FERTILIZER MATERIAL FROM APATITE

Abstract: Apatite is conventionally made into a fertilizer material by treatment with a strong acid, requiring capital-intensive industry. Hydroxyapatite is treated above 1100C with alkali but world reserves are problematic. The invention treats apatite at 900C with sodium aluminosilicate/carbonate and siliceous material in quantities to keep the composition in terms of CaO, SiO<u2>u, Na<u2>uO and P<u2>uO<u5>u in or near the ternary system Ca<u2>uSiO<u4>u - Ca<u3>u(PO<u4>u)<u2>u - CaNaPO<u4>u.

Description: FERTILIZER MATERIAL FROM APATITE This invention relates to making a fertilizer material from apatite. Apatite is an insoluble phosphorus-containing mineral, approximating to Ca (PO.) (F,OH,CI,|-CO ), 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 1 described in British Patent Specification 301022. The apatite is mixed with Na CO to give a molar ratio of 2 3 Na CO /P 0 ~ 1.0, while at the same time sufficient Si0 o is **, 3 25 2 also added to combine with excess CaO. The reactants are ground together and calcined in a rotary kiln at 1100C - 1200C for approximately 2 hours. Fluorine is said to be retained in the process, although steam is sometimes admitted to the kiln before lOOOC 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 Na CO solution, giving either Na PO. or Ca (P0, ) . This 3m J 3 ** 3 *± 2 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 - 1200C 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 K 0, 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 P 0 ): alkali 2 5 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 SiO ) is preferably from 1:0.75 to 1:1.0. The roasting temperature is preferably below 1000C, and desirably at least 8θθC, more preferably at least 85OC, most preferably from 88OC 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 tetracalcium 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 OMPI Ca.P 0„ is all soluble, i.e. it is at first sight an ideal 4 2 9 fertilizer material. However, Ca.P 0 is liable to conversion 42 9 in the presence of water vapour, which is likely in a fossil- fuel-fired tunnel kiln, to CaO (or Ca(OH) ) plus insoluble hydroxyapatite, one of the very materials which the present invention was devised to solubilise. In practice, the quantities of Na 0/K 0 and SiO (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 P 0 -rich regions 25 of the system CaO-Na O-P 0 -SiO . The plane of compositions lying between Ca SiO , Ca PO. ) and CaNaPθ, just fulfils the 2 - 3 ** 2 % condition that CaO and Ca.P 0 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: % available P„0 = 100 x 29^-i-c*-i J -t*-r-*i- L c*--a*-*c*-i-*-d"-s ~ ~ oul m .u u .ble~_ P r £0 > 2 -5 — J total P 0 2 _. The experimental method for determining the 2%-citric-acid- soluble P 0 is given in the Appendix. Ground mineral apatite 25 is poorly soluble: typically only 17 - 18% of its P 0 content 2 5 is 'a-vailable' . Amongst the phases having 100% available P 0 are Ca.P 0 , nagelschmidtite, silicocarnotite, rhenanite (a range of solid solutions around CaNaPO, ) , and an α Ca SiO, solid solution 2 4 containing typically 30 wt % Ca-z(P0. ) . 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 α and β Ca (PO. ) ? give less than 100 -available P °5* Moreover - while Ca*P o 0 has 100% availability, it is (as- already mentioned) *-- ■ 2 9 readily converted to hydroxyapatite by annealing in air, whereby the available P 0 falls to 20%. Attempts to form a solid 2 5 + ++ solution, substituting two Na ions for Ca ions, in the hope that Ca, Na P 0 would be less reactive to water vapour than 4-x 2x 2 9 the Ca.P O , were unavailing. " . __ y CaNa-P 0 was found to be 100% extractable, but is hygroscopic and therefore undesirable. TABLE 1. RESULTS OF SOLUBILITY STUDIES SAMPLE P O SOLUBLE IN " SOLUBILITY 2% CITRIC ACID . AS PERCENT OF TOTAL P 2°5 « Ca 3 (P0 4 ) 2 36.6 80 100 Ca 4 P 2° 9 38.9 Ca.P heated at 1000°C in air 7-8 20 4 2 9 Ca 5 (Si0 4 ) (P*V2 30.1 100 Ca 7 (Si0 4 ) 2 (P0 4 ) 2 21.9 100 α -Ca SiO. ss containing Ca (PO, )„ 2 4 a 3 42 Composition = 70.0 wt % Ca SiO, 14.0 100 Phase A - Ca Na ( PO, ), 40.1 90 5 2 44 Phase A ss containing SiO [Composition = 20 wt % Ca SiO, , 40% Ca (PO.) , 40% CaNaPoT] 36*.4 100 β CaNaPO 4, 45.0 100 α CaNaPO, ss containing SiO [Composition = 10 wt % Ca SiO, , 10% Ca, (P0 4 ) , 80% CaNaPO 71 40.5 100 Cont ' d. , TABLE 1. (Cont'd) SAMPLE P O SOLUBLE IN SOLBUBILITY 2% CITRIC ACID AS PERCENT OF TOTAL P 2°5 CaNa^P O 100 6 2 9 36.9 Sri Lanka Apatite Sample (1) 6.3 17 Sir 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 eliminated 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-Na O-P 0 -SiO . The plane Ca (PO ) -Ca SiO -CaNaPO, has been marked out. Compositions on this plane contain neither CaO nor Ca, P O , and accordingly are desirable. Mineral apatite (minus 100 mesh BS) was reacted with Na CO and SiO (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. - b - TABLE 2. CHEMICAL COMPOSITION OF SRI LANKA APATITE wt % Sample EP/N/J Sample used in (a) from the leached zone the Examples CaO 55-30 SrO 1.18 MgO 0.01 MnO 0.01 Fe O, 0.08 23 SiO„ 0.40 2 P 0 40.75 38.10 25 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 ~ 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 SiO , mixtures of apatite and Na CO react 2 2 to produce large 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: SiO fulfils this-rble. 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 Na CO from 1:1.5 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 Na CO between 20 and 27%. 2 3 _ OMPI ~~ Some excess sodium carbonate was tolerable. If the optimum proportions of all three components are considered, molar ratios of apatite : Na CO : SiO close to 1:2:1 are favourable for 2 3 2 reaction. If the SiO content is reduced slightly below this optimum, for example to 1:2:^, 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 by sintering 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 accord¬ ing to the invention. It will be seen that few of Examples A to o J gave any significant reaction below 1100 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 P 0 CONTENT OF SINTERS MADE WITH APATITE COMPOSITION Example No Molar ratio Weight per cent Apatite:Na CO :Si0 Apatite Na_C0 SiO 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 CO D 1 :10: 0 32.9 67.1 0.0 E 3 .2:8 78.6 6.3 14.9 F *+ .2:3 83.1 8.1 6.8 G 10 8:3 82.9 14.2 2.9 H 13 10 : 11 79 -A 12.7 7.9 J 1 1 : 1 73.5 15.6 8.9 1 k 6 :3 71.5 22.2 6.3 K 2 3:3 67.3 20.9 11.8 2 k 8:3 66.6 27-3 5.9 3 1 2: 1 65.3 27-0 7.7 -& 0 RE OMPI TABLE 3. (CONT ' D) EX. TEMP. 2% CITEIC ACID SOLUBILITY PHA. 3ES PRESENT NO. (Se< 3 abbreviations Wt Per cent Per- cent out of °C he: Low) P -°5 Total P ■ °5 A 1100 20.5 55 β E + CaO + F ap 1300 28.5 76 β E +C.Q-R + CaO + F ap B 900 23.0 67 - β R + αR + CaO + F ap 1100 27.8 81 β + fl{R + CaO + F ap 1300 29.5 86 β E + ctR + CaO + F ap C 900 31 -A 90 β E + CaNagP-0 + CaO + tr. F ap 1100 35.0 100 β R + CaO 1300 3^.7 100 β R + CaO D 900 16.6 100 CaO + Ca aPO E 1100 8.0 23 F ap + α β + βR 1300 14.4 42 F ap + A F 1100 9.8 27 F ap + αfi . βR 1300 15.8 43 F ap + A G 1100 18.1 ^9 β R + F ap + CaO 1300 18.8 51 β E + F ap + CaO H 900 10.0 36 F ap ÷ βR 1100 16.5 59 β E + F ap 1300 20.4 73 A + β C P + F ap J 1100 18.8 55 β E + F ap 1300 25.1 7 A + βR + F ap 1 900 21 .7 67 β B + α E + F ap 1100 29.7 91 β + α R + F ap 1300 30.0 92 β + 0 E + F ap K 1100 21 .8 70 β E + F ap ^URE O TABLE 3. (CONT 'D) TEMP. 2% CITRIC ACID SOLUBILITY PHA;- 3ES PRESENT (See -- Abbreviations Wt Percent Percent out of be]Low) °C Total _?- * O c P 2°5 25 1300 24.9 80 β R + F ap 900 27.0 92 oE + β R + CaO + tr. F ap 1100 30A 100 β _ +<-E + CaO 1300 31.1 moo -SH +α R + CaO 900 25.5 91 αR +/3 + tr. F ap 1100 28.1 100 a R 1300 28.6 100 a R Abbreviations:- R = Rhenanite, CaNaPO, ; F ap = Fluorapatite, Ca ς (PO, ) F (all remaining apatite having this composition); tr. = trace, α and β are the high and low temperature forms respectively. C,P = tricalcium phosphate. Phase A is believed to • approximate to Ca-.Na„(PO, ), and is the crystalline phase defined by Ando and Matsuno (J. Ando and S..Matsuno, Bull. Chem. Soc. Japan 41 (1968) 3^2.) In the present context, Phase A solid solutions may also contain silicon. . 10 - APPENDIX Determining the 2 -citric-acid-soluble O,- 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 P ? 0 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 HClO, 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/n . O c 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 6C0 colorimeter against a reagent blank solution. The standard phosphate solution (iii) is used in calibration.

Claims: CLAIMS 1. 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 P ? 0 ):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 SiO ) is. from lsθ.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 7ι wherein the said temperature is from 88OC 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.

Inventors: GLASSER F, GUNAWARDANE R
Applicants: GLASSER F, GUNAWARDANE R, NAT RES DEV, NAT RES DEV CORP
Application year: 1980
Main classification: C05B13
IPC codes: C05B13