GB2059935A - Fertilizer from apatite - Google Patents

Abstract

Apatite is conventionally made into a fertilizer material by treatment with a strong acid, requiring capital- intensive industry. Hydroxyapatite is treated above 1100 DEG C with alkali but world reserves are problematic. The invention comprises roasting apatite at up to 1100 DEG C with a carbonate and/or aluminosilicate of an alkali metal in an amount such that the molar ratio apatite (as P2O5); alkali metal is 1: at least 3 and in the presence of sufficient siliceous material to keep the free line content below 2% by wt. and to inhibit the formation of tetrocalcium phosphate.

Description

SPECIFICATION Fertilizer material from apatite This invention relates to making a fertilizer material from apatite. Apatite is an insoluble phosphoruscontaining mineral, approximating to Ca5(PO4)3 (F, OH, CI, 2iCOs), and the phosphate content must be rendered soluble for fertilizer use.

Apatite, the most abundant terrestial form of phosphorus, is conventionally treated with a strong acid such as nitric or sulphuric acid to render the phosphate soluble. This requires a capital-intensive industry.

Another known way of treating apatite is the 'Rhenania process' described in British Patent Specification 301022. The apatite is mixed with Na2CO3 to give a molar ratio of Na2CO3/P205 = 1.0, while at the same time sufficient SiO2 is also added to combine with excess CaO. The reactants are ground together and calcined in a rotary kiln at 11 00C - 1 200C for approximately 2 hours. Fluorine is said to be retained in the process, although steam is sometimes admitted to the kiln before 1000C is reached in an attempt to remove at least part of the fluorine.The sintered product may be used directly as a slow-release source of P or it may be subsequently extracted with hot aqueous Na2CO3 solution, giving either Na3PO4 or Ca3(PO4)2. This process requires high temperatures, and hydroxy-rich apatite, which is indigenous to central Europe and occurs in a few other regions of the world, some of which are of questionable reliability.

Hard mineral apatite (richer in chlorine/fluorine) is widely available in Sri Lanka, India and East Africa and is often a by-product from other mining operations. The present invention seeks to make fertilizer material from such apatite at a lower temperature than the 1100C - 1 200C required in the Rhenania process and without using acid. Sri Lanka is understood to have an indigenous alkali (NaOH) industry. (While it does also produce hydrochloric acid, this is not a suitable acid for treating apatite). The alkali is readily convertible to sodium carbonate. To make fertilizer material from apatite according to the present invention requires siliceous material, and this is also widely available as quartz, sand or potash felspar (an alkali metal aluminosilicate).Addition of the last-named in small proportions also has the advantage of introducing available K2O, and the same might be said of mica.

Accordingly, the present invention is a method of making a fertilizer material from apatite, by roasting apatite at up to 1100C with a carbonate and/or aluminosilicate of an alkali metal in an amount such that the molar ratio apatite (as P205): alkali metal is 1: at least 3 and in the presence of sufficient siliceous material to keep the free-lime content of the fertilizer material below 2 weight % and to inhibit formation of tetracalcium phosphate.

The molar ratio apatite : alkali metal is preferably from 1:3 to 1:10, more preferably 1:3 to 1:5, for example 1:4.

The molar ratio apatite : siliceous material (as SiO2) is preferably from 1:0.75 to 1:1.0.

The roasting temperature is preferably below 1 000C, and desirably at least 800C, more preferably at least 850C, most preferably from 880C to 950C, for example 900C. The duration of roasting need not exceed 2 hours, and is preferably at least 1 hour.

Preferably the apatite, the siliceous material and the carbonate and/or aluminosilicate are pressed together (e.g. pelletised) before the roasting. This appears to enhance the rate of reaction.

The invention extends to the fertilizer material made as set forth above, optionally admixed with other agriculturally acceptable components.

The reasons for avoiding excessive free lime and .etracalcium phosphate (i.e. why siliceous material is added) are as follows: Free lime is capable of causing skin burns, reacts with moisture thereby causing caking and may make the fertilizer, and hence the soil, too alkaline. The phosphate in tetracalcium phosphate Ca4P2Og is all soluble, i.e. it is at first sight an ideal fertilizer material. However, Ca4P2Og is liable to conversion in the presence of water vapour, which is likely in a fossil-fuel-fired tunnel kiln, to CaO (or Ca(OH)2) plus insoluble hydroxyapatite, one of the very materials which the present invention was devised to solubilise.

In practice, the quantities of Na2O/K2O and SiO2 (which must be added in order to eliminate the above undesirable phases) are desirably the minimum, as an excess would result in too much dilution of the phosphate phases. A way of determining these is to consider the CaO- and P205-rich regions of the system CaO-Na2O-P205-SiO2. The plane of compositions lying between Ca2SiO4, Ca3(PO4)2 and Ca NaPO4 just fulfils the condition that CaO and Ca4P2Og should be absent and, furthermore, we find that this plane of compositions constitutes a true ternary system at subsolidus temperatures. Its position within the quaternary system is shown in the accompanying drawing.

Table 1 records the results of solubility determinations made on pure single-phase preparations. For present purposes availability is defined by the relation: 0/ available PO - 100 x 2%-citric-acid-soluble P205 25 available P205 = 100 x total P205 The experimental method for determining the 2%-citric-acid-soluble P205 is given in the Appendix. Ground mineral apatite is poorly soluble: typically only 17 - 18% of its P205 content is 'available'.

Amongst the phases having 100% available P205 are Ca4P2O9, nagelschmidtite, silicocarnotite, rhenanite (a range of solid solutions around CaNaPO4), and an a Ca2SiO4 solid solution containing typically 30 wt. % Ca3(PO4)2. The presence of silica in solid solution in rhenanite appears to activate the dissolution of phosphate. Moreover, phase A is not completely soluble unless it too contains silica in solid solution; Phase A is explained in the footnote to Table 3. Both a and ss Ca3(PO4)2 give less than 100% available P205.

Moreover, while Ca4P2O9 has 100% availability, it is (as already mentioned) readily converted to hydroxyapatite by annealing in air, whereby the available P205 falls to 20%. Attempts to form a solid solution, substituting two Na+ ions for Ca++ ions, in the hope that Ca4~xNa2xP209 would be less reactive to water vapour than the Ca4P2Og, were unavailing.

CaNa5P2O9 was found to be 100% extractable, but is hygroscopic and therefore undesirable.

TABLE 1 Results of Solubility Studies Sample P205 Soluble in Solubility 2% citric acid as percent of total P205 p Ca2(PO4)2 34.0 74 a Ca3(PO4)2 36.6 80 Ca4P2Og 38.9 100 Ca4P2O9 heated at 1000"C in air 7.8 20 Ca5(SiO4) (PO4)2 30.1 100 Ca7(SiO4)2(PO4)2 21.9 100 a-Ca2SiO4 ss containing Ca3(PO4)2 [Composition =70.0wt%Ca2SiO4] 14.0 100 PhaseA-Ca5Na2(PO4)4 40.1 90 Phase Ass containing SiO2 [Composition = 20 wt. % Ca2SiO4, 40% Ca3(P04)2, 40% CaNaPO4 36.4 100 t3CaNaPO4 45.0 100 a CaNaPO4 ss containing SiO2 [Composition = 10wt%Ca2SiO4, 10% Ca3(PO4)2,80% CaNaPO4] 40.5 100 CaNa6P209 36.9 100 Sri Lanka Apatite Sample (1) 6.3 17 Sri Lanka Apatite Sample (2) 6.2 18 Note: ss = solid solution; Sri Lanka apatite sample (1) is a sample of pure apatite from the "leached zone" in the deposit at Eppawela, Sri Lanka. Sample (2) is a commercially beneficiated sample of apatite from Eppawela, Sri lanka.

For explanation of Phase A, see footnote to Table 3.

As for the influence of halogens, chlorine is almost entirely during the firing of apatite-containing batches, although most of the fluorine is retained. The fluorine is believed to be present in solid solution in phases which are soluble in citric acid. When these phases are dissolved in citric acid, fluorine is probably present in the solution in the form of fluorosilicate complexes.

The invention will now be described by way of example. The accompanying drawing shows a corner of the quaternary system CaO-Na2O-P205-SiO2. The plane Ca3(PO4)2-Ca2SiO4-Ca NaPO4 has been marked out.

Compositions on this plane contain neither CaO nor Ca4P2Og, and accordingly are desirable.

Mineral apatite (minus 100 mesh BS) was reacted with Na2CO3 and SiO2 (quartz, minus 120 mesh). The apatite was taken from the "leached zone" of the deposit: Table 2 gives a complete analysis typical of the concentrate as well as partial analysis of the particular batch of apatite concentrate used in this study.

Microscopically, the apatite occurs as anhedral grains, most of which are monocrystals.

TABLE 2 Chemical Composition of Sri Lanka Apatite v,,t% Sample EP/N/J Sample used in from the leached zone (a) the Examples CaO 55.30 SrO 1.18 MgO 0.01 MnO 0.01 Fe2O3 0.08 SiO2 0.40 P205 40.75 38.10 F 1.78 1.70 Cl 2.29 2.20 (a) Analysis reported by the Geological Survey Department, Colombo 2, Sri Lanka (1973). The two samples were believed to be essentially identical.

Reaction batches were prepared by blending these raw materials and firing following a heating rate of 5C/min at a constant temperature for 2 hours. Table 3 records the results of the annealing treatments. The phases present were determined by X-ray powder diffraction but the limit of detection of unreacted apatite was as high as 5%.

In the absence of SiO2, mixtures of apatite and Na2CO3 react to produce iarge amounts of free CaO.

Therefore, as free CaO is deemed to be an undesirable constituent, it is essential to add something to combine with it: SiO2 fulfils this role.

Much of the reaction is completed swiftly, even though this apatite is comparatively coarse-grained. As a rough guide, batches having a molar ratio of apatite to Na2CO3from 1 :1.5two to 1:2.0 (i.e. apatite : alkali metal = from 1:3 to 1:4) gave the most rapid reaction at low temperatures. These batch proportions correspond to a weight percentage of Na2CO3 between 20 and 27%. Some excess sodium carbonate was tolerable. If the optimum proportions of all three components are considered, molar ratios of apatite: Na2CO3 : SiO2 close to 1:2:1 are favourable for reaction. If the SiO2 content is reduced slightly below this optimum, for example to 1 :2:4, a high yield of available phosphorus is obtained, but free CaO is also developed.Similarly, reduction of the sodium carbonate content leads to incomplete reaction and the appearance of unreacted apatite which can only be removed bysintering at 1100'- 1300"C.

Table 3 shows the various compositions tried. Examples 1 to 3 are according to the invention. Examples A to J are not according to the invention. It will be seen that few of Examples A to J gave any significant reaction below 1 100"C, and of those which did, either inadequate phosphate was solubilised (as H) or free lime and sometimes a hygroscopic product resulted (as C and D). The phases present are given roughly in the order: most first.

TABLE 3 Available P205 Content ofSinters made with Apatite Composition Example No. Molar ratio Weight per cent Apatite: Na2CO3:SiO2 Apatite Na2CO3 SiO2 A 1:1:0 82.9 17.1 0.0 B 2:3:0 76.3 23.7 0.0 C 1:2:0 71.0 29.0 0.0 D 1:10:0 32.9 67.1 0.0 E 5:2:8 78.6 6.5 14.9 F 4:2:3 85.1 8.1 6.8 G 10:8:3 82.9 14.2 2.9 H 13:10:11 79.4 12.7 7.9 J 1:1:1 75.5 15.6 8.9 1 4:6:3 71.5 22.2 6.3 K 2:3:3 67.3 20.9 11.8 2 4:8:3 66.6 27.5 5.9 3 1:2:1 65.3 27.0 7.7 EX. TEMP. 2% CITRIC ACID SOLUBILITY PHASES PRESENT NO.Wt Percent Percent out of (See abbreviations C P205 Total P205 below) A 1100 20.5 55 p R+CaO+Fap 1300 28.5 76 ss R + ααR + CaO + F ap B 900 23.0 67 p R+aR+CaO+Fap 1100 27.8 81 p R+aR+CaO+Fap 1300 29.5 86 p R + aR + CaO + F ap C 900 31.4 90 p R+CaNa6P2O5+CaO + tr.F ap 1100 35.0 100 p R+CaO 1300 34.7 100 p R+CaO D 900 16.6 100 CaO + CaNa6P2Og E 1100 8.0 23 Fap+aR+pR 1300 14.4 42 Fap+A F 1100 9.8 27 F ap + aR + ssR 1300 15.8 43 Fap+A G 1100 18.1 49 p R+Fap+CaO 1300 18.8 51 p R+Fap+CaO H 900 10.0 36 Fap + ssR 1100 16.5 59 p R+Fap 1300 20.4 73 A + ssC3P + F ap J 1100 18.8 55 p R + Fap 1300 25.1 74 A+pR+Fap 1 900 21.7 67 p R+aR+Fap 1100 29.7 91 p R+aR+Fap 1300 30.0 92 p R+aR+Fap K 1100 21.8 70 ss R+Fap 1300 24.9 80 ss R+Fap 900 27.0 92 a R + PR + CaO + tr.

Fap 1100 30.4 100 ss R + αR + CaO 1300 31.1 100 ss R+aR+CaO 900 25.5 91 α a R+pR+tr.Fap 1100 28.1 100 a R 1300 28.6 100 a R Abbreviations:- R = Rhenanite, CaNaPO4; F ap = Fluorapatite, Ca5(PO4)3F (all remaining apatite having this composition); tr. = trace. a and pare the high and low temperature forms respectively.

C3P = tricalcium phosphate. Phase A is believed to approximate to Ca5Na2(PO4)4 and is the crystalline phase defined byAndo and Matsuno (J. Ando and S. Matsuno, Bull. Chem. Soc. Japan 41 (1968) 342.) In the present context, Phase A solid solutions may also contain silicon.

Appendix Determining the 2%-citric-acid-soluble P205 in a sample: The sample is ground to pass a 100 mesh BS sieve. A 1.0 g sample is extracted with 100 ml of 2% citric acid in a mechanical shaker operating at 260 oscillations per minute for 30 minutes at 18C. The resultant solution is filtered under vacuum using a sintered glass crucible (porosity No.4) and the filtrate P205 content is determined by the vanadomolybdate method, in which the following reagents are used: (i) Ammonium metavanadate solution, prepared by dissolving 1.12 g of ammonium metavanadate in a mixture of 240 ml of concentrated He104 acid and 260 ml of water.

(ii) Ammonium molybdate solution, prepared by dissolving 35 g of ammonium molybdate in 500 ml of water.

(iii) Standard phosphate solution, 0.2 mg/ml P2O5, prepared by dissolving 0.3835 g of dried potassium dihydrogen phosphate in 1 litre of water.

Solutions (i) and (ii) are stable and will keep for some months.

To a 2 ml aliquot of sample filtrate are added 10 ml of the vanadate solution (i) and 10 ml of the molybdate solution (ii) successively, mixing well after the addition of each reagent. The resultant solution is diluted with water in a 100 ml volumetric flask.

After 30 minutes the absorbance is measured at 460 nm using a Unicam SP 600 colorimeter against a reagent blank solution. The standard phosphate solution (iii) is used in calibration.

Claims (13)

1. A method of making a fertilizer material from apatite, by roasting apatite at up to 11 OOC with a carbonate and/or aluminosilicate of an alkali metal in an amount such that the molar ratio apatite (as P205):alkali metal is 1: at least 3 and in the presence of sufficient siliceous material to keep the free-lime content of the fertilizer material below 2 weight % and to inhibit formation of tetracalcium phosphate.

2. A method according to Claim 1, wherein the molar ratio apatite:alkali metal is from 1:3 to 1:10.

3. A method according to Claim 2, wherein the molar ratio apatite:alkali metal is from 1:3 to 1:5.

4. A method according to any preceding claim, wherein the molar ratio apatite:siliceous material (as SiO2) is from 1:0.75 to 1:1.0.

5. A method according to any preceding claim, wherein the temperature of roasting the apatite is below 1000C.

6. A method according to any preceding claim, wherein the temperature of roasting the apatite is at least 800C.

7. A method according to Claim 6, wherein the said temperature is at least 850C.

8. A method according to Claim 7, wherein the said temperature is from 880C to 950C.

9. A method according to any preceding claim, wherein the duration of the roasting does not exceed 2 hours.

10. A method according to any preceding claim, wherein the duration of the roasting is at least 1 hour.

11. A method according to any preceding claim, further comprising pressing together the apatite, the siliceous material and the carbonate and/or aluminosilicate before the roasting.

12. A method according to Claim 1 substantially as hereinbefore described with reference to Example 1, Example 2 or Example 3.

13. Afertilizer material made by a method according to any preceding claim, and optionally being admixed with other agriculturally acceptable components.