 5 The present invention relates to mixtures of alkali metal polysulfides and to mixtures of alkali metal polysulfides and alkali metal thiocyanates, to processes for preparation thereof, to the use thereof as heat transfer or heat storage fluids, and to heat transfer or heat storage fluids which comprise the mixtures of alkali metal polysulfides or the mixtures of alkali metal polysulfides and alkali metal thiocyanates. 10 Fluids for transferring thermal energy are used in various fields of industry. In internal combustion engines, mixtures of water and ethylene glycol convey the heat of combus tion into the radiator. Similar mixtures convey the heat from solar roof collectors into heat stores. In the chemical industry, they convey the heat from electrical or fossil-fuel 15 heating systems to chemical reactors or out of the latter to cooling apparatus. According to the field of use, the profile of requirements for heat transfer or heat stor age fluids varies very greatly, and therefore a multitude of fluids is used in practice. The fluids should be liquid and have low viscosities at room temperature or even lower 20 temperatures. For higher use temperatures, water is no longer an option; its vapor pressure would become too great. Therefore, hydrocarbon-based mineral oils are used up to approximately 320*C, and synthetic aromatics-containing oils or silicone oils for temperatures up to 400*C (VDI W~rmeatlas, VDI-Gesellschaft Verfahrenstechnik und Chemieingenieurwesen, Springer Verlag Berlin Heidelberg 2006). 25 A new challenge for heat transfer fluids is that of thermal solar power plants, which generate electrical power on a large scale (Butscher, R., Bild der Wissenschaft 2009, 3, pages 84 to 92). To date, such power plants have been built with an installed power of a few hundreds of MW, and many others are planned, especially in Spain, but also in 30 North Africa and the USA. The solar radiation is focused, for example, by means of parabolically shaped mirror troughs into the focus line of the mirrors. At the focus line is a metal tube present within a glass tube to prevent heat losses, the space between the concentric tubes being evacuated. A heat transfer fluid flows through the metal tube. Currently, a mixture of diphenyl ether and diphenyl is used here. The heat transfer 35 agent is heated to a maximum of 400 0 C, and is used to operate a steam generator in which water is evaporated. This steam drives a turbine and this in turn drives the gen erator as in a conventional power plant. Thus, peak efficiencies of approximately 30 percent are achieved, based on the energy content of the solar irradiation. The effi ciency of the steam turbines at this entrance temperature is approximately 37 percent. 40 Both constituents of the mixture of diphenyl ether and diphenyl used as a heat transfer agent boil at approximately 256 0 C under standard pressure. The melting point of 2 diphenyl is 68-72*C, and that of diphenyl ether 26-390C. The mixing of the two sub stances lowers the melting point to 120C. The mixture of the two substances can be used up to a maximum of 400*C; at higher temperatures, decomposition occurs. The steam pressure is about 10 bar at this temperature, a pressure which is still tolerable in 5 industry. In order to obtain higher turbine efficiencies than 37 percent, higher steam inlet tem peratures are necessary. The efficiency of a steam turbine rises with the turbine inlet temperature. Modern fossil fuel-fired power plants work with steam inlet temperatures 10 up to 6500C and thus achieve efficiencies around 45%. It would be technically entirely possible to heat the heat transfer fluid in the focus line of the mirrors to temperatures around 650"C and thus likewise to achieve such high efficiencies; this, however, is pre vented by the limited thermal stability of the currently used heat transfer fluid. 15 Higher temperatures than in parabolic trough power plants can be achieved in solar tower power plants, in which one tower is surrounded by mirrors which focus the sun light onto a receiver in the upper part of the tower. In this receiver, a heat transfer agent is heated, which is utilized to raise steam by means of a heat exchanger and to operate a turbine. In tower power plants (Solar II, California), a mixture of sodium nitrate 20 (NaNO 3 ) and potassium nitrate (KNO 3 ) (60:40) has already been used as a heat trans fer. This mixture can be used up to 5500C without any problem, but has a very high melting point of 2400C. There are to date no known organic substances which permanently withstand tempera 25 tures above 4000C without decomposition. Some dimethylsilicone- or diphenylsilicone based oils can likewise be used up to temperatures of 400 0 C or even at somewhat higher temperatures. However, the very high cost thereof opposes the use thereof in thermal solar power plants. 30 Another option is the use, known from nuclear technology, of liquid sodium or sodium potassium alloy as a heat transfer agent. However, the preparation of these metals is very expensive, and they react with traces of water to give hydrogen gas, which consti tutes a safety challenge. 35 In addition, low-melting solder metals, for example Wood's metal (Bi-Pb-Cd-Sn alloy, melting point approximately 75*C), are known. However, the very high specific weight opposes use as a heat transfer fluid. Further possible high-temperature heat transfer agents based on sulfur have been pro 40 posed, which is used, for example, in a mixture with smaller amounts of selenium and/or tellurium (WO 2005/071037). Liquid sulfur is problematic as a heat transfer agent since it has high viscosity in the range of 150 to 2000C and cannot be pumped in 3 this form. The viscosity can be reduced by additives such as bromine or iodine (US 4 335 578), but they are highly corrosive. It is technically also possible to use water under correspondingly high pressure. This is 5 opposed, however, by the extremely high vapor pressure of more than 270 bar at tem peratures of more than 500*C, which would make the many kilometers of pipelines in a thermal solar power plant uneconomically expensive. The steam itself, as a heat trans fer agent, is disadvantageous owing to its comparatively low thermal conductivity and the low heat capacity per unit volume compared to a liquid. 10 A further option is the use of inorganic salt melts as heat transfer fluid. Such salt melts are state of the art in processes which work at high temperatures. The eutectic mixture of potassium nitrate, sodium nitrate and sodium nitrite has a melting point of 1460C and is commercially available. However, the upper use temperature is limited to 4500C, 15 since considerable decomposition of the nitrite to nitrous gases, alkali metal oxides and elemental nitrogen takes place above this temperature. The eutectic mixture of sodium nitrate and potassium nitrate can be used up to temperatures of 600*C. However, the use of this mixture as a heat transfer fluid in solar power plants is problematic owing to the high melting point of approx. 2200C. Lowering of the temperature below the melting 20 point, for example in the night or during periods of low solar irradiation, would result in solidification of the salt in the pipelines. This has to be prevented because local stress es would arise in the course of remelting, which would result in damage to the plant. Antifreeze protection in the form of trace heating would be conceivable, but is techni cally very difficult to implement and additionally expensive for such high temperatures. 25 The melting point of the mixture of sodium nitrate and potassium nitrate can be lowered by adding lithium nitrate or calcium nitrate (Bradshaw, R. W., Meeker, D. E., Solar En ergy Materials 1990, Vol. 21, page 51 to 60). However, mixtures with lithium nitrate are uneconomic owing to the high cost, while the presence of calcium promotes the de composition of the nitrate to nitrite and oxygen, and hence the upper application tem 30 perature is lowered ever further with rising calcium content. Furthermore, the use of metal halides as a heat transfer fluid would be possible. The problem arises here that halogenated fluids, especially at elevated temperatures, often cause corrosion problems for the metallic materials to be used. 35 Mixtures of alkali metal polysulfides, especially of sodium and potassium polysulfides, should theoretically have low melting points and could be usable at temperatures of up to 500*C and higher. The phase diagram for the ternary sodium sulfide-potassium sul fide-sulfur system should, according to calculations, have invariant points with low melt 40 ing temperatures for the compositions Ko.8 4 Nao.

2 6 S3.

6 1 (78 *C), Ko.77Nao.

23 S3.

7 5 (73*C) and Ko.

7 Nao.

2 1

S

3

.

95 (830C) (Lindberg, D., Backman, R., Hupa, M., Chartrand, P., J. Chem. Therm. 2006, vol. 38, pages 900 to 915). There are no experimental data for 4 this ternary system. In the potassium sulfide-sulfur system, the melting point can be lowered to approximately 1200C (Sangster, J., Pelton, A. D., J. Phase Equil. 1997, vol. 18, page 82). One disadvantage of the alkali metal polysulfides is the relatively high viscosity thereof in the molten state, especially that of sodium polysulfides (Cleaver, B., 5 Davis, A. J., Electrochimica Acta 1973, vol. 18, pages 727 to 731). DE 3824517 describes the use of mixtures of the alkali metal thiocyanates as heat transfer fluids, especially of potassium thiocyanate and sodium thiocyanate. Potassium thiocyanate melts at 173 0 C, sodium thiocyanate at 3100C. The eutectic mixture of the 10 two salts with a ratio of 73 mol% of potassium thiocyanate to 27 mol% of sodium thiocyanate has a melting point around 130*C. The melt is of low viscosity and hence pumpable without increased energy expenditure. One disadvantage of the alkali metal thiocyanates is that they already begin to decom 15 pose at temperatures above 450'C. With the exclusion of sulfur, the higher-melting alkali metal cyanides are formed (Gmelins Handbuch der Anorganischen Chemie 1938, vol. 22, page 899). The melting point of the alkali metal thiocyanates can be lowered further by adding 20 further salts. Especially the addition of nitrites or nitrates lowers the melting point. However, the addition of the oxidizing nitrites or nitrates at elevated temperature caus es an explosive decomposition, which can additionally be accelerated by any dissolved heavy metal traces. The use of such mixtures for industrial use is therefore ruled out. 25 A further problem arises from the fact that the aim is to operate a solar power plant continuously. This can be achieved by storing heat during periods of high solar irradia tion, which can be utilized for power production after sunset or during phases of poor weather. Heat can be stored directly by storing the heated heat transfer medium in well-insulated reservoir tanks, or indirectly by transferring heat to another storage me 30 dium. The indirect method is implemented in the 50 MW Andasol I power plant in Spain, where approx. 28 000 t of a melt of sodium nitrate and potassium nitrate (60:40; wt.-%) are used. The melt is pumped during the periods of solar irradiation from a colder tank 35 (approximately 280*C) through an oil-salt heat exchanger into a hotter tank, in the course of which it is heated to about 380*C. In periods of low solar irradiation and at night, the power plant can be run under full load with the store fully charged for about 7.5 h (www.solarmillennium.de/upload/Download/Technologie/Andasol1-3deutsch.pdf). However, it would be advantageous also to use the heat transfer fluid as a storage flu 40 id, since it would thus be possible to dispense with the corresponding oil-salt heat ex changers. This is not being considered to date owing to the high vapor pressure of the oil and the high cost compared to the nitrate salts.

5 It is an object of the invention to provide a readily available, improved heat transfer and heat storage fluid. The fluid should be usable at higher temperatures than 400*C, pref erably above 500*C. At the same time, the melting point should be at a minimum, pref 5 erably below 200 0 C. The liquid should additionally have a techically controllable, mini mal vapor pressure, preferably less than 10 bar. The object is achieved in accordance with the invention by mixtures of alkali metal polysulfides. 10 The invention therefore provides mixtures of alkali metal polysulfides of the general formula

(M

1 xM 2 (1-x)) 2 Sy 15 where M 1 , M 2 = Li, Na, K, Rb, Cs, and M 1 is not the same as M 2 , and 0.05 5 x 5 0.95 and 2.0 5 y 5 6.0. In a preferred embodiment of the invention, M 1 = K and M 2 = Na. 20 In a further preferred embodiment of the invention, 0.20 5 x 5 0.95. In a particularly preferred embodiment of the invention, 0.50 5 x S 0.90. In a further preferred embodiment of the invention, 3.0 5 y 5 6.0. In a particularly pre 25 ferred embodiment of the invention, y = 4.0, 5.0 or 6.0. In a particularly preferred embodiment of the invention, M 1 = K, M 2 = Na, 0.20 5 x s 0.95 and 3.0 5 y 5 6.0. 30 In a very particularly preferred embodiment of the invention, M 1 = K, M 2 = Na, 0.50 5 x 5 0.90 and y = 4.0, 5.0 or 6.0. A further embodiment relates to alkali metal polysulfides of composition (K(lx)Nax) 2 Sz, with x = 0 up to 1 und z = 2.3 up to 3.5, preferably x = 0.5 up to 0.7 and z = 2.4 up to 35 2.9. A further embodiment relates to alkali metal polysulfides (Nao.5-c.6 5 Ko.

5

-

0

.

3 5

)

2 2.4 -2.a or such having the composition (Nao.

6 Ko.

4

)

2

S

2

.

6 . 40 The inventive mixtures are notable for particularly low melting points. In a preferred embodiment of the invention, the melting point of the inventive mixture is below 200*C, and in a particularly preferred embodiment below 160 0

C.

6 The inventive mixtures have a high thermal stability. In a preferred embodiment of the invention, the inventive mixtures are stable up to a temperature of 4500C, in a particu larly preferred embodiment up to a temperature of 500*C, and in a very particularly 5 preferred embodiment even at temperatures above 500*C. In a preferred embodiment of the invention, the inventive mixtures at 5000C have a vapor pressure of below 5 bar, more preferably of below 2 bar. 10 The preparation of alkali metal polysulfides is known and can be effected, for example, by reaction of alkali metal sulfides with sulfur. One alternative is the direct reaction of alkali metals with sulfur, as described in US 4 640 832 for sodium. The reaction of alka li metals in liquid ammonia with sulfur has likewise been described. A further synthesis option is the reaction of alkali metal hydrogensulfides or alkali metal sulfides with sulfur 15 in alcoholic solution. The invention further provides a process for preparing the inventive mixtures of alkali metal polysulfides, which comprises heating corresponding alkali metal sulfides with sulfur or corresponding alkali metal polysulfides with or without sulfur, under protective 20 gas or under reduced pressure. In a preferred embodiment of the process according to the invention, the starting mate rials are heated to at least 4000C for at least 0.5 hour. 25 Suitable protective gases are noble gases, preferably argon, or nitrogen. The invention further provides a process for preparing the inventive mixtures of alkali metal polysulfides, which comprises reacting a solution of corresponding alkali metals in liquid ammonia with sulfur under protective gas. 30 The invention further provides for the use of the inventive mixtures of alkali metal polysulfides as heat transfer or heat storage fluids. In a preferred embodiment of the invention, the inventive mixtures of alkali metal 35 polysulfides are used with exclusion of air and moisture, preferably in a closed system of, for example, pipelines, pumps, control units and vessels, in order to prevent hydro lytic reactions or the oxidation of the heat transfer or heat storage fluid in the course of operation. 40 The invention further provides heat transfer or heat storage fluids which comprise the inventive mixtures of alkali metal polysulfides.

7 The field of application of the inventive mixtures of alkali metal polysulfides can be ex tended further when they are mixed with alkali metal thiocyanates. The invention further provides mixtures of alkali metal polysulfides and alkali metal 5 thiocyanates of the general formula ((M'xM2(1--x))2Sy)m (M3zM4(1-.z)SCN)(1--m) where M 1 , M 2 , M 3 , M 4 = Li, Na, K, Rb, Cs, and M 1 is not the same as M 2 , M 3 is not the 10 same as M 4 , and 0.05:s x 5 1, 0.05:s z s 1, 2.0s y s6.0, and m is the molar propor tion, where 0.05 5 m 5 0.95. In a preferred embodiment of the invention, M 1 and M 3 = K and M 2 and M 4 = Na. 15 In a further preferred embodiment of the invention, 0.20 x 5 1. In a particularly pre ferred embodiment of the invention, 0.50 s x s 1. In a further preferred embodiment of the invention, 3.0 5 y : 6.0. In a particularly pre ferred embodiment of the invention, y = 4.0, 5.0 or 6.0. 20 In a further preferred embodiment of the invention, 0.20 5 z 1. In a particularly pre ferred embodiment of the invention 0.50 s z s 1. In a further preferred embodiment of the invention, 0.20 s m s 0.80. In a particularly 25 preferred embodiment of the invention, 0.33 s m 5 0.80. In a particularly preferred embodiment of the invention, M 1 and M 3 = K, M 2 and M 4 = Na, 0.

2 05 x:5 1, 0.20:5 z:5 0.95, 3.0:5 y5 s6.0 and 0.205 m s 0.95. 30 In a very particularly preferred embodiment of the invention, M 1 and M 3 = K, M 2 and M 4 = Na, 0.50 s x s 1, 0.50s5 z5 s0.95, y = 4.0, 5.0 or 6.0 and 0.33:s m:5 0.80. In a further particularly preferred embodiment of the invention, M 1 and M 3 = K, x = 1, z = 1, y = 4.0, 5.0 or 6.0 and 0.33 s m 5 0.80. 35 In a further particularly preferred embodiment of the invention, M 1 and M 3 = K, x = 1, z = 1, y = 4 and m = 0.5. In a further particularly preferred embodiment of the invention, M 1 and M 3 = K, x = 1, z 40 = 1, y = 5 and m = 0.5.

8 In a further particularly preferred embodiment of the invention, M 1 and M 3 = K, x = 1, z = 1, y = 6 and m = 0.5. It has been found that, surprisingly, the inventive mixtures of alkali metal polysulfides 5 and alkali metal thiocyanates are more thermally stable than the alkali metal thiocyanates alone. In addition, the viscosity of the inventive mixtures of alkali metal polysulfides and alkali metal thiocyanates is lower than that of the alkali metal polysul fide mixtures without alkali metal thiocyanates. 10 The preparation of alkali metal thiocyanates is known and is performed on the industrial scale. The invention further provides a process for preparing the inventive mixtures of alkali metal polysulfides and alkali metal thiocyanates by co-melting alkali metal polysulfides 15 and alkali metal thiocyanates. The process can also be performed while stirring the melt. The inventive mixtures of alkali metal polysulfides and alkali metal thiocyanates are generally suitable for high-temperature applications which require a heat transfer agent 20 with a broad liquid temperature range. The invention further provides for the use of the inventive mixtures of alkali metal polysulfides and alkali metal thiocyanates as heat transfer or heat storage fluids. 25 In a preferred embodiment of the invention, the inventive mixtures of alkali metal polysulfides and alkali metal thiocyanates are used with exclusion of air and moisture, preferably in a closed system of, for example, pipelines, pumps, control units and ves sels, in order to avoid hydrolytic reactions or the oxidation of the heat transfer or heat storage fluid in the course of operation. 30 The invention further provides heat transfer or heat storage fluids which comprise the inventive mixtures of alkali metal polysulfides and alkali metal thiocyanates. Examples: 35 1. Synthesis of sodium-potassium polysulfides (KxNa 1

-)

2 Sy a) By melting mixtures of alkali metal polysulfides and sulfur 40 The K 2

S

3 and Na 2

S

4 starting materials were prepared by literature methods.

9 Synthesis of Nao.

4 64K 1

.

5 36

S

3

.

745 3.51 g of K 2

S

3 , 0.43 g of sulfur and 1.06 g of Na 2

S

4 were heated to 400C in a closed, evacuated quartz glass ampoule for 30 minutes, and the melt was then cooled to room temperature. The ampoule was opened in an argon glovebox and the red to reddish 5 yellow solid was pulverized by pestling (quantitative yield). The solid melts in the range of 151-157*C. Synthesis of Nao.

42

K

1 .58S 3

.

80 3.65 g of K 2

S

3 , 0.49 g of sulfur and 0.95 g of Na 2

S

4 were heated to 4000C in a closed, 10 evacuated quartz glass ampoule for 30 minutes, and the melt was then cooled to room temperature. The ampoule was opened in an argon glovebox and the red to reddish yellow solid was pulverized by pestling (quantitative yield). The solid melts in the range of 158-1670C. 15 Synthesis of Nao.

325

K

1

.

675

S

3

.

6 1 3.87 g of K 2

S

3 , 0.38 g of sulfur and 0.75 g of Na 2

S

4 were heated to 4000C in a closed, evacuated quartz glass ampoule for 30 minutes, and the melt was then cooled to room temperature. The ampoule was opened in an argon glovebox and the red to reddish yellow solid was pulverized by pestling (quantitative yield). The solid melts in the range 20 of 157-163*C. b) By reaction of alkali metals with sulfur in liquid ammonia Synthesis of Nao.

4 6

K

1

.

54

S

3

.

7 5 25 The synthesis was carried out under an atmosphere of argon with the aid of Schlenk and glovebox techniques. 63.6 g (1.98 mol) of sulfur were initially charged in liquid ammonia at -30*C in a glass flask. Subsequently, a blue solution of 5.50 g (0.24 mol) of sodium metal and 32.0 g (0.81 mol) of potassium metal in approx. 800 ml of liquid ammonia (-300C) were added dropwise while stirring. The resulting mixture was 30 warmed to room temperature and stirred until the ammonia had evaporated. The result ing orange solid was subsequently freed of ammonia residues at 1500C under reduced pressure (approx. 1 mbar). The solid melts in the range of 166-1690C. Synthesis of Nao.

23

K

1

.

77

S

3

.

75 35 The synthesis was carried out under an atmosphere of argon with the aid of Schlenk and glovebox techniques. 43.0 g (1.34 mol) of sulfur were initially charged in liquid ammonia at -300C in a glass flask. Subsequently, a blue solution of 1.82 g (0.079 mol) of sodium metal and 24.9 g (0.63 mol) of potassium metal in approx. 800 ml of liquid ammonia (-30*C) were added dropwise while stirring. The resulting mixture was 10 warmed to room temperature and stirred until the ammonia had evaporated. The result ing orange solid was subsequently freed of ammonia residues at 1500C under reduced pressure (approx. 1 mbar). The solid melts in the range of 165-166*C. 5 2. Synthesis and properties of mixtures of (KxNa 1

-)

2 Sy with alkali metal thiocyanates a) Synthesis Method 1: 10 Corresponding amounts of potassium polysulfide (K 2 Sx) or potassium sodium polysul fide ((KxNalx) 2 Sy) and potassium thiocyanate (KSCN) were heated to 4000C in a closed, evacuated quartz glass ampoule for 30 minutes, and the melt was then cooled to room temperature. The ampoule was opened in an argon glovebox, and the fusion product was pulverized by pestling. This afforded orange solids, the melting ranges of 15 which are shown in tab. 1. Method 2: Corresponding amounts of potassium polysulfide (K 2 Sx) or potassium sodium polysul fide ((KxNa 1

-)

2 Sy) and potassium thiocyanate (KSCN) were mixed and heated to 20 1800C in a glass flask under an argon atmosphere. The mixture was stirred until a ho mogeneous melt had formed, and then cooled to room temperature. This afforded or ange solids, the melting ranges of which were identical to those of the solids prepared according to method 1 (cf. tab. 1).

Tab. 1 Composition Melting range

(K

2

S

4 )0.

67

(KSCN)

0

.

33 123-125

(K

2

S

4 )0.

50

(KSCN).

5 o 110-112

(K

2

S

4 )0.

33

(KSCN)

0

.

67 128-130

(K

2

S

5 )0.

50

(KSCN)

0

,

50 150-158

(K

2

S

6 )0.

50 (KSCN)o.

5 o 146-153 (N aO.

46 K 1 .5 ~S 3

.

75 ) o. 5 o(KS CN) o.

5 o 92-100 (NaO.

46 Kl.54S 3

.

75 )0.

45

(KSCN)

0

.

55 94-110 (NaO.23Kl.

77

S

3

.

75

)O.

67 (KSC N).

33 100-108 (NaO.

23

K,.

77

S

3 .75)0.

53

(KSCN).

47 98-102 (Nao 23 Kl.77S 3

.

75 )0.

50 (KSCN)o.

5 o 82-96 (NaO.

23

K

1

.

77

S

3

.

75 )0.

4

(KSCN)

0

.

52 80-90 (NaO.2 3

K,.

77

S

3

.

75 )o.

33 (KSCN)0.

67 80-96 12 b) Viscosity The viscosity of the melts was determined by means of rotational viscometry. 5 Tab.2 Composition Viscosity [mPa*s] 1600C 1800C

K

2

S

4 1000 390

(K

2

S

4 )o.

50

(KSCN)

0

.

50 235 113

(K

2

S

4 )o.

33

(KSCN)

0

.

67 93 46 Nao.

46

K

1 .4S 3

.

75 8400 780 (Nao.4 6

K

1 .54S3.

7 s)o.

4 7 (KSCN)o.

53 229 110 (Nao.

4 6Kl.

54

S

3

.

75

).

33 (KSCN)o.

67 103 59 Nao.

23

K

1 .77S 3

.

75 2388 752 (Nao.

23

K

1

.

77

S

3

.

75 )o.

53

(KSCN)

0

.

47 106 57 (Nao.

23

K

1

.

77

S

3

.

75 )o.so(KSCN)o.so 109 59 (Nao.

23

K

1

.

77

S

3

.

7 r 5

).

4 8(KSCN)o.

52 96 55 (Nao.

23

K

1

.

77

S

3

.

75 )o.

33

(KSCN)

0

.

67 47 28 c) Thermal stability The examination of thermal stability was examined using the mixtures 10 (K2S 4 )o.

5 (KSCN)o.

5 (melting range 110-112 *C), (K 2

S

5 )o.

5

(KSCN)

0

.

5 (melting range 150 158 0C) and (K 2

S

6 )o.s(KSCN) 0

.

5 (melting range 146-153 0 C). Stability at 400*C: 15 3 g of a mixture of the composition (K 2

S

4 )o.

5

(KSCN)

0 . were stored at 4000C in an evac uated quartz glass ampoule for 28 days. The melting range of the mixture was un changed. 3 g of a mixture of the composition (K 2

S

5 )o.

5 (KSCN)o.

5 were stored at 4000C in an evac 20 uated quartz glass ampoule for 28 days. The melting range of the mixture was un changed.

13 3 g of a mixture of the composition (K 2

S

6 )o.

5 (KSCN)o.

5 were stored at 4000C in an evac uated quartz glass ampoule for 28 days. The melting range of the mixture was un changed. 5 Stability at 450C: 3 g of a mixture of the composition (K 2

S

4 )o.

5

(KSCN)

0

.

5 were stored at 450*C in an evac uated quartz glass ampoule for 28 days. The melting range of the mixture was un changed. 10 3 g of a mixture of the composition (K 2

S

5 )o.

5 (KSCN)o.

5 were stored at 4500C in an evac uated quartz glass ampoule for 28 days. The melting range of the mixture was un changed. 15 3 g of a mixture of the composition (K 2

S

6 )o.

5 (KSCN)o.

5 were stored at 4500C in an evac uated quartz glass ampoule for 28 days. The melting range of the mixture was un changed. Stability at 500*C: 20 3 g of a mixture of the composition (K2S 4 )o.

5 (KSCN)o.

5 were stored at 5000C in an evac uated quartz glass ampoule for 28 days. The melting range of the mixture was un changed. 3 g of a mixture of the composition (K 2

S

5 )o.

5 (KSCN)o.

5 were stored at 500 0 C in an evac 25 uated quartz glass ampoule for 28 days. The melting range of the mixture was un changed. 3 g of a mixture of the composition (K 2

S

6 )o.

5 (KSCN)o.

5 were stored at 5000C in an evac uated quartz glass ampoule for 28 days. The melting range of the mixture was un 30 changed. Stability at 6000C: 3 g of a mixture of the composition (K2S 4 )o.

5 (KSCN)o.

5 were stored at 6000C in an evac 35 uated quartz glass ampoule for 28 days. The melting range of the mixture was un changed. 3 g of a mixture of the composition (K 2

S

5 )o.

5 (KSCN)o.

5 were stored at 6000C in an evac uated quartz glass ampoule for 28 days. The melting range of the mixture was un 40 changed.

14 3 g of a mixture of the composition (K 2

S

6 )o.

5 (KSCN)o.

5 were stored at 6000C in an evac uated quartz glass ampoule for 28 days. The melting range of the mixture was unchanged.

Claims

1. A mixture of alkali metal polysulfides of the general formula 5 (M 1 xM 2 (1-x))

2 Sy where M 1 , M 2 = Li, Na, K, Rb, Cs, and M 1 is not the same as M 2 , and 0.05 5 x s 0.95 and 2.0 5 y5 s6.0. 10 2. The mixture according to claim 1, wherein M 1 = K and M 2 = Na.

3. The mixture according to either of claims 1 and 2, wherein 0.20 5 x s 0.95.

4. The mixture according to any of claims 1 to 3, wherein 3.0 5 y 5 6.0. 15

5. The mixture according to any of claims 1 to 4, wherein M 1 = K, M 2 = Na, 0.20 s x s 0.95 and 3.0 s y s 6.0.

6. The mixture according to any of claims 1 to 5, wherein M 1 = K, M 2 = Na, 0.50 s x s 20 0.90 and y = 4.0, 5.0 or 6.0.

7. A process for preparing the mixtures according to any of claims 1 to 6, which comprises heating corresponding alkali metal sulfides with sulfur or corresponding alkali metal polysulfides with or without sulfur, under protective gas or under 25 reduced pressure.

8. A process for preparing the mixtures according to any of claims 1 to 6, which comprises reacting a solution of corresponding alkali metals in liquid ammonia with sulfur under protective gas. 30

9. The use of the mixtures according to any of claims 1 to 6 as heat transfer or heat storage fluids.

10. A heat carrier or heat storage fluid which comprises a mixture according to any of 35 claims 1 to 6.

11. A mixture of alkali metal polysulfides and alkali metal thiocyanates of the general formula 40 ((M 1 xM 2 (1-x)) 2 Sy)m (M 3 ,M 4 (1-z)SCN)(1-m) 16 where M 1 , M 2 , M 3 , M 4 = Li, Na, K, Rb, Cs, and M 1 is not the same as M 2 , M 3 is not the same as M 4 , and 0.05 5 x s 1, 0.05 s z s 1, 2.0 s y s 6.0, and m is the molar proportion, where 0.05 s m 5 0.95. 5

12. The mixture according to claim 11, wherein M 1 and M 3 = K, and M 2 and M 4 = Na.

13. The mixture according to either of claims 11 and 12, wherein 0.20 s x s 1.

14. The mixture according to any of claims 11 to 13, wherein 3.0 5 y 6.0. 10

15. The mixture according to any of claims 11 to 14, wherein 0.20 5 z 5 1.

16. The mixture according to any of claims 11 to 15, wherein 0.20 5 m s 0.80. 15

17. The mixture according to any of claims 11 to 15, wherein M 1 and M 3 = K, M 2 and M 4 = Na, 0. 2 05 x: 1, 0.20 5 z 0.95, 3.0s y5 s6.0 and 0.20 s m s 0.95.

18. The mixture according to any of claims 11 to 16, wherein M 1 and M 3 = K, M 2 and M 4 = Na, 0.50S5 x s 1, 0.50 s z! s0.95, y = 4.0, 5.0 or 6.0 and 0.335s m5 0.80. 20

19. A process for preparing mixtures according to any of claims 11 to 18, which comprises co-melting corresponding alkali metal polysulfides and alkali metal thiocyanates. 25

20. The use of the mixtures according to any of claims 11 to 18 as heat transfer or heat storage fluids.

21. A heat transfer or heat storage fluid which comprises a mixture according to any of claims 11 to 18. 30

22. Mixtures of alkali metal polysulfides of composition (K(1.x)Nax) 2 Sz with x = 0 up to 1 and z = 2.3 up to 3.5.

23. The use of the mixtures according to claim 22 as heat transfer or heat storage 35 fluids.

24. A heat transfer or heat storage fluid which comprises a mixture according to claim 22.