JP2005523227A - Purification of cyanamide-containing solutions - Google Patents

Abstract

The subject of the present invention is a process for the purification of cyanamide-containing solutions, in which case the starting solution is subjected to membrane filtration at a temperature of -20 ° C to + 80 ° C, in which case the separation membrane has a MWCO value of 80 Da to 60 kDa. The starting solution is then exposed to a pressure of 2.5-60 bar. In the case of such a method, which is preferably operated according to the principle of feeding and discharging and the solution retained by the membrane is applied to the circulating stream in front of the membrane, it preferably has a cyanamide content of 5 to 60% by weight. A starting solution is supplied. In this way, in particular calcium-containing compounds, iron-containing compounds, nickel-containing compounds and silicon-containing compounds can be removed from solutions in which more than 80% is used.

Description

  The subject of the present invention relates to a method for purifying cyanamide-containing solutions.

  Cyanamide is produced industrially from lime nitrogen (calcium cyanamide), which is obtained from lime, calcium carbide and nitrogen on one side.

  Due to the raw lime nitrogen, the metals contained therein, such as iron, nickel and calcium, reach the cyanamide production process. In this case, the amount or concentration of contaminated metal depends exclusively on the starting coke and lime quality used in the production of lime nitrogen. In this case, lime impurities are mainly caused by the type and composition of the fuel gas and petroleum used in the combustion of lime. However, when producing calcium carbide, metal impurities are carried into the product by burning blast furnace electrodes used on coke, in which case the impurities come from the (steel) jacket of the electrodes.

  In fact, when producing a cyanamide solution, coarse impurities such as graphite, FeO, CaCO, SiO and silicon steel are carried out on a rotating filter, but this rotating filter is already described, for example, iron, Fine particulate impurities, including nickel and calcium, remain in the product solution, causing quality degradation in the form of, for example, typical discoloration, turbidity or precipitation of ions.

  To overcome this problem, the use of ion exchangers is recommended (Ullmanns Encyclopedia of Industrial Chemistry, 6 Edition, 2001), but this does not produce the required results. In this connection, a test for purifying a cyanamide solution with a cation exchanger is also known (Zh. Prikl. Khim (Leningrad) 1966, Vol. 39, No. 11, 2581/2).

  However, attempts have also been made to remove iron and calcium ions from cyanate solutions stabilized by phosphates with weakly acidic cation exchangers. Thereby, the iron ions remain in the cyanamide solution, although calcium ions could be at least partially removed (Applicant's unpublished test).

Numerous membrane filtration methods are known from the state of the art. In this case, as is well known, the principle of separation is important, in which case the substance is retained due to the size of the particles, molecules or ions. In the case of membrane filtration, in principle two variants are distinguished:
In the case of the pore type, when the particle diameter is larger than the pore diameter, the liquid particles are retained on the pores of the membrane. Only liquids and small particles can pass through. A filter cake or a liquid stream (concentrate) having an increased solids content is retained.

  The membrane separation method based on the principle is a microfiltration method using a pore size of about 0.05 to 2.0 μm and an ultrafiltration method using a pore size of about 0.003 to 0.1 μm.

  In this case, the driving force is the pressure difference between the so-called feedstock side and the permeate side.

  In the case of the solution diffusion type, the membrane acts as a tight separation layer. The feed stream and its dissolved contents dissolve in the membrane and diffuse through the membrane. The retained material dissolves only poorly in the used membrane and / or has a low diffusion rate, so that at the same time a significant amount of the desired material passes through the membrane.

  This type of membrane separation is called reverse osmosis, reverse osmos or ultrafiltration.

  The separation limit is defined as the molecular size for this model, where the Mass Weight Cut Off (MWCO) determines the molecular weight of the normalized polysaccharide and the size of this molecular weight. Defines the membrane separation limit in a standardized manner. In the case of reverse osmosis, typically 30-300 g / mol of molecules are retained. In part, the membrane used for this is also characterized by its ability to retain NaCl in water.

  In this case, the driving force is the pressure difference between the feedstock side and the permeate side.

A similarly known mixed form of the two said types is the nanofiltration technique:
Nanofiltration works in the boundary area between ultrafiltration and reverse osmosis and combines the features of the two types.

  The separation limit is usually a diameter of 0.8 to 600 nm or a molecular weight of 80 to 15000 g / mol.

  From the suspension or solution, the solvent is removed on the membrane directly during filtration and the retained content is concentrated. This effect is called concentrated polarization; a coating layer is formed on the membrane.

  For the purpose of keeping this coating layer as thin as possible and thus resisting concentrated polarization, an attempt is made to generate as strong a turbulent flow as possible on the membrane in the feed stream.

  For this purpose, an overflow flows through the membrane at the same time, so that a flow called concentrate, which is enriched with dirt at the end of the membrane, exits the membrane module. In this principle, it is called “cross flow”.

  In the case of this cross flow, a relatively large concentrate flow is generated, so that the membrane module is driven in a “feed and discharge system”. In this case, the concentrate is returned in front of the membrane module. A small amount of flow called concentrated liquid passes through this circulation path through the pressure holding valve. In this case, the high-pressure pump has to transport only the volume flow, where the volume flow results from the sum of the permeate and the concentrate. The recirculation pump transports a large volume flow impinging on the membrane module. This recirculation pump overcomes the pressure loss on that side, which pressure loss occurs during the flow through on the feed side of the module.

  However, it is difficult to adapt the processing conditions to different molecular and ionic sizes, especially the fact that colloidal components simply occupy the membrane pores is considered a disadvantage of the method.

  Accordingly, the present invention provides a method for purifying cyanamide-containing solutions that removes iron, nickel and calcium from the solution as long as the adverse effects on product quality are eliminated when using disturbing ionic impurities. The issue was imposed.

  This object is achieved according to the method according to the invention by subjecting the starting solution to membrane filtration at a temperature of −20 ° C. to + 80 ° C., in this case using a separation membrane having a MWCO value of 80 Da to 60 kDa, Is solved by exposure to a pressure of 2.5-60 bar.

Surprisingly, according to this method, more than 90% of iron, nickel and calcium can be removed as major impurities in the cyanamide solution, and other interfering substances such as SiO ₂ are regularly more than 80%. It was shown that can be removed. This was even more surprising than in the case of silicates where known strong colloidal compounds are important, which are usually not easily removed by membrane filtration. This is because the silica gel that is formed adheres tightly to the membrane pores, thus hindering or significantly hindering the separation of other ions. Therefore, in principle, this result could not be expected.

  It has been found preferable for the process to use a starting solution having a cyanamide content of 5 to 60% by weight. Particularly suitable in this case are aqueous or alcoholic starting solutions.

  The membranes used may be used in different forms: flat membranes (similar to a filter press in a flat plate), hollow fiber modules (thin tube bundles), tubes It may be used as a shaped module (thick tube) or as a wound module (rolled). Due to the small construction volume and the relatively low production costs, wound modules are usually used.

  In this case, the feed liquid flows into the membrane module at one front surface, where the permeate is branched through the membrane. The remaining amount leaves the module as a concentrate at another front.

  In the case of the process according to the invention, it has been found that the MWCO value of the separation membrane is 150 to 1000 daltons, particularly preferably 200 to 500 daltons.

  Further, the present invention gives priority to a separation membrane containing polyamide as a separation layer.

  In most uses, the membrane is not only composed of one layer (symmetric membrane) but also asymmetric. In this case, a very thin active separating layer is applied on the thick carrier layer. The support layer is often coarse and porous and does not contribute to the separation. This carrier layer simply ensures pressure stability. In this case, it is desirable that another layer functioning as a kit exists between the separation layer and the carrier layer.

  In the case of the present invention, the active separating layer of the membrane used consists of polyamide and the carrier can be, for example, polyethylene. Therefore, there is a composite membrane.

  It has proven to be suitable for the pressure to be between 10 and 40 bar within the pressure range to which the starting solution is exposed, which can be essentially preferred for the present invention.

  In normal operation, the described coating layer is formed on the membrane. Most of the deposited material can then be removed during cleaning, but a small portion of the material can remain and gradually degrade the flow-through efficiency of the membrane.

  Usually, it is sufficient to expose the clear layer of the membrane, for example water, in order to decompose the coating layer. However, a detergent may be added to the water so that the cleaning liquid becomes acidic or alkaline. Furthermore, salts, enzymes or surfactants for pH buffering may be present in the added detergent. The solubility of impurities present on the membrane is even higher when it is cleaned at higher temperatures. Therefore, it is preferable to use a hot cleaning liquid up to 40 ° C. In general, the membrane is not cleaned on the permeate side. This is because substances that reach the permeate side during operation are washed away with the permeate from the membrane module.

  It has therefore proved advantageous to apply the solution retained by the membrane to the circulating flow in front of the membrane. This sets the present invention as a preferred variant as well.

  In this connection, it has proved particularly advantageous that the method according to the invention is carried out according to the principle of supply and discharge.

  Within the scope of the preferred process, including the process according to the invention, one preferred variant is considered, in which case calcium-containing compounds, iron-containing compounds and / or nickel-containing compounds or silicon-containing compounds, in particular from the starting solution, Silicic acid or a mixture thereof is separated.

  The following example demonstrates the advantages of the process for purifying cyanamide-containing solutions according to the invention.

Examples The tests described below use two wound modules having dimensions of about 10.16 cm x about 101.6 cm (4 x 40 inches), where all modules are 4.5 m ^2. Had a membrane area of.

  The feedstock liquid was continuously fed into the receiver by prefiltration from the circulation path of the cyanamide solution storage container. Pre-filtration consists of a bag-shaped filter (deep bottom filter with a nominal pore size of 1 μm: Amafilter HPM97-01-1SS) and a candle type filter (6 candles with a pore size of 0.5 μm: Amafilter Part # T8619508246 WS 0.5-20U-X9 246).

  The apparatus was operated at high pressure levels by recirculation, in which the permeate and concentrate were continuously discharged into the corresponding storage tank. In the permeate circulation path, there was a four-way double tube heat exchanger operated with cooling water.

The following operating parameters were selected:
The operating pressure was established in three pressure stages: 20 bar, 30 bar and 40 bar.

  As the concentration ratio (permeate to feed ratio), select 50%, 75%, 82% and 90%, which means 100%, 33%, 22% or 11% concentrate: permeate Corresponded to the ratio.

Specific measured values as feed side and concentrate side pressure, concentrate temperature, pressure reduction with bag filter, volumetric flow in the case of permeate and concentrate, and in feed (after each pre-filtration), permeate The concentrations of the substances calcium, iron, nickel and SiO ₂ to be retained in and in the concentrate were measured.

result:
A permeate flow of 3.3 liters / (h · m ² · bar) at 20 ° C. was measured as the flow-through efficiency of the membrane using water as the feedstock.

Permeate of 0.85 liters / (h · m ² · bar) at a pressure of 20 to 40 bar and a temperature of about 17 ° C. using a membrane that has been freshly cleaned with a 50% cyanamide solution. Achieved the flow. This value decreased by about 0.25 liters / (h · m ² · bar) within the cleaning interval.

Purification efficiency:
Table 1 contains the analytical values and the resulting retention of the observed substances Ca, Fe, SiO ₂ and nickel resulting therefrom. In this case, various retention rates are measured from the analysis value of the impurity. In particular, in the case of SiO ₂ , the retention rate is significantly reduced. This is because SiO ₂ can exist in a wide variety of forms due to its colloidal properties.

Claims (9)

  In the purification method of the cyanamide-containing solution, the starting solution is subjected to membrane filtration at a temperature of −20 ° C. to + 80 ° C. In this case, a separation membrane having a MWCO value of 80 Da to 60 kDa is used. A method for purifying a cyanamide-containing solution, characterized by being exposed to a pressure of 5 to 60 bar.   2. The process as claimed in claim 1, wherein a starting solution having a cyanamide content of 5 to 60% by weight is used.   3. A process according to claim 1 or 2, wherein an aqueous or alcoholic starting solution is used.   4. The process as claimed in claim 1, wherein the MWCO value of the separation membrane is 150 to 1000 Da, particularly preferably 200 to 500 Da.   The method according to any one of claims 1 to 4, wherein the separation membrane as the separation layer contains polyamide.   6. Process according to any one of claims 1 to 5, wherein the starting solution is subjected to a pressure of 10 to 40 bar.   7. A method according to any one of claims 1 to 6, wherein the solution retained by the membrane is applied to the circulating flow in front of the membrane.   8. The method according to claim 1, wherein the method is carried out according to the principle of supply and discharge.   9. The process as claimed in claim 1, wherein calcium-containing compounds, iron-containing compounds and / or nickel-containing compounds or silicon-containing compounds, in particular silicic acid, or mixtures thereof are separated from the starting solution.