WO2005002702A1

WO2005002702A1 - Process for separating granular size crystals

Info

Publication number: WO2005002702A1
Application number: PCT/EP2004/006803
Authority: WO
Inventors: Geert Ekkelenkamp; Andreas Oliver Mittenzwei; Johannes Elisabeth Pierre Debeij
Original assignee: Dsm Ip Assets B.V.
Priority date: 2003-07-03
Filing date: 2004-06-21
Publication date: 2005-01-13
Also published as: TW200512037A

Abstract

The invention is related to a process for separating granular size crystals from a slurry of granular size crystals produced in a crystallizer by crystallization comprising the steps of (a) discharging the slurry of granular size crystals from the crystallizer, (b) transporting the slurry from the crystallizer to a separator, (c) separating the granular size crystals in the separator from the slurry and (d) transporting the separated granular size crystals from the separator to a storage tank wherein the slurry is transported in step (b) by using gravity and the granular size crystals are transported in step (d) by using gravity and/or one or more elevator Oevices. An example of an elevator device is a conveyer belt or a bucket elevator.

Description

PROCESS FOR SEPARATING GRANULAR SIZE CRYSTALS

The invention is related to a process for separating granular size crystals from a slurry of granular size crystals produced in a crystallizer by crystallization comprising the steps of (a) discharging the slurry of granular size crystals from the crystallizer, (b) transporting the slurry from the crystallizer to a separator, (c) separating the granular size crystals in the separator from the slurry and (d) transporting the separated granular size crystals from the separator to a storage tank. Granular size crystals, which for example have a size greater than 1 mm, are amongst others preferred in fertilizer applications. An example of a fertilizer application is spreading fertilizer crystals on a farmland. Granular size crystals are preferred as they can be spread further away. Granular size crystals are also preferred as raw material for bulk blending. Granular size crystals can be applied more uniformly and segregate less from crystals of other components of bulk blend. A process for separating granular size crystals from a slurry of granular size crystals is described in WO 93/19826. This patent publication describes a crystallization process in a draft tube baffle crystallizer, resulting in an increased yield of granular size crystals. The product comprising granular size crystals is discharged from the crystallizer as a slurry. The granular size crystals are separated from the slurry in a centrifuge, dried and screened. However, although the process of WO 93/19826 results in an improved yield of granular size crystals, the amount of granular size crystals is partly reduced again during the separation process of the granular size crystals from the slurry comprising those granular size crystals. In the known processes the slurry is normally transported with a slurry pump whereas the separated product is normally transported with pneumatic transport systems. The separated product comprises, next to the granular size crystals, an amount of smaller-sized crystals, so- called fines. Such fines can be recycled as for example described in US 2,867,523. US 2,867,523 describes a process for producing granular ammonium sulfate particles by agglomeration in an agitation zone, drying the obtained granules and classifying them into an intermediate product size fraction, an undersize and an oversize fraction. The undersize fraction is being recycled. However, such a recycle has the disadvantage of increasing the production time as well as decreasing the production capacity. The object of the present invention is to provide a process for separating granular size crystals from a slurry of granular size crystals wherein the amount of granular size crystals is only slightly or even not reduced. A further object of the present invention is to eliminate or at least minimize the necessity to recycle fines. This object is achieved in that in a process for separating granular size crystals from a slurry of granular size crystals produced in a crystallizer by crystallization comprising the steps of (a) discharging the slurry of granular size crystals from the crystallizer, (b) transporting the slurry from the crystallizer to a separator, (c) separating the granular size crystals in the separator from the slurry and (d) transporting the separated granular size crystals from the separator to a storage tank, the slurry is transported in step (b) by using gravity and the granular size crystals are transported in step (d) by using gravity and/or one or more elevator devices. Compared to known processes the amount of granular size crystals is only slightly or even not reduced by using gravity and/or one or more elevator devices for transporting the granular size crystals according to the process of the present invention. A further advantage of the process performed according to the present invention is that a recycle of fines can be omitted or is at least reduced. Furthermore performing a process according to the present invention by using one or more elevator devices has the advantage that the process can be managed in a plant with a limited amount of floors. In general any kind of an elevator device can be used for transporting the granular size crystals. Good results are obtained with a continuous conveyer. Preferably, a conveyer belt and/or a bucket elevator are used as elevator device. Optionally, the in step (c) separated granular size crystals are transported to a dryer before being transported to a storage tank. In general, any dryer may be used for drying the separated granular size crystals. A fluid bed dryer or a tumbling dryer are examples of a suitable dryer. Preferred final moisture contents are between 0 and 1% by weight, more preferable between 0 and 0.3 % by weight, most preferable between 0 and 0.15 % by weight. Transportation of the separated granular size crystals from the separator to the dryer and of dried granular size crystals from the dryer to the storage tank is performed by using gravity and/or one or more elevator devices. Also in this case good results are obtained when a continuous conveyer is used as elevator device, such as a conveyer belt or a bucket elevator. A further option is to transport separated and/or dried granular size crystals to one or more screens and subsequently to one or more storage tanks in order to get granular size crystals within a certain preferred range. Again, this transportation is performed by using gravity and/or one or more elevator devices, whereby good results are obtained when a continuous conveyer is used as elevator device, such as a conveyer belt or a bucket elevator. In the process of the present invention a slurry of granular size crystals is used which is obtained from a crystallizer wherein the granular size crystals are produced by crystallization. Any crystallizer capable to produce granular size crystals by crystallization may be used. A draft tube baffle crystallizer or a fluidized bed crystallizer is an example of such a crystallizer. Preferably, the solids content of the slurry of granular size crystals varies between 5 and 55 % by weight. More preferable the solids content of the slurry of granular size crystals varies between 10 and 50 % by weight and most preferable it varies between 15 and 45 % by weight. With solids content in % by weight is meant the total weight of the slurry minus the weight of the liquid part of the slurry divided by the total weight of the slurry times 100%. Optionally the solids content of the slurry of granular size crystals is increased in a thickener, such as a hydro cyclone. It is also possible to lower the solids content of a slurry of granular size crystal, for example by adding a desired amount of the liquid part of the slurry after the granular size crystals are separated from the slurry. In a preferred embodiment the solids content of the slurry of granular size crystals is such that before separating the granular size crystals in a separator no thickening step is required. In general between 30 and 100 % by weight of the solids content of the slurry of granular size crystals are crystals having a size between 1 and 3 mm. Preferably, more than 50 % by weight of the solids content of the slurry of granular size crystals are crystals having a size between 1 and 3 mm. More preferable, more than 60 % by weight, and most preferable, more than 70% by weight of the solids content of the slurry of granular size crystals are crystals having a size between 1 and 3 mm. Any slurry of granular size crystals obtained from a crystallizer wherein the granular size crystals are produced by crystallization may be used in the process of the present invention. Preferably the granular size crystal slurry comprises granular size salt crystals, more preferable it comprises granular size ammonium sulphate crystals. In general the liquid part of the slurry may comprise any solvent. Examples of solvents are benzene, toluene, alcohols, such as isopropyl alcohol, ethanol and methanol, and water. More preferable solvents are ethanol, methanol or water. Most preferable water is used as solvent. In general any kind of a separator may be used to separate the granular size crystals from the liquid part of the slurry in the process of the present invention. Good results are obtained by using a centrifuge. Examples of centrifuges to be used as separator are a slide centrifuge, a swing centrifuge, a tumbler centrifuge and a scroll screen centrifuge. Preferably, a scroll screen centrifuge is used. A scroll screen centrifuge is especially preferred in the case that the process according to the present invention is applied to slurries having a solids content higher than 20% by weight. More preferable, a centrifuge having a conical screen is used for separating the granular size crystals from the liquid part of the slurry in the process of the present invention whereby the slurry is fed to the centrifuge comprising the conical screen in such a way that the slurry moves over the screen from the side having the smallest diameter towards the side of the conical screen having the largest diameter. During movement of the slurry of granular size crystals over the screen the granular size crystals will retain on the screen whereas the liquid part of the slurry can flow through the openings of the screen. In this way granular size crystals are separated from the liquid part of the slurry and are being discharged from the screen at the side of the conical screen having the largest diameter. The conical screen of the centrifuge is for example perforated with openings having a diameter between 0.1 and 0.4 mm. The total area of the openings is for example between 4 and 10% of the total screen area of the conical screen. The diameter of the conical screen at the feed point of the slurry of granular size crystals is for example between 300 and 600 mm, whereas the diameter of the conical screen at the discharge point of the granular size crystals is for example between 500 and 1000 mm, whereby the diameter of the conical screen at the feed point of the slurry of granular size crystals on the conical screen is always smaller than the diameter of the conical screen at the discharge point of the granular size crystals from the conical screen. Preferably a conical screen with an angle of inclination between 5 and 30 degrees is used. More preferably the angle of inclination is between 15 and 25 degrees. The movement of the slurry of granular size crystals and/or the granular size crystals over the conical screen is facilitated by the angle of inclination, a larger angle of inclination facilitates the movement of the slurry of granular size crystals and/or the granular size crystals more than a smaller angle of inclination. Preferably the conical screen is rotated with a speed between 800 and 2000 rounds per minute (RPM). Resulting centrifugal forces may vary between 100g and 1000g (with g is meant the standard force of gravity). More preferred resulting centrifugal forces vary between 150g and 600g. The throughput of the slurry of granular size crystals through the centrifuge depends a.o. on the kind of centrifuge used. A throughput through the centrifuge of 100 m³/h is feasible, however, in general the throughput varies between 10 m³/h and 60 m³/h. Preferably, the throughput through the centrifuge varies between 20 m³/h and 50 m³/h. The temperature during the process of the present invention is for example between 0 and 150 °C. Preferable, the temperature is between 30 and 130 °C, more preferable, the temperature is between 40 and 120 °C. In a preferred embodiment, the temperature during the process of the present invention is equal to the temperature used to produce the slurry of granular size crystals. For example in the case that the slurry of granular size crystals is produced in a crystallizer, the temperature is equal to the temperature of the crystallizer.

Brief description of the drawing Fig. 1 represents a flow chart comprising apparatus for employing the process of the present invention.

Description of a preferred embodiment In a preferred embodiment (see Fig. 1) an aqueous slurry of granular size ammonium sulphate crystals is discharged through line 1 from a crystallizer (A) and fed to a centrifuge (B) by using gravity. In centrifuge B granular size ammonium sulphate crystals are separated from the liquid part of the slurry. The liquid part of the slurry leaving the centrifuge through line 2 may be recycled into crystallizer A or into line I .The granular size crystals leaving centrifuge B through line 3 are transported to a dryer (C) using an elevator device (D), preferably a conveyer belt. Dried crystals leaving the dryer through line 4 are transported by using an elevator device (E), preferably a bucket elevator, in order to be transported further to one or more screens (F). The screened granular size crystals are transported further through line 5 to one or more silos (G) by using gravity.

The invention is illustrated by the following examples without being limited thereto.

Comparative Experiment A A slurry of ammonium sulphate crystals comprising of 29 % by weight of ammonium sulphate obtained from a draft tube baffle crystallizer was fed to a scroll screen centrifuge by using gravity. The in the centrifuge separated ammonium sulphate crystals were subsequently fed to a fluid bed dryer and transported by a pneumatic conveying system from the fluid bed dryer to a storage tank. In Table 1 the % of crystals, having a crystal size between 1 and 3 mm are shown before and after transport by the pneumatic conveying system. From the results presented in Table 1 it can be calculated that the loss of ammonium sulphate crystals between 1 and 3 mm is 11 %.

Example 1 Comparative experiment A was repeated whereby the pneumatic conveying system was replaced by a bucket elevator.

Table 1 also shows the percentage of ammonium sulphate crystals between 1 and 3 mm, analysed before and after transport by the bucket elevator, showing a decrease in granular size crystals between 1 and 3 mm of only 5 wt% by weight.

Table 1 Loss of granular size crystals during transport after drying.

Claims

1. Process for separating granular size crystals from a slurry of granular size crystals produced in a crystallizer by crystallization comprising the steps of (a) discharging the slurry of granular size crystals from the crystallizer, (b) transporting the slurry from the crystallizer to a separator, (c) separating the granular size crystals in the separator from the slurry and (d) transporting the separated granular size crystals from the separator to a storage tank characterized in that the slurry is transported in step (b) by using gravity and the granular size crystals are transported in step (d) by using gravity and/or one or more elevator devices.

2. Process according to claim 1 characterized in that in step (d) the separated granular size crystals are transported to one or more screens and subsequently to one or more storage tanks.

3. Process according to claim 1 characterized in that in step (d) the separated granular size crystals are transported to a dryer before being transported to a storage tank.

4. Process according to claim 3 characterized in that in step (d) the dried separated granular size crystals are transported to one or more screens and subsequently to one or more storage tanks.

5. Process according to any one of claims 1-4 characterized in that a conveyer belt and/or a bucket elevator is used as elevator device.

6. Process according to any one of claims 1-5 characterized in that the slurry of granular size crystals has a solids content between 5 and 55 % by weight and/or more than 30 % by weight of the solids content of the slurry are crystals having a size between 1 and 3 mm.

7. Process according to any one of claims 1-6 characterized in that the slurry of granular size crystals comprises ammonium sulfate crystals.

8. Process according to any one of claims 1-7 characterized in that the slurry of granular size crystals is an aqueous slurry comprising ammonium sulfate.

9. Process according to any one of claims 1-8 characterised in that a centrifuge comprising a conical screen is used for separating the granular size crystals from the slurry of granular size crystals whereby the slurry is fed to the centrifuge in such a way that the slurry moves over the screen from the side having the smallest diameter towards the side of the conical screen having the largest diameter.

10. Process according to claim 9 characterised in that the centrifuge used is a scroll screen centrifuge.