WO2003089372A2

WO2003089372A2 - Method for purifying solutions containing cyanamide

Info

Publication number: WO2003089372A2
Application number: PCT/EP2003/004086
Authority: WO
Inventors: Thomas GÜTHNER; Johann Fasenacht
Original assignee: Degussa Ag
Priority date: 2002-04-19
Filing date: 2003-04-16
Publication date: 2003-10-30
Also published as: CN1646428A; DE10217530A1; EP1497227A2; WO2003089372A3; CN100333824C; JP2005523227A; DE10217530B4

Abstract

The invention relates to a method for purifying solutions containing cyanamide, according to which the parent solution is subjected to a membrane filtration at temperatures of between -20 and +80 °C, whereby the separation membrane has an MWCO value of between 80 Da and 60 kDa and the parent solution is subjected to a pressure of between 2.5 and 60 bar. In said method, which preferably operates according to the feed-and-bleed principle and whereby the solution retained by the membrane is subjected to a circulatory flow in front of the membrane, parent solutions having a cyanamide content of between 5 and 60 wt. % are used. This permits over 80 % of compounds containing in particular calcium, iron, nickel and silicon to be eliminated from the solutions used.

Description

Process for the purification of solutions containing cyanamide

description

The present invention relates to a method for purifying solutions containing cyanamide.

Cyanamide is manufactured on an industrial scale from calcium cyanamide, which in turn is obtained from lime, calcium carbide and nitrogen.

Metals contained therein, such as e.g. Iron, nickel and calcium, in the manufacturing process for cyanamide. The quantity or concentration of the contaminating metals depends mainly on the quality of the raw materials coke and lime used in the production of lime nitrogen. The lime contamination mainly results from the type and composition of the gas and oil used in lime burning. During the production of the calcium carbide, metallic impurities are introduced into the product via the coke but also by burning off the blast furnace electrode used, in which case the impurities originate from the (steel) sheathing of the electrode.

In the production of cyanamide solutions, coarser impurities such as graphite, Fe ₂ O ₃ , CaCO ₃ , SiO ₂ and ferrosilicon are removed via rotary filters, but the fine-particle impurities already mentioned, which contain iron, nickel and calcium, for example, remain in the Product solution and cause quality disturbances, for example in the form of ion-typical discolouration, cloudiness or precipitation.

To overcome this problem, the use of ion exchangers is recommended (Ullmanns Encyclopedia of Industrial Chemistry, 6 Edition, 2001), but this does not bring the required success. Attempts to cyanamide solutions using cation exchange scissors to clean (Zh. Prikl. Khim (Leningrad) 1966, v. 39, No. 11, page

2581/2).

However, attempts have also been made to free iron and calcium ions from phosphate-stabilized cyanamide solutions using weakly acidic cation exchangers. While calcium ions could thereby be at least partially removed, the iron ions remained in the cyanamide solution (unpublished attempts by the applicants in).

Several membrane filtration methods are known from the prior art. As is known, this is a separation principle in which substances are retained due to their particle, molecule or ion size. There are basically two different types of membrane filtration:

In the pore model, particles of a liquid are retained on the pores of a membrane if the particle diameter is larger than the pore diameter. Only the liquid and smaller particles can pass through (permeate). What remains is either a filter cake or a liquid stream with an increased solids content (retentate).

Membrane separation processes based on this principle are microfiltration with pore sizes of approx. 0.05 to 2.0 μm and ultrafiltration with pore sizes of approx. 0.003 to 0.1 μm.

The driving force here is the pressure difference between the so-called feed side and the permeate side.

In the solution diffusion model, the membrane acts as a dense separation layer. The feed stream and its dissolved ingredients dissolve in the membrane and diffuse through it. The substances that are to be retained are only poorly soluble in the membrane used and / or have one low diffusion speed, so that noticeably more of the desired substances pass through the membrane in the same time.

This type of membrane separation is known as reverse osmosis, reverse 5 osmosis or hyperfiltration.

In this model, the separation limit is given as the molecular size, the mass weight cut off (MWCO) indicating the molecular weight of a standardized polysaccharide, the size of which defines the separation limit i o of the membrane using a standardized method. Reverse osmosis typically retains molecules between 30 and 300 g / mol. A membrane used for this is also characterized in part by its retention of NaCl in water.

The driving force in this model is the pressure difference between feed side 15 and permeate side.

Another well-known hybrid of the two models presented is nanofiltration technology:

20 Nanofiltration works in the border area between ultrafiltration and reverse osmosis and combines features of both models.

The separation limit is usually at diameters from 0.8 to 600 nm or at molecular sizes from 80 to 15,000 g / mol.

25

Solvent is removed from the suspension or solution directly on the membrane during the filtration, the retained contents concentrate on. This effect is called concentration polarization; a cover layer forms on the membrane. In order to keep this cover layer as thin as possible and thus counteract the concentration polarization, an attempt is made to generate as much turbulence as possible in the feed stream on the membrane.

For this purpose, the membrane is flowed over in parallel, so that at the end of the membrane a stream enriched in contaminants emerges from the membrane module, which is referred to as retentate. This principle is called "crossflow".

Since a relatively large retentate stream is produced in this crossflow mode of operation, the membrane module is operated in a "feed-and-bleed system". Here, the retentate is fed back in front of the membrane module. From this circuit, a small stream is discharged via the pressure control valve, which is referred to as concentrate. The high-pressure pump only has to deliver the volume flow resulting from the sum of permeate and concentrate. A recirculation pump promotes the large volume flow that hits the membrane module. This in turn only has to overcome the pressure loss that arises when the module flows through the feed side.

However, the disadvantage of this method is considered that the process conditions are difficult to adapt to different molecular or ion sizes and that colloidal components in particular easily occupy the membrane pores.

The object of the present invention is therefore to provide a process for the purification of solutions containing cyanamide, with which the disruptive ionic impurities iron, nickel and calcium are removed from the solution to such an extent that their negative effects occur the product quality can be eliminated.

This object was achieved with the method according to the invention, in which the starting solution is subjected to membrane filtration at temperatures between -20 and +80 ° C., the separating membrane having a MWCO value between 80 Da and 60 kDa and the starting solution is exposed to a pressure of 2.5 to 60 bar.

Surprisingly, this method has shown that iron, nickel and calcium as the main impurities in the cyanamide solutions can be removed to more than 90%, and that more contaminants such as SiO ₂ can also be removed regularly to more than 80%. This was all the more astonishing since silicates are known to be strongly colloidal compounds that are usually not so easy to remove by membrane filtration, since the silica gels that form seal the membrane pores and thus also prevent or even prevent the separation of other ions. Basically, this success was not to be expected.

It has been shown to be advantageous for the present process if a starting solution is used which has a cyanamide content between 5 and 60

% By weight. Aqueous or alcoholic starting solutions are particularly suitable.

The membrane used can be used in different forms: either as a flat membrane (structure similar to a filter press in flat plates), as a hollow fiber module (a bundle of thin tubes), as a tubular module (thick tubes) or as a winding module (rolled up). Because of their small construction volume and the comparatively low manufacturing costs, winding modules are usually used.

The feed solution flows into the membrane module at one end, where the permeate is branched off through the membrane. The rest leave the module on the other end as a retentate.

Has been shown to be advantageous in the method according to the invention if the

MWCO value of the separation membrane is 150 to 1000 Daltons and particularly preferably 200 to 500 Daltons. The present invention also prefers a separating membrane which contains a polyamide as the separating layer.

In most applications, membranes do not consist of just one layer (symmetrical membranes) but are constructed asymmetrically. The very thin active separation layer is applied to a thicker carrier layer. The carrier layer is usually coarse-pored and does not contribute to the separation. It is only intended to ensure pressure stability. It is recommended that there is another layer between the separating and carrier layers that functions as a putty.

In the present case, the active separating layer of the membrane used consists of a polyamide; the carrier can be polyethylene, for example. There is therefore a composite membrane.

Within the pressure range to be regarded as essential to the invention, to which the starting solution is exposed, pressures have proven to be suitable which are between 10 and 40 bar.

In normal operation, the cover layer described forms on the membrane. A large part of the deposited substances can be removed during cleaning, but a small part remains and can slowly but permanently deteriorate the flow rate of the membrane.

Typically, simply overflowing the membrane with clear solvent such as water is sufficient to break down the top layer. A cleaning agent can also be added to the water, which makes the cleaning solution acidic or alkaline. The added cleaning agents can also contain salts for pH buffering, enzymes or surfactants. The solubility of impurities that are on the membrane is higher when cleaning at a higher temperature. It can therefore be beneficial to use cleaning solutions that are up to 40 ° C warm. As a rule, the Membrane not cleaned on the permeate side, since substances that reach the permeate side during operation are flushed out of the membrane module with the permeate.

It has therefore proven to be advantageous if the solution retained by the membrane is subjected to a circulation flow in front of the membrane, which the present invention also provides as a preferred variant.

In this context, it has proven to be particularly advantageous if the method according to the invention is carried out according to the feed-and-bleed principle.

In the context of the advantageous possibilities which the method according to the invention comprises, it takes into account a preferred variant in which calcium-containing, iron- or or nickel-containing or silicon-containing compounds, in particular silica or mixtures thereof, are separated from the starting solution ,

The following examples illustrate these advantages of the process for the purification of cyanamide-containing solutions according to the present invention.

Examples

For the experiments described below, two 4 x 40 inch winding modules were used, each module having a membrane area of 4.5 m ² .

The feed solution was fed continuously from the circulating circuit of a cyanamide solution storage container via a prefiltration into a storage container. The pre-filtration consisted of a bag filter (depth filter with a nominal pore size of 1 μm: Amafilter HPM97-01-1SS) and a candle filter (6 candles with a pore size of 0.5 μm: Amafilter Part # T8619508246; WS 0.5-20U-X9 246).

This system was operated with a recirculation at a high pressure level, with permeate and concentrate flowing continuously into the corresponding storage tanks. In the retentate circuit there was a four-pass double-tube heat exchanger operated with cooling water.

The following operating parameters were selected:

The operating pressure was set in three pressure levels of 20, 30 and 40 bar.

The concentration ratio (ratio of permeate to feed) was chosen to be 50, 75, 82 and 90%, which corresponded to a concentrate: permeate ratio of 100%, 33%, 22% and 11%, respectively.

The characteristic measured variables were the pressures on the feed and retentate sides, the retentate temperature, the pressure drop across the bag filter, the volume flows for permeate and concentrate and the concentrations of the substances to be retained calcium, iron, nickel and SiO ₂ in the feed (each after prefiltration), in the permeate and in the concentrate. Results:

As flow rates of the membrane were used with process water as feed

20 ° C determined permeate fluxes of 3.3 ltr./(h ^• m ^{2 •} bar).

In operation with a 50% cyanamide solution with freshly cleaned membranes at pressures between 20 and 40 bar and a temperature of approx. 17 ° C permeate flows of 0.85 ltr./(h ^• m ^{2 •} bar) were achieved. The value dropped to about 0.25 liters / (h ^• m ² • bar) within a cleaning interval.

Purification services:

Table 1 contains the analytical values and the resulting retention for the monitored substances Ca, Fe, SiO ₂ and nickel. Different retention values were determined from the analysis values for the contaminants under consideration. In the case of SiO _{2 in} particular, the retention fluctuates greatly, since the colloidal nature of the SiO ₂ allows it to be in a wide variety of forms.

(N

Claims

Expectations

1. A process for the purification of cyanamide-containing solutions, characterized in that the starting solution is subjected to membrane filtration at temperatures between -20 and +80 ° C, a separating membrane being used which has a MWCO value between 80 Da and 60 kDa and the starting solution is exposed to a pressure of 2.5 to 60 bar.

2. The method according to claim 1, characterized in that the starting solution is used with a cyanamide content between 5 and 60 wt .-%.

3. The method according to any one of claims 1 or 2, characterized in that an aqueous or alcoholic starting solution is used.

4. The method according to any one of claims 1 to 3, characterized in that the MWCO value of the separation membrane is 150 to 1000 Da and particularly preferably 200 to 500 Da.

5. The method according to any one of claims 1 to 4, characterized in that the separating membrane contains a polyamide as the separating layer.

6. The method according to any one of claims 1 to 5, characterized in that the starting solution is exposed to a pressure of 10 to 40 bar.

7. The method according to any one of claims 1 to 6, characterized in that the solution retained by the membrane is subjected to a circulation flow in front of the membrane.

8. The method according to any one of claims 1 to 7, characterized in that it is carried out according to the feed-and-bleed principle.

9. The method according to any one of claims 1 to 8, characterized in that calcium-containing, iron and / or nickel-containing or silicon-containing compounds, in particular silica, or mixtures thereof are separated from the starting solution.