JPH0193415A - Production of calcium thiocyanate - Google Patents

Abstract

PURPOSE:To enable mass production of calcium thiocyanate at a low cost, by subjecting a rhodax solution formed in a decyanogenation step of coke oven gas to decarbonation treatment and then metathetic reaction with calcium hydroxide, etc. CONSTITUTION:A solution (rhodax solution), containing ammonium thiocyanate and obtained in a decyanogenation step for washing coke oven gas with a sulfur suspension or polysulfide-containing solution is heated until water, e.g. in a given amount, is distilled and then subjected to decarbonation treatment. Ca(OH)2 or CaO both are subsequently added to the solution obtained after the treatment to carry out metathetic reaction with the ammonium thiocyanate. Thereby calcium thiocyanate expectable in demand as a cement hardening promoter can be supplied in a large amount at a low cost by this method.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to ammonium thiocyanate-containing solutions (generally This invention relates to a method for producing calcium thiocyanate from (known as Rodax liquid).

(Prior art) Calcium chloride has traditionally been widely used as a cement hardening accelerator, but as the negative effects of chloride ions on concrete structures have become clearer, the use of chloride has been regulated in recent years. . Calcium thiocyanate can be used as a non-chloride cement hardening accelerator that can replace calcium chloride, and is a material that is expected to be in high demand in the future. However, because it is a substitute for calcium chloride, it must be supplied cheaply and in large quantities, and the cement-related industry is demanding the development of mass production technology that can supply calcium thiocyanate at a low cost.

Conventionally, calcium thiocyanate has been produced by the reaction between ammonium thiocyanate and calcium nitrite shown in the following reaction formula (1): Reaction formula (■) and 2NH4SCN + Ca(Not)g -Ca(SC
N)* + 2Ng + 41!0 or the following reaction formula (
Metathesis reaction between ammonium thiocyanate and calcium hydroxide or calcium oxide shown in I
It has been synthesized by N)* + 2NHs + 2HgO. However, conventionally, an industrial reagent has been used as ammonium thiocyanate, and there has been a limit to the reduction in manufacturing costs.

On the other hand, as a solution containing ammonium thiocyanate, there is a solution generally called a Rodax liquid, which is produced in a decyanization process in which coke oven gas is washed with a sulfur suspension or a polysulfide-containing solution.

This cleaning is performed using hydrogen cyanide (ICN) in coke oven gas.
This is done for the purpose of removing the Rodax liquid, and the obtained Rodax liquid
Contains ~20% by weight ammonium thiocyanate. A large amount of this Rodax liquid is generated in a coke manufacturing factory where coke oven gas is cleaned using this method, but conventionally it has been treated by burning, for example by stiffening calcium sulfate (ceracola); Because it requires energy and this by-product is cheap,
The reality is that waste liquid treatment is expensive. In addition, since it was processed by combustion, there was also the problem of air pollution. Therefore, it would be very advantageous if high value-added products could be produced from this Rodax liquid by a wet process (ie, a closed system).

The present inventors focused on the above-mentioned Rodax liquid as a raw material for producing calcium thiocyanate in large quantities at low cost, and conducted repeated studies on a method for producing calcium thiocyanate from this liquid.

As a result, when we added calcium hydroxide, calcium oxide, or both (hereinafter simply referred to as calcium hydroxide, etc.) to this solution and attempted a metathesis reaction with ammonium thiocyanate according to the above reaction formula (II), It was found that a significantly larger amount of calcium hydroxide, etc. was required than calculated from the thiocyanate ion and the small amount of thiosulfate ion and sulfide ion present in the solution, and furthermore, a large amount of sludge was generated in the reaction mixture. ,
It turned out that the manufacturing costs were quite high, partly because the processing was also expensive.

(Problems to be Solved by the Invention) Therefore, an object of the present invention is to provide a method for producing calcium thiocyanate at low cost from the above-mentioned ammonium thiocyanate-containing solution known as Rodax solution.

(Means for Solving the Problems) Here, the present invention provides a decarboxylation treatment of an ammonium thiocyanate-containing solution obtained in a decyanization process in which coke oven gas is washed with a sulfur suspension or a polysulfide-containing solution. rear,
This is a method for producing calcium thiocyanate, which is characterized by carrying out a metathesis reaction with calcium hydroxide or the like.

(Function) The present inventors used the RODAX liquid to conduct a double decomposition reaction with calcium hydroxide, etc. as the summer progressed, and the cause of excessive consumption of calcium hydroxide and the generation of a large amount of sludge. Upon investigation, we found that the cause was that the carbonate ions contained in this liquid consumed calcium hydroxide, etc., and produced calcium carbonate precipitates.

According to conventional chemical knowledge, in the presence of ammonium thiocyanate, which is a salt of thiocyanic acid, a strong acid, calcium carbonate, which is a salt of carbonic acid, a weak acid, dissolves, and a double decomposition reaction produces thiocyanic acid according to the following reaction formula (III). Calcium should be produced.

and 1st section 2NLSCN + CaCO5 = Ca (SCN) snow +
(NHa)tcOsAs is well known, for example, the ammonium salt of sulfuric acid ((NH#)gS
Between O*) and CaCO5, the above reaction formula (m)
The reaction corresponding to the above proceeds when carried out under conditions of heating and reduced pressure while distilling off ammonia, and calcium carbonate is dissolved by the metathesis reaction with ammonium sulfate.

It is also well known that a similar double decomposition reaction proceeds between ammonium thiocyanate and sodium carbonate according to reaction formula (IV) below.

2NHaSCN + NazCO* → 2Na
SCN + (NH11)gCOs Therefore, even if calcium carbonate is generated due to the presence of carbonate ions in the RODAX solution, it is dissolved and metathesized with ammonium thiocyanate to form thiocyanate, as shown in reaction formula (III) above. Since it is converted to calcium, it was expected that calcium hydroxide etc. would not be consumed excessively by carbonate ions and that calcium carbonate precipitates would not be formed. Ammonium carbonate ((
NH#)tCO3), that is, ammonia and carbonic acid are
Calcium thiocyanate (Ca
A solution containing only (SCN) t ) will be obtained.

However, unexpectedly, according to the experiments conducted by the present inventors, the reaction represented by the above reaction formula (I [ Even if the reaction is carried out under conditions where ammonia and carbonic acid are removed by heating or reduced pressure so as to shift the equilibrium of reaction formula (II[) to the right, the above reaction still does not substantially proceed and calcium carbonate precipitates. It remained as it was.

Therefore, in the method of the present invention, it is essential to remove carbonate ions from the Rodax liquid in advance, that is, to perform decarboxylation treatment prior to the metathesis reaction with calcium hydroxide or the like. This prevents the above-mentioned unexpected overconsumption of calcium hydroxide and the like due to the presence of carbonate ions and the generation of a large amount of sludge that is troublesome to dispose of.

Decarboxylation of the Rodax liquid, that is, removal of carbonate ions (dissolved carbon dioxide), can be carried out by any of the well-known decarboxylation methods. be able to. Alternatively, decarboxylation can also be carried out by lowering the pH to preferably 8 or less, more preferably 6 or less, using an acid such as sulfuric acid, nitric acid, formic acid, or acetic acid. After lowering the pH, if reduced pressure, heating, rapid stirring, bubbling, etc. are used in combination, carbonate ions can be removed more efficiently and completely.

The Rodax liquid decarboxylated as described above is then double decomposed using calcium hydroxide or the like according to the method of the present invention. Before carrying out this metathesis reaction, it is desirable to remove free sulfur suspended in the Rodax liquid by suitable means such as filtration.

This is because when free sulfur is present, it reacts with calcium hydroxide, etc., and is converted into periosulfate and sulfide, which may lead to an increase in the amount of impurities in the liquid and an increase in the consumption of calcium carbonate, etc. It is from. This removal of free sulfur can also be carried out before the decarboxylation treatment.

The double decomposition reaction of Rodax liquid with calcium hydroxide etc.
It is convenient to carry out the reaction under elevated pressure, normal pressure or reduced pressure while removing ammonia from the system by heating the reaction system.In the case of normal pressure, the reaction temperature is -Jl within the range of 100 to 110°C. The preferred amount of calcium hydroxide, etc. to be added is the amount calculated above, that is, the amount of thiocyanate ion and small amounts of thiosulfate ion and sulfide ion present in the RODAX liquid, since it has been decarboxylated as described above. The amount may be the stoichiometric amount determined from the content or a slightly excess amount, whereby ammonium thiocyanate in the liquid is almost completely converted to calcium thiocyanate, and a solution of calcium thiocyanate is obtained.

According to the present invention, the formation of precipitates (calcium carbonate) due to carbonate ions during this metathesis reaction does not substantially occur, but the precipitation of calcium sulfite formed by the reaction between thiosulfate ions and calcium hydroxide etc. A small amount may be produced, and this can be removed by simple separation operations such as filtration.

In addition, the polysulfides existing in the Rodax liquid and the polysulfides and sulfides generated during the metathesis reaction cause the obtained calcium thiocyanate solution to have a hydrogen sulfide odor. At the same time as double decomposition,
This decomposition of sulfides and polysulfides, which is preferably decomposed and removed at the same time as or after metathesis, can be achieved by oxidation,
For example, this can be carried out by air bubbling or by adding an oxidizing agent such as hydrogen peroxide.

The calcium thiocyanate solution obtained in this way can be recovered as a 30-40% solution by concentration during the reaction, and if the above-mentioned coexisting substances are removed, a large amount of the calcium thiocyanate solution can be used as is. A pure calcium thiocyanate solution is obtained. When used as a cement hardening accelerator, it is more convenient to be in the form of a solution, so the above concentrate can be used as a product as it is.

If desired, if it is desired to obtain it in crystalline form, it may be further concentrated if necessary and then cooled and crystallized.
The mother liquor after separation of the crystals is advantageously recycled to the metathesis step or to the concentration step for obtaining crystals.

Next, the present invention will be specifically illustrated by examples.

In the examples, all percentages and parts are by weight unless otherwise specified.

The sulfur powder obtained from the wet, sulfur recovery desulfurization process for coke oven gas is suspended in water, and the undesulfurized and decyanized coke oven gas is washed with this suspension to form a Rodax liquid. Obtained. After removing free sulfur by filtration, the Rodax liquid contains 10.3% ammonium thiocyanate, 0.26% ammonium thiosulfate, and 0.5% sulfide ions.
It contained 3.2% of carbonate ions (as carbon dioxide gas) and had a pH of 8.7.

1,000 parts of this Rodax liquid (containing 1.36 moles of ammonium thiocyanate) was placed in a glass distillation flask, heated to 105°C under normal pressure, and heated until 200 parts of water was distilled out to undergo decarboxylation treatment. did.

By this heating, the carbonate ion concentration in the liquid was reduced to 0.1% or less.

65.6 parts of 95% calcium hydroxide was added to the liquid obtained after the decarboxylation treatment. The amount added is 1.1 equivalents based on the total amount of thiocyanate ions, thiosulfate ions, and sulfide ions in the liquid, that is, about 10 equivalents from the stoichiometric amount.
% excess. The obtained solution was heated to 100 to 11
The mixture was heated to 0° C. for 4 hours to carry out a double decomposition reaction between ammonium thiocyanate and calcium hydroxide while distilling off ammonia together with water. In this way, 300 parts of a solution containing 35% calcium thiocyanate, 0.6% calcium thiosulfate, and 0.7% sulfide ions were recovered. That is, calcium thiocyanate was obtained at a yield of approximately 100% based on ammonium thiocyanate in the Rodax liquid. The residual ammonia concentration in this solution is 500■
/kg (0.05%).

When this solution was subjected to air bubbling treatment at 80° C. for 1 hour to decompose the sulfide ions in the solution, the sulfide ions were removed to an undetectable level.

The free ammonia concentration after this treatment was reduced to 100.8 g.

This lu91 Rodax liquid 1000 before decarboxylation used in Example 1
65.6 parts of calcium hydroxide was added to the part, and under normal pressure,
It was heated to 105-110°C and concentrated to 300 parts. When the free ammonia concentration in the obtained solution was measured, it was found to be 8.
%, and a large amount of sludge was generated in the liquid.

Therefore, 60 parts of calcium hydroxide was added to this and heated under the same conditions as above for 4 hours to volatilize the ammonia.Then, the sludge was removed by filtration, the filter cake was washed with water, and the washing liquid was added to the filtrate. By reconstitution, 250 parts of a solution containing 40% calcium thiocyanate was obtained. The residual ammonia concentration at this time was 0.12%.

(Effects of the Invention) As is clear from the above Examples and Comparative Examples, according to the method of the present invention, the required amount of calcium hydroxide, etc.
This is very economically advantageous, and the production of sludge is also extremely small, which simplifies the operation.

Furthermore, one of the advantages of the method of the present invention is that the residual ammonia concentration of the obtained solution (ammonium imparts a characteristic ammonia odor to the solution, so it is desirable to reduce it as much as possible) is very low. be.

As explained above, by the method of the present invention, high value-added calcium thiocyanate can be produced economically and easily from Rodax liquid, which was conventionally processed at a high cost, and it can be used as a cement hardening accelerator. This makes it possible to inexpensively supply large amounts of calcium thiocyanate, which is expected to be in high demand. In addition, when viewed as a treatment method for Rodax liquid, the method of the present invention is a wet treatment, so there is no concern about air pollution, and energy and equipment costs are significantly lower than conventional treatment methods that employ combustion. Moreover, the by-product is calcium thiocyanate, which has high added value.

Therefore, the method of the present invention can be said to be a very advantageous method that simultaneously solves the two technical problems of effective utilization of Rodax liquid and inexpensive mass production of calcium thiocyanate.

Claims (4)

[Claims] (1) After decarboxylation, the ammonium thiocyanate-containing solution obtained in the decyanization process in which coke oven gas is washed with a sulfur suspension or a polysulfide-containing solution is subjected to a metathesis reaction with calcium hydroxide and/or calcium oxide. A method for producing calcium thiocyanate, characterized by: (2) The method according to claim 1, wherein the decarboxylation treatment is performed by heating, reducing pressure, adding an acid, or a combination thereof. (3) Claim 1 or 2 further includes a step of removing polysulfides and sulfides from the solution by oxidation before or at the same time as the metathesis reaction.
The method described in section. (4) The method according to any one of claims 1 to 3, further comprising the step of removing free sulfur suspended in the solution before the metathesis reaction.