ZA200901756B

ZA200901756B - Calcium nitrate

Info

Publication number: ZA200901756B
Application number: ZA2009/01756A
Authority: ZA
Inventors: Torstein Obrestad; Peter Mutsaers; Ingar Wallestad
Original assignee: Yara Int Asa
Priority date: 2008-04-16
Filing date: 2009-03-11
Publication date: 2010-02-24
Also published as: CN101559966A; NO20081853L; RU2414426C2; CZ2009222A3; RU2009112498A; PL220612B1; CO6210122A1; CN104085910A; NO332615B1; PL387815A1

Description

Calcium nitrate

Field of invention :

The present invention relates to a process for the purification of calcium nitrate,

CN, solutions/melts.

Background

In the nitrophosphate process rock phosphate is dissolved in excess nitric acid , the digested liquid is cooled down to approx. 0 °C to precipitate calcium nitrate tetra hydrate.

The acidic precipitate is removed by centrifugation and neutralized by ammonia.

The water content is adjusted by evaporation prior to granulation or prilling.

Rock phosphate minerals e.g. apatite contain high concentrations of ions like Si*",

Fe, Fe’, Al’ F, etc in addition to calcium and phosphor. Calcium nitrate crystals originating from digested rock phosphates will therefore comprise various amounts of impurities of fluorides, phosphates and silicates. Also particles of sand, silicates and sludge from the digestion stage may be present. Most of the impurities have to be removed to obtain a calcium nitrate suitable for technical applications or drip irrigation systems in green houses.

Present purification processes; the production stages of CN in the nitrophosphate process is schematically shown in figure 1. The purification of the CN is carried out in stages 5 to 7 and includes diluting the CN melt with water to a density of 1,45- 1,48 kg/l at 60 °C prior to neutralization with ammonia to a pH of 5-8.

The neutralization brings about precipitation of inorganic components like apatite, calcium fluoride, silicates and silica etc.

To reduce the amount of insoluble components the solution from stage 5 is mixed with flocculants. Flocs are formed and much of the inorganic material is removed by use of decanter centrifuges.

The supernatant is transferred to stage 8 where water is adjusted and a melt is formed containing 77% Calcium nitrate, CaN , 7% ammonium nitrate, AN, and 15 % water. ~ 30 Granulation or prilling results in a solid product, (solid CN) having insolubles in the range 2400-900 ppm. Insolubles are a mixture of insoluble compounds like e.g. silica, apatite, calcium fluoride, Al-Fe-silicates etc.

This purification procedure is well established and the process is easy to operate.

However, the above described purification process as some disadvantages and

° insufficiencies. The purification method requires addition of much water to reduce density from approx. 1,6 (70 °C) to approx. 1,45 (70 °C). This water then has to be . evaporated prior to granulation/prilling and significant additional energy consumption is required. Further, a purification level between 2400-900 ppm is not sufficient for some technical applications. In the green house market this level of insolubles results in so much sedimentation in storage tanks over time that cleaning procedures for the equipment are required. And finally the described process requires operation of several decanter centrifuges for which maintenance is costly.

Other purification techniques: Purification by filter press (plate and frame)or other 10 . types of filter equipment (like Candle filter) are well-known methods for purification of salt solutions to high levels of purity. However, for use in purification of calcium nitrate from apatite, these filter systems will require the use of a filter aid (i.e. diatomite) to maintain an acceptable flux through the filter cake.

The amount of filter aid limits the use of this equipment to smaller production volumes, as the filter cake must be disposed of in an environmentally sound way, and thus induces a high cost. Most filters are also operated batch-wise, which is a disadvantage in an otherwise continuous process. Another disadvantage is the necessity to dilute the CN solution to be purified to below 1.5 kg/l to get an acceptable viscosity and filtration rate. This again increases cost as all water added has to be removed in order to make solid CN material.

Other purification methods are available for instance micro filtration by use of ceramic filters and disc-centrifuges. The micro filtration method requires large investments.

Further disc-centrifuges in trials turned out to give too much scaling resulting in clogging of the equipment.

RU 2228906 (abstract) disclose a method for refining calcium nitrate melt or solution by isolating calcium nitrate tetrahydrate crystals on cooling of phosphate ore nitric acid extract, melting of crystals, dilution of calcium nitrate melt/solution with 0.5-60% ammonium nitrate solution, and neutralization of dilute solution with ammonia to pH 6.1-7.6 followed by separation of solid impurities by settling resulting suspension in settler in one step.

RU 2154045 disclose a method for preparation of complex mineral fertilizer wherein phosphorite calcined at 850-1050 C is decomposed with non-concentrated nitric acid, and suspension resulting from decomposition is added into water in amount of 0.5-2.5 volume per volume of suspension, insoluble residue is removed by allowing it to settle, part of calcium nitrate is removed by cooling, solution 18 ammonized and processed to form NP or NPK type fertilizer by evaporation, granulation and drying, added water being removed by evaporation prior to

3 ~~. an crystallization of calcium nitrate. The process of settling insoluble precipitate is accelerated by many times, and standardized fertilizer is prepared from phosphite.

JP 2006225175 concerns a method of manufacturing transparent liquid fertilizer containing magnesium nitrate and calcium nitrate as main components wherein nitric acid is added into water in which dolomite is assed while stirring and mixing the water, so as to be neutralized to produce magnesium nitrate and calcium nitrate, and next, at least one kind of a component selected from a group comprising a component to be a potassium component, and a compound to be a nitrogen component and a compound to be a trace-element component is added and dissolved and after that, a high molecular flocculant is added and mixed under a temperature condition of 40-80 oC and the mixture is allowed to stand to settle and separate an undissolved portion.

Brief description of the figures

Figure 1 schematically shows CN production stages in the nitrophosphate process.

Figure 2 shows a pilot plant for continuous purification of CN melt/solution by settling.

Figure 3 shows an example of Lamella equipment.

Figure 4 shows a cross-section of a pilot plant i.e. cross-section of the Lamella unit - with connections. :

Figure 5 shows the water insoluble content of purified melt exiting the lamella separator depending on the density of the melt (at 60-70 °C, including flocculant) and the feed flow of the melt.

Figure 6 shows the amount of water insoluble material in CN solution exiting the lamella unit and the mol P/mol F ratio in crude CN over a period of 3 days

Figure 7 schematically shows the purification of CN originating from the nitrophosphate process

Summary of invention

One object of the present invention is to provide an improved purification of CN : solutions/melts, i.e. the method provides a CN solution/melt wherein the amount of water insolubles is lowered.

Another purpose of the invention is to reduce energy consumption of the CN purification stage.

PY 4

The present invention provides a method for purification of a CN solution ~ originating from the nitrophosphate process comprising the step of settling insoluble "material. The insoluble material partly originates from the digested rock phosphate and partly from the neutralization of the acidic crude CN solution.

The settling may be carried out in any equipment suitable for that purpose, particularly a tank containing plates or lamellas.

The invention particularly provides a method for purification of a CN solution by settling after having adjusted the ratio mol P/mol F to above 0.4 by use of phosphoric acid or another phosphor source soluble in acidic CN solution.

Detailed description of the invention

The present invention provides a method for the purification of calcium nitrate solutions or melts, wherein the step of settling is comprised. The settling step is followed by centrifugation of the sludge phase. Further the method involves that the sludge is recycled by return to a step of digestion of rock phosphate.

The method according to the present invention comprises the steps of a) digestion of rock phosphate, b) cooling/ crystallization, ¢) filtration, d) washing of crystals, e) dissolve and dilute to 1.3-1.75 kg/l at temperatures between 20-90 C °C f) neutralization : g) addition of flocculant and adjustment of mol P/mol F ratio h) settling i) centrifugation of sludge phase and possibly recycling of sludge to step a) j) evaporation k) particulation.

In an embodiment of the method, step €) above involves that the melt is diluted to about 1.6 kg/1 at 70°C.

In another embodiment of the method, step g) above the mol P/mol F is adjusted to above 0.30 preferably above 0.45

In yet another embodiment of the method the settling in step h) is carried outina settling tank equipped with thin plates, lamellas. 40

In a further embodiment of the invention a calcium nitrate solution/melt is provided that comprise less than3 % insolubles.

PY 5

The calcium nitrate produced by the method of the invention is particulated by granulation, prilling or any other way of particulation known in the art.

From x-ray diffraction analyses it is known that the insoluble material that precipitates when neutralizing an acidic CN solution mainly consists of fluoroapatite (Cas(PO4)3F (70-95%), and minor amounts of SiO, and CaF,. The densities of these minerals are in the range 2.2-3.2 g/cm’ and should therefore settle in a CN water solution. It is observed that insolubles slowly settle and form a sludge phase at the bottom of a container when a neutralized solution is left resting.

The settling rate is among other factors dependent on the density of the solution and the mol P/mol F ratio. Generally the more diluted the solution is the quicker the settling speed is, and a low mol P/mol F ratio cause low settling rate.

The fact that the more diluted the solution is the faster the settling is follows directly from Stokes'law;

Vs= K(pins-psol)d,’ /n wherein

Vs is the settling speed

K is a constant pins is the density of insolubles ~~ psol is the density of solution n is the viscosity of solution d,’ is diameter of particle

The second factor, low mol P/mol F ratio causes low settling rate, arise from the - fact that main impurity that precipitates during neutralization is fluoroapatite (Cas(PO4)3F). Ideal ratio of mol P to the mol F for precipitation of the mentioned compound is 3. If the ratio is too low the precipitated apatite crystals become few and small, i.e. low rate of crystal growth for apatite, the remaining F precipitates as

CaF,, and the whole mixture of insoluble material settles significantly more slowly.

The CaF, and SiO; crystals formed are small, with large surface area and a more fluffy behaviour.

Investigations have shown that low F content and high mol P/mol F (2 0,7) in a CN solution gives large apatite crystals that settles quickly. On the other hand high F 40 concentrations and low mol P/mol F (< 0,4 ) often results in small crystals with low rate of settling.

® 6

As mentioned above the rock phosphates contain various amounts of F, Si, Al, Fe and other species in addition to Ca and P. The amounts will vary with type of rock phosphates and the washing procedures applied, Figure 1, step 4.

Consequently the mol P/mol F of the CN crystals going into the dilution and neutralization step will vary with the crude phosphate used.

Some typical rock phosphates used are e.g. Kola phosphates, Boucraa, (Marocco) and Youssoufia (Marocco) or mixtures thereof. The content of Si, F, Fe, Al, Ca, P etc varies from type to type and also within the same type. Consequently the mol

P/mol F in a CN solution originating from the nitrophosphate process will also vary to a large extent. During neutralization of the CN melt the consistency of the . formed insolubles and the amount of each compound will vary and could influence on a settling process. :

In a continuous settling process a flocculant is added to the CN solution/melt before entering into the settling tank. The addition of a flocculant improves the purification in that the using amount of water insolubles is lowered and the solid content of the sludge increases that is a more compacted sludge is obtained. To obtain a better purification the density of the CN solution/melt is decreased and the ratio mol P: mol F is increased.

The quantities of insolubles of the purified solutions are surprisingly far below what is obtained by the purification method used in a current CN plant, figure 1, wherein a centrifuge giving 500-2000 g (centrifugal force) is used to remove insoluble particles.

A purification method comprising settling of insolubles could therefore be utilized for the purification of CN in the nitrophosphate process.

To further improve the purification of a CN solution/melt a settling tank equipped with metal plates or lamellas is used. Such settling equipment is developed and used in water purification. A settling unit with lamellas is connected to a feeding tank for

CN solution/melt, e.g. the neutralization tank of a CN plant, (step 6, figure 7).

Before entering the settling unit the CN solution /melt is optionally added a flocculant and/or the mol p/mol F ratio is adjusted. The sludge resulting from the settling is discharged to a decanter centrifuge for dewatering the sludge phase.

The lamella settling unit is in principle a tank equipped with a lot of thin metal 40 plates (lamellas), assembled some 0.05-0.1 m apart. The slope of the plates is 40-70 degrees. The lamellas increase the total settling area in the tank and the settling is more efficient. The space in between the plates allows the liquid to move freely

7 ~~ . upwards, but insoluble particles are hampered and tend to deposit on the plates and slide downwards into the coned sludge tank.

As mentioned above a flocculant may optionally be added to the CN solution/melt before the solution is transferred to the settling tank. The flocculant added solution is first transferred to a first mixing compartment wherein the flocculant is mixed well into the solution/melt. This compartment is equipped with a rotating agitator or other suitable mixing equipment. The solution/melt mixed with flocculant overflows to a second compartment which has a slow moving agitator to keep the flocculated particles formed suspended.

From the second compartment the melt/solution flows into the lamella settling tank and enters at the bottom of the plates. The solution moves upwards in between the plates and leave the lamella separator via discharge flumes placed over the plates.

The discharge flumes have a number of holes through which the liquid has to flow in order to exit the lamella. In this way the flumes ensure that the solution distributes evenly over the plates.

The solid particles settle on the plates and slide down into the sludge hopper that is equipped with a very slowly rotating scraper. The sludge level in the hopper is kept constant or varied by pumping the sludge phase to a decanter centrifuge for dewatering.

In one embodiment of the present invention a method for the purification of CN solutions/melts in the nitrophosphate process is provided wherein the purification step comprises settling of insolubles and discharge of said insolubles from the bottom of the settling tank.

In another embodiment of the invention a method for the purification of CN solutions/melts in the nitrophosphate process is provided wherein the purification step comprises settling of insolubles in a lamellar settling tank.

In another embodiment of the invention a flocculant is added to the CN solution/melt before entering the settling tank. Various types of flocculation agents may be applied; preferably the flocculation agent is selected from Fennopol A3304,

Nordfloc A172 and Superfloc AF126.

In yet another embodiment of the invention, the sludge formed by the settled insolubles is discharged from the bottom of the settling tank and optionally exposed 40 to further treatment like centrifugation. In an additional centrifugation step the sludge is further separated into, a precipitate i.e. the concentrated pellet of

Sinsolubles, and a supernatant i.e. the purified melt.

PY 8

The density of the CN solution/melt to be purified is in the range of 1.3-1.75 kg/ 1, preferably 1.45-1.65 kg/l, and the temperature of the solution/melt is held in the range 25-90 °C, preferably 40-80°C.

The mol P/mol F is a characteristic of the crude CN melt and can be varied by changing rock phosphates in the NPK plant or by adding concentrated phosphoric acid to the crude CN melt. The mol P/mol F can also be varied by adding another phosphate source soluble in the acidic crude CN solution. The amount of added concentrated phosphoric acid will vary according to mol P/mol F in the CN solution/melt, however, a dosage of 0-9 kg /m> CN solution would normally be sufficient when based on 50% CaN in the solution.

Purification of CN in the nitrophosphate process according to the present invention is illustrated schematically in figure 7.

The purification includes an additional stage 8, purification by settling. Stage 7 optionally includes addition of a flocculant and adjustment of the ratio, mol P/mol

F, with addition of phosphoric acid or another phosphate source. :

In modifying stage 7 to include adjustment of the mol P/mol F ratio and including a : settling stage comprising a decanter centrifuge for sludge treatment several : advantages, compared to the process given in figure 1, are achieved.

Among these advantages are saving of a substantial amount of energy since the CN solution to be treated has a density up to 1.67. In the existing purification process the CN melt (from stage 4, fig. 1) has to be diluted to a density of around 1.47 kg/l.

In the process of the present invention dilution can normally be omitted.

The settling stage provides a solid CN comprising insoluble material in a concentration of typically 350 ppm. Compared with the present process, as illustrated in figure 1, giving solid CN comprising 1000 ppm insoluble material this is a substantial improvement.

With the same energy consumption as in the process illustrated in figure 1 the use of the process according to the invention can reduce the content of insoluble material to values between 50 and 100 ppm.

Maintenance costs for the decanter centrifuges will be substantially reduced since only 1 small centrifuge will be used in the process according to the invention compared to three large centrifuges in the present process. 40

The invention will now be illustrated by the following non-limiting examples.

PS 9

Examples

Unless it is indicated that other flocculant solutions are used, all the examples have been performed using Nordfloc A172 (supplied by SNF). The flocculant solution was prepared in batches by dissolving! kg of flocculant in Im’ of water in the flocculant tank. The resulting 0.1% solution was fed both to the crude melt and the sludge phase entering the decanter centrifuge.

Example 1

In laboratory tests various neutralised CN solutions (approx. 1000 ml) from the plant were mixed with a flocculant and transferred to a graded cylinder (1000 ml) and allowed to rest for 2 minutes. The volume of the sludge phase, i.e. settled insoluble material, was observed. The results are given in table 1 below: :

S1 and S2 indicate sample 1 and 2 from the plant.

Table 1 CN solution neutralized to pH 6-7 (1+10) with ammonia

Density of | Flocculant | Volume %P %F molP/molF | Amount solution added of sludge of insol.

Superfloc in liquid (Cytec Ind) above 1% sludge (g/cm?) (ml/l (ml) phase solution) (ppm)

TEE Je [®0 [ons fo [om [NA

ESA CN EC (SE KLE XA 1.49 190 0.15 0.12 0.77 72 rn | I 1.49 180 0.15 0.12 0.77

Fr ll i

EE 1 CN CI [0 [AC LR 1.48 750 0.065 0.14 0.28 55 i

NA = not available

The results of table 1 clearly indicate that the insolubles settle rather quickly provided some preconditions are fulfilled. Nevertheless, the following conclusions may be drawn: i) the more diluted the solution the quicker the settling speed, ii) at low mol P/mol F ratios the settling rate is low, and iii) the amount of insolubles in solution above sludge are very low.

® —

Example 2

Testing in pilot plant

The test described in examplel is a batch process. To further test the use of settling to obtain a purified melt, i.e. to efficiently purify a CN melt from the nitrophosphate process by settling, a pilot plant was designed as shown in figure 2. The pilot plant - had a volume of approx. 1.5 m’ and was made of Plexiglas to visually observe the settling.

Description of the equipment:

Tank A: Thorough mixing of flocculant and crude CN solution

Tank B: Gentle mixing of flocculant and crude CN solution

Volume D: Settling area for water insoluble sludge

Volume C: Purified CN solution

Crude CN solution was pumped into tank A where it was thoroughly mixed with a suitable flocculant, Nordfloc A172 (high speed mixer, short residence time).

Thereafter the solution was stirred gently in tank B for 2-6 minutes in order to allow aggregation of larger flocs. The solution flows into area D by gravity and distributes throughout the whole tank, and moves slowly upwards into area C. :

The flocs (insoluble material) migrated down into area D, the coned part of the tank and were drained off at the bottom. From area C the solution was forced to the top and overflows a wall, into a discharge chamber before leaving through an outlet nozzle “clean melt”.

Neutralized CN solution from the plant was used (“KS smelte” in figure 2) and several test runs were made under various conditions. The results are given in table 2 below.

® 11

Table 2. Results from purification by settling in pilot plant

CN sol. | solut F tank n agent |4 in 1% ion : of sludge) + CN

Insolub AmolP | sol. (min) /F 63% | 62 NA| NA| NA| 86 57] 74] NA| 484] 50 | (oao|ois|oar| 36] 80] 801 Nal 250

Gs00 | 61] | [050] 36] 106] 80] +0.I| 189] — rr 1° 1

SiS | 61010] Gre 04a] | Na ®3| NA| 203] [eT 0n | 0 [057] 8] NA| 82] Nal 150]

S| ei] NA] NAO] e010 90] 02] 120

NA= not available

Example 3

In a further example the pilot plant of Figure 2 was replaced by more advanced settling equipment developed for water purification. A Lamella settling unit, (model

LF, supplied by Nordic Water) was connected to the neutralization tank of the CN plant (stage 5 in figure 1) and a decanter centrifuge for dewatering the sludge phase was connected.

In principle the Lamella unit is very similar to the settling equipment of fig 2 but the unit is equipped with several) plates (lamellas), assembled some 0.05-0.1 m apart from each other. The sloop of the plates is 55 degrees. The total projected horizontal settling area is 5 m>. The space in between the plates allows the liquid to move freely upwards, but insoluble particles are hampered and deposit on the plates and slide downwards into the coned sludge region.

A cross-section of the pilot plant showing the Lamella unit with connections is shown in fig 4

At the upper, left side of the “Lamella unit” there is a flocculator consisting of two compartments with a total volume of 1m>. The first compartment is small and

® 12 equipped with an agitator having high rotating speed in order to mix the flocculant and the melt well.

The melt overflows to the second compartment which has a slow moving agitator to keep the formed flocculated particles suspended.

From the second compartment the melt/solution flows into the Lamella tank and enters at the bottom of the plates. The solution moves upwards in between the plates and leave the lamella separator via discharge flumes placed over the plates. The discharge flumes have lot of holes through which the liquid has to flow in order to exit the lamella. In this way the flumes ensure that the solution distributes evenly over the plates.

Results wherein the retention time and density of the crude CN solution are varied are given in figure 5. Figure 5 shows the amount of water insoluble material in the purified CN melt leaving the lamella unit depending on the density of the melt (at 60-70 °C, including flocculant) and the feed flow of the melt.

These results show that the lower the density of the crude CN solution the lower the ‘content of insoluble material of the purified solution exiting the lamella unit. At densities between 1.58 and 1.60 kg/1 ( 70 °C) the content of insoluble material will normally be lower than 300 ppm. At densities around 1.46-1.47 kg/l content of insolubles below 50 ppm is achievable.

Variation of retention time between 60 and 35 min does not significantly influence on the purification of the CN solution.

Example 4

Effect of the ratio mol P/mol F of the crude CN solution

The lamella settling unit was connected to the CN plant to execute a continuous : settling trial over three days. The ratio mol P/mol F of the crude CN was varied by changing rock phosphate in the NPK plant and by adding phosphoric acid to the crude CN liquor. The content of insoluble material of purified CN solution was measured. 40

The following conditions were used:

Flow: 2.5 m3 CN solution/h

® 13

Retention time: 34 min

Flocculant dosage: 20 ppm (0.1% solution)

Density of melt: Average 1580 kg/m3 at 80 °C (1550-1620)

Content of insoluble material in crude CN: 1-1.6% pH of crude CN solution was between 5.5 and 6.5.

Flow of sludge phase: 250 liter/hour

Results are given in figure 6. The amount of water insoluble material in CN solution exiting the lamella unit and the mol P/mol F ratio in crude CN over a period of 3 days is shown.

Figure 6 shows that the water insolubles were reduced to between 150 and 350 ppm in the period up to May 23, 1800 hours. In this period the ratio mol P/mol F values were between 0.4 and 0.5.

From May 23, 1800 hours the water insolubles started to increase and the ratio mol

P/mol F of the crude CN decreased to 0.3. From May 24, 0400 and so forth the water insolubles declined from approximately 1000 ppm to approx. 200 ppm as the ratio mol P/mol F steadily moved towards 0.7.

The latter development of the water insolubles was disrupted between 1200 and 1600, May 24 because the pH values developed outside the acceptable range, (above 7 due to HNOs3 blockage in the plant).

When the mol P/mol F of the crude CN solution (and the acidic CN solution from the NPK plant) decreased the water insoluble crystals formed in the neutralization stage became smaller and fluffier. Consequently more voluminous sludge was formed in the settling unit. Such sludge settled much slower and the insolubles of the CN exiting the Lamella increased.

The mol P/mol F is a characteristic of the crude CN and can be varied by changing rock phosphates in the NPK plant or by adding concentrated phosphoric acid.

Treatment of sludge phase:

The sludge phase exiting the lamella unit was connected to a decanter centrifuge to remove the dry material of the sludge phase. The decanter centrifuge has a g-force of about 550.

Prior to entering the centrifuge, 25-35 ppm flocculant was added to the sludge phase. 40 Concentrated sludge (approx. 15% of -lamella feed) exiting the centrifuge was recycled to the digestion stage i.e. stage 1 in figure 1.

Ps | 14

CN solution exiting the centrifuge (approx. 85% of centrifuge feed) had below 450 ppm insolubles and were mixed with purified CN stream exiting the lamella unit or recycled to CN crude stream entering the lamella unit.

The results show that a crude CN solution originating from the nitrophosphate process and having a density up to 1620 kg/m3 at 70 °C, can be purified to between 200 and 300 ppm insoluble material by means of a settling tank, preferably a settling tank having lamella plates, provided retention time is up to 35 min and the ratio mol P/mol F of the CN solution is above 0.4, preferably above 0.5.

If the ratio mol P/mol F is below 0.4 the rate of sludge settling decreases and makes the purification less efficient. Purification can still be carried out but the required sludge flow exiting the settling tank will be much higher.

The mol P/mol F can easily be varied by adding concentrated phosphoric acid or : another phosphate source soluble in the acidic crude CN solution.

Claims

® CLAIMS

1. A method for the purification of calcium nitrate solutions or melts, characterized in that it comprises the step of settling.

2. The method according to claim 1, characterized in that the settling is followed by centrifugation of the sludge phase.

3. The method according to claim 1 or 2, characterized in that the sludge is recycled by return to the step of digestion of rock phosphate.

4. The method according to claim 1, 2, or 3, characterized in that it comprises the steps of a) digestion of rock phosphate, b) cooling/ crystallization, c) filtration, d) washing of crystals, e) dissolve and dilute to 1.3-1.75 kg/l at temperatures between 20-90 C °C f) neutralization g) addition of flocculant and adjustment of mol P/mol F ratio h) settling i) centrifugation of sludge phase and possibly recycling of sludge to step a) j) evaporation k) particulation.

5. The method according to claim 4, characterized in that in step e) the melt is diluted to about 1.6 kg/l at 70°C.

6. The method according to claim 4 or 5, characterized in that in step g) the mol P/mol F is adjusted to above 0.30 preferably above 0.45

7. The method according to claim 4, S or 6, characterized in that the settling in step h) is carried out in a settling tank equipped with thin plates, lamellas.

8. A calcium nitrate solution/melt, characterized in that it comprise less than3 % insolubles.

$0. The calcium nitrate solution/melt according to claim 8, characterized in that itis prepared by a method according to claims - 1t07.

10. The method according to claim 1, substantially as herein described with reference to and/or as illustrated by any of the examples and/or any of the accompanying figures.

11. The calcium nitrate solution/ melt according to claim 8, substantially as herein described with reference to and/or as illustrated by any of the examples and/or any of the accompanying figures. . Dated this 11" day of March 2009 ~~ Adams & Adams Applicants Patent Attorneys