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 process for crystallization in a draft tube baffle crystallizer. In this process cycling in the particle size distribution of the product withdrawn, is reduced and the rate of granular size crystals is increased. The product comprising granular size crystals is withdrawn from the crystallizer as a slurry by the suction of a pump. Further steps for separating the crystals from the slurry of granular size crystals are thickening the slurry of granular size crystals in a thickener, separating the crystals from the liquid part, the mother liquor, in a centrifuge, drying wet crystals in a dryer and screening the crystals to obtain the appropriate product size. However, although WO 93/19826 provides a process to increase the production of granular size crystals the final separated product still comprises, next to the granular size crystals, an amount of smaller-sized crystals, so- called fines. Due to the presence of fines the final separated product has to be screened in order to obtain the appropriate product size, comprising mainly granular size crystals.

The object of the present invention is to provide a process for separating granular size fertilizer crystals from a slurry of granular size fertilizer crystals wherein the amount of granular size fertilizer crystals is only slightly or even not reduced.

Surprisingly we have found that the final separated product comprises mainly granular size fertilizer crystals and hardly any fines when for separating granular size fertilizer crystals from a slurry of granular size fertilizer crystals, obtained from a crystallizer producing granular size fertilizer crystals by crystallization, said slurry is being fed to a centrifuge comprising a conical screen whereby granular size fertilizer

crystals separated from said slurry by the screen are discharged from the screen at the side of the screen having the largest diameter. Therefore, the final separated product no longer necessarily has to be screened. A further advantage of the process according to the present invention is that granular size fertilizer crystals are obtained which have a low moisture content. Furthermore an advantage of the process according to the present invention is that a step for thickening the slurry of granular size fertilizer crystals in a thickener and/or a step for drying wet crystals in a dryer can be shortened or even eliminated.

The slurry of granular size fertilizer crystals used in the process of the present invention is obtained from a crystallizer wherein the granular size fertilizer crystals are produced by crystallization. Any crystallizer capable to produce granular size fertilizer crystals by crystallization may be used. A draft tube baffle crystallizer or a fluidized bed crystallizer is an example of such a crystallizer.

In the process of the present invention the centrifuge comprising a conical screen may comprise any conical screen. The slurry of granular size fertilizer crystals 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 fertilizer crystals over the screen the granular size fertilizer 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 fertilizer 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 fertilizer crystals is for example between 300 and 600 mm, whereas the diameter of the conical screen at the discharge point of the granular size fertilizer 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 fertilizer crystals on the conical screen is always smaller than the diameter of the conical screen at the discharge point of the granular size fertilizer 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 fertilizer crystals and/or the granular size fertilizer 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 fertilizer crystals and/or the granular size fertilizer 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.

In general any kind of a centrifuge comprising a conical screen can be used in the process of the present invention. Examples of centrifuges which can be used 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. 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%.

The solids content of the slurry of granular size fertilizer crystals preferably varies between 5 and 55 % by weight, more preferable it varies between 10 and 50 % by weight and most preferable it varies between 15 and 45 % by weight. The solids content of the slurry of granular size fertilizer crystals may be lowered or increased in the process according to the present invention. To lower the solids content of the slurry a desired amount of the liquid part of the slurry separated at the centrifuge may be added to the slurry anywhere in the process between discharging the slurry from the crystallizer and feeding it to the centrifuge. To increase the solids content of the slurry the slurry of granular size fertilizer crystals may be thickened in a thickener, for example in a hydro cyclone.

In general between 30 and 100 % by weight of the solids content of the slurry of granular size fertilizer 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 fertilizer 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 fertilizer crystals are crystals having a size between 1 and 3 mm.

The slurry of granular size fertilizer crystals may comprise any

granular size fertilizer crystals. Preferably, it comprises granular size fertilizer 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.

The throughput of the slurry of granular size fertilizer 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.

Optionally the granular size fertilizer crystals discharged from the centrifuge may be dried further. Drying may be performed any way, e. g. in a fluid bed dryer or a tumbling 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.

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 fertilizer crystals. For example in the case that the slurry of granular size fertilizer crystals is produced in a crystallizer, the temperature is equal to the temperature of the crystallizer.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 represents a flow chart comprising apparatus for employing the process of the present invention.

Fig. 2 shows a cross-section of an example of a conical screen present in a centrifuge comprising a conical screen as used in the present application.

DESCRIPTION OF A PREFERRED EMBODIMENT In a preferred embodiment an aqueous slurry of granular size fertilizer crystals is separated from a crystallizer (A) through line 1, subsequently transported through line 1 and line 2 to centrifuge (C), see Fig. 1. In this preferred embodiment centrifuge C is a scroll screen centrifuge. Optionally the slurry separated from

crystallizer A passes through a hydro cyclone (B) before it is fed to the centrifuge. In the centrifuge the slurry of granular size fertilizer crystals is fed to the conical screen (1) at the feed point (2), see Fig. 2. The slurry of granular size fertilizer crystals moves over the screen from the feed point (2) to the discharge point (3). During moving the granular size fertilizer crystals will retain on the screen whereas the liquid part of the slurry can flow through openings in the screen into line 4. The movement of the slurry of granular size fertilizer crystals over the screen is facilitated by a screw (5) which fits into the cavity of the bowl formed by the conical screen and which turns round rotating axis (6) in such a direction that the slurry moves from feeding point to discharge point.

The screen turns round rotating axis (7) with a rotating direction of the screen in the same or in the opposite direction compared to the rotating direction of the screw and with a speed between 800 and 2000 rounds per minute (RPM). The difference in speed between screen and screw may vary between 10 and 35 RPM. Corresponding centrifugal forces vary between 100g and 1000g (with g is meant the standard force of gravity). In a preferred embodiment this centrifugal force varies between 150g and 600g. The centrifugal force will be the lowest at the feed point of the slurry of granular size fertilizer crystals and the highest at the discharge point of the granular size fertilizer crystals. Due to the different speed in movement of screen and scroll the slurry of granular size fertilizer crystals and/or the granular size fertilizer crystals move in a spiral way over the screen. The spiral way of moving has an effect on the residence time of the slurry of granular size fertilizer crystals and/or the granular size fertilizer crystals on the screen as well as on the way of movement of the granular size fertilizer crystals over the screen. Residence times lie for example between 1 and 25 seconds, whereas the throughput through the centrifuge may vary between 28 and 36 m3/h. Due to this spiral way of moving the slurry of granular size fertilizer crystals and/or the granular size fertilizer crystals move in a tumbling way and over a longer distance over the screen compared to moving in a straight line from feeding point to discharge point from the screen, which results in granular size fertilizer crystals having lower moisture contents. The gap (8) between the screw and the inner surface of the bowl may vary between 1 and 2 mm. The granular size fertilizer crystals leaving the centrifuge through line 3 (see Fig. 2) are transported to a dryer (D) in order to obtain dry granular size fertilizer crystals. Optionally the liquid part of the slurry leaving the centrifuge through line 4 is recycled into the crystallizer A or into line 1.

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

Comparative Experiment A<BR> A slurry of ammonium sulfate crystals comprising 30% by weight of ammonium sulfate obtained from a draft tube baffle crystallizer was fed to a typical pusher centrifuge with an entrance velocity of 30 m/s. The velocity of the pusher was 5 m/s. Table I shows the % of ammonium sulfate crystals, present in the slurry of ammonium sulfate crystals, having a crystal size between 1 and 3 mm for the crystals fed to the centrifuge as well as for the crystals discharged from the centrifuge. From the results presented in Table I it can be calculated that in the centrifuge the % of ammonium sulfate crystals having a crystal size between 1 and 3 mm is reduced with 50%.

Example I Comparative Experiment A was repeated whereby a scroll screen centrifuge (Siebtechnik, Conturbex H700 comprising a conical screen with an angle of inclination of 20°) was used instead of a pusher centrifuge and a slurry of ammonium sulfate crystals comprising 35% by weight of ammonium sulfate. The screen of the centrifuge was rotated with a speed of 880 RPM, the difference in speed between screen and screw was 25 RPM. Table I also shows the data for Example 1, the % of ammonium sulfate crystals, present in the slurry of ammonium sulfate crystals, having a crystal size between 1 and 3 mm for the crystals fed to the centrifuge as well as for the crystals discharged from the centrifuge. From the results presented in Table I it can be calculated that in the scroll screen centrifuge the % of ammonium sulfate crystals having a crystal size between 1 and 3 mm is reduced only with 9%. Thus, this Example shows that using a centrifuge comprising a conical screen the loss of granular size fertilizer crystals having a crystal size between 1 and 3 mm is decreased from 50 to 9%.

Table I Loss of granular size crystals between feed point and discharge point of the centrifuge.

Comparative Example 1 Experiment A % of crystals having a crystal size between 1 and 3 mm at the 40 79 feed point of the centrifuge % of crystals having a crystal size between 1 and 3 mm at the 20 72 discharge point of the centrifuge Loss of granular size crystals 50% 9%