STUDIES ON COMPLETE AND PARTIAL ACIDULATION OF EPPAWELA APATITE

Abstraa : Acidulation of Eppawela apatite with hydrochloric acid and sulphuric acid has been investigated in an attempt t o convert this rock phosphate to an acceptable P-fertilizer using a simple and low cost method. Although direct acidulation with hydrochloric acid leads to a highly hygroscopic product, physical condition of this material can be improved by subsequent treatment with ammonia. Hydrochloric acid acidulation with 15-20% HC1 followed by pH adjustment using an alkali, leads to the precipitation of dicalcium phosphate,CaHP04, which can readily be used as a P-fertilizer. Use of ammonia and ammonium sal& has an added advantage in that the product contains nitrogen in addition to phosphorus. Physical condition of H SO4 acidulated material is superior and the fertilizer value of this product is depen2dent on ,tHe extent of acidulation. Since the cost of this product depends mainly on the cost of H2S04, partial acidulation with sulphuric acid seems t o be apotentially useful and economical method. 50% acidulation with sulphuric acid t o produce partially ac.idulated phosphate rock (PAPR-50) containing 17% wt available P205 and 25% wt total P 0 appears to be suitable. 2 5

Rock phosphate reserves6 at Eppawela in ,Sri.Lanka can be represented as chlorfluorapatite Ca5(P04)9(Cl,F).I t has been reported8 that the use of Eppawela apatite as a phosphatic fertilizer is limited due to its very low solubility.Its water solubility is about 0.5% wt P 0 5 , while 2% citric acid solubility3 is in the range 5-6% wt P 05.&us, it.is not generally recommended for direct application especi & y for short term crops such as paddy.As such, attempts have been made t o convert this mineral to more soluble phosphate fertilizers by low temperature sintering method^.^*^*^Acidulation is one of the common methods1' to convert'rock phosphate t o more soluble phosphate fertilizers.Most commonly rock phosphate is treated with either H2S04 or H3P04 t o produce singlesuperphosphate (SSP) or triplesuperphosphate (TSP) respectively.SSP contains about 20% wt available P O5 while TSP possesses much higher available phosphorus content ( N 4 25 % wt P205).
.Eppawela rock hosphate is believed t o be of igneous origin6 and 5 contains more chlorine than fluorine.Its total P 0 content is rather high, In the present study complete and partial acidulation of Eppawela apatite with hydrochloric acid and sulphuric acid has been investigated in an attempt to convert Eppawela rock phosphate to a more soluble fertilizer grade phosphate material.

Experimental
Two rock phosphate samples labelled I and I1 have been used in the present study.Sample I was collected from one of the hillocks in the northern part of the "leached zone" of the apatite deposit at Eppawela, Sri Lanka.This rock sample was crushed, powdered and sieved (100 mesh) for subsequent investigation.Sample I1 is a sample of commercially available powdered (90% passing through 100 mesh) Eppawela rock phosphate.

Chemical analysis
Analysis of the metallic elements and silicon has been performed using a X-ray fluorescence spectrometer.Total P205 has been determined by the X-ray fluorescence as well as by the vanadomolybdate m e t h ~d .~ Analyses for' fluoride have been made using Orion model 94-09 fluoride ion electrode1 against a Beckman type R.L.B. calomel reference electrode.The samples were dissolved in 5M hydrochloric acid for this analysis.Chlorine analyses were made by dissolving the samples in 1:l nitric acid and titrating it with standard 0.05M silver nitrate to a potentiometric end point.Table 1 shows the results of chemical analysis of the two samples for their major constituents and trace elements.Phases present in some products were identified by using powder X-ray diffraction with Cu Kct radiation.

HCl acidulation
Rock phosphate samples were treated with hydrochloric acid df known strength and stirred vigorously for 30 minutes in beakers.The resulting slurry waS allowed to stand for 6 hours and then transferred to orcelain 8 dishes for curing.After curing period, the product was dried at 100 C.
For dicalcium phosphate precipitation, the rock samples have been mechanically stirred with hydrochloric acid for 6 -8 hours.The pH of the resulting solution was adjusted in the .range 5 -7 using aqueous sodium hydroxide, Lime or ammonia.After the precipitation is complete, the product was filtered under suction and dried at 100°C prior to analysis.

H2S04 acidulation
Sulphuric acid acidulation was performed similar to hydrochloric acid acidulation but in this case drying was not necessary.During the curing period the acidulated product gradually transforms into a dry powder.

Estimation of phosphorus
Water and 2% citric acid soluble P205 contents of the samples have been determined1' by using about l g samples.These were extracted in 250 ml reagent bottles with 100 ml of distilled water or 2% citric acid solution using a mechanical shaker operating at about 250 oscillations per minute for 30 minutes.The total P O5 contknts have been determined by extracting the samples with conc.&I.The extracts were analysed for phosphorus by the vanadomolybdate9 method using Corning colorimeter model 253 at a wavelength of 460 nm.

Direct acidulation with HCl
Acidulation reaction for the complete (100%) acidulation of apatite'may be represented as follows.
~C ~; Several side, reactions.,mayalso occur depending on the other components and the ,impurities present in the rock.As such, the acid requirement for acidulation in general is calculated by.considering the nature and amounts of impurities present and the total P205 content of the rock.
Variation of available P205 with the acid concentration for 100% and 60% acidulations is shown in Figure 1.In both cases the available P O5 increases with increase in concentration up to about 18%'and fur ?h er increase in concentration does not have any effect on the available phosphorus content.Thus, the optimum concentration for HCl acidulation is in the range 15 -20%.Therefore, it appears that dilute hydrochloric acid solution could be conveniently used for the acidulation process.Completely acidulated product of rock sample I contains r, 19% available P20 j while commercial apatite (sample 11) gave a product containing N 17% wt P205.However, a product containing a maximum of 20% available P205 can be obtained by increasing the amount of acid added.On the other hand partially acidulated (60%) product contains 14 -16% wt available P205.

C
Major problem of HCl acidulation is due to the fact that the product contains a large amount of highly hygroscopic calcium chloride.The presence of calcium chloride leads to problems of drying, storage and transport of the fertilizer.However, it has been observed in the present study that the hygroscopic nature of the product can be considerably reduced and the physical properties improved by the treatment of the HC1 acidulated product with aqueous ammonia or a suitable ammonium salt.
Since both monocalcium phosphate, Ca(H2P04)2, and calcium chloride are highly water soluble it is rather difficult to separate one from the other by a simple and inexpensive technique.As an alternative, it may be possible to convert apatite to &calcium phosphate (CaHP04) which is water insoluble (Table 5) but citric acid soluble makihg its phosphorus available to the plants.Thus, extremely water soluble calcium chloride tan be conveniently removed from the product.

CaHP04 precipitation
The optimum conditions suitable for the precipitation of dicalcium phosphate, CaHP04, from acidic phosphate solutions have been.determined7 previously.The optimum pH for this precipitation is found to be in the range 5 -7.The variation of available P O5 of the product with the precipitation pH is shown in Figure 2.This con %-~r m s the optimum pH for the precipitation as 5 -7.The precipitation begins around pH 4 and when, the pH is increased beyond 7, citric acid solubility of the product decreases indicating the conversion of CaHP04 t o Ca3(P04)2.
pH adjustment in the acid extracts has been done using aqueous NaOH lime or ammonia.Table 2 shows the results of dicalcium phosphate precipitation after 60% and 100% acidulation followed by pH adjustment using aqueous NaOH.Both sets of results show that there is'an increase of 2% citric acid solubility of the product with increase in acid concentration up to 18%.60% acidulation with 18% HCl followed by neutralization with aqueous NaOH yielded a product containing 23-25% available P205.The rock sample 11, however, yielded a product containing slighdy lower ( N 21%) available P 2 0 content.The major constituent in the dried product was identified as &calcium phosphate (CaHPO4) by powder X-ray diffraction.Tricalcium phosphate, Ca3(P04)29 and apatite also have been identified in the product.Possibility of using lime to adjust the pH has been attempted.But in this case the product was found to contain about 16% available P 05.Rela-I tively large quantities of Ca3(P04)2 and hydroxyapatite in a dition t o CaHPO have been observed in the product obtained using lime.Thus i t is rather Afficult to control the reaction for the preferential precipitation of CaHP04 in the presence of lime.On the other hand, ammonia was found to be an acceptable pH controlling agent.I t has an added advantage in that the product contains appreciable amount of nitrogen in addition to phosphorus ( 26% wt available P 05).The following reaction may occur during the ammoniation facilitating &e precipitation of CaHP04.

~cidulation with sulphuric acid
Figure 3 shows the variation of the extent of conversion with the concentration of H SO4 used in the acidulation.All three curves representing loo%, 75% and %0% acidulation reactions show a maximum around 70% H2S04.Thus the optimum concentration for complete and partial acidula~on of apatite with H2S04 is in the range 65 -70%.As such, all the H2S0 aciclulation experiments have been performed using 70% sulphuric acia in the present study.P205 contents of the products obtained by complete and partial aciddabon of Eppawela apatite are given in Table 3.The variation of total, available and water soluble P O5 contents with the extent of acidulation are a shown in the Figure 4. SS produced (100% acidulation) from apatite contains 20.8% available P205, of which 19.6% is water soluble.The available P205 contents decrease with decrease in the extent of acidulation.50-60% acidulated product contains a reasonably high vdue of available P 0 (17-18% wt), although the amount of acid added is reduced by 4%-80%.Furthermore, partially acidulated product contains much higher total P205 content (Figure 4) which will eventually be available to the soil and to the plants.
Available P2O-5 contents of the product obtained after different " curing periods are shown in Table 4. Effect of curing period on the product for 100% and 60% acidulations axe shown in Figure 5.The result.indicate that for 60% acidulation the optimum curing period is about 3-4 weeks while for SSP production 5-6 weeks curing is required.Thus, partial acidulation has an added advantage of having a less curing period,.
50% acidulated product was found to contain unreacted apatite, monocalcium phosphate and trace amounts of dicalcium phosphate by powder X-ray diffraction.The product was not found to be sticky.However, a suitable treatment may be necessary if the product obtained in a commercial scale preparation is found to be sticky.   .acidulated product.
Phosphorus contents of the starting materials are compared with those of the major constituents in the fertiIizer products in Table 5. Conditions of acidulation of Eppawela apatite are summarised in Table 6.For the production of 50% acidulated product, 30 kg H2S04 (100% basis) diluted to 70% is required for 100 kg powdered apatite.
Direct acidulation of Eppawela rock phosphate with HC1 to produce SSP is not feasible owing to the presence of large amounts of highly hygroscopic calcium chloride in the product.However, physical conditions of this product may be improved considerably by the subsequent treatment with ammonia or ammonium salts.A similar situation is anticipated in nitric acid acidulation to produce nitrophosphates.Nevertheless, HCl acidulation followed by alkali treatment produces CaHP04 which can readily be used as P-fertilizer.Use of CaHP04 may be advantageous in areas where there is considerable leaching of phosphates.
Although-lime is the cheapest alkali available, the use of lime in the pH adjustment is not recommended..Arnmonia is an efficient neutralization agent because it facilitates the formation of dicalcium phosphate and the product contains another important plant nutrient, N.However, considering the availability and economic factors aqueous NaOH seems to be the most suitable.
In the case of sulphuric acid acidulation the major factor seems'to be ' the cost of sulphuric acid.The cost of 'the final product will'be dependent mainly on the cost and the amount of sulphuric acid used (extent of acidulation) in the process.When the extent of acidulation is less the cost will be lower ind ,the total P 0 content of the product will be more.On the.other hand, t h e available %i+d5.content or the, fertilizer value increases with increase in the extent o acidulation.
On account of the large saving due tocutting down of acid requirement, and due to the presence of reasonably high phosphorus content of the partially acidulated product, this material can be considered as ,a potentially useful and economical phosphate fertilizer for Sri.Lanka.Thus, 50% acidulation of Eppawela apatite with H2S04 t o produce partially acidulated phosphate rock (PAPR) containing about 17% wt available P205 and 25% total P205 appears to be suitable.

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Free phosphoric acid content in the acidulated product 'plays an important role in fixing phosphates as aluminium and iron phosphates.Since the free phosphoric acid content is minimal in the partially acidulated product, the effect of the presence of A1203 and Fe203 is negligible in comparison with100% acidulation to produce SSP.
Presence of high chlorine content in Eppawela apatite leads to corrosion problems in an industrial acidulation process.As such, mechanical mixers and the -acidulation tanks should be lined with corrosion resistant material.Furthermore, pollution factors also should be taken into consideration in deciding the location of acidulation plants.
Although chemical tests and laboratory evaluations indicate that these products can be used as P-fertilizers, it is essential to examine the crop response of the products and the financial viability of these processes.Therefore, pilot plant trials together with comprehensive financial evaluation of the processes and long-term field trials have to be carried out prior to commencement of any commercial production.

2 5 the
average being about 36%.It has a relatively high and a variable content 18410%) of total A.1203 and Fe203.Although these special chemical features of Eppawela apatite could make it difficult t o use it for conventional acidulation processes, it is of interest to investigate the basic aspects of its acidulation reaction with locally produced mineral acids such as hydrochloric and sulphuric acids.

Figure 1 .
Figure 1.Variation of available phosphorus content of HC1-acidulated product with the acid concentration.

Figure 2 .
Figure 2. Effect of precipitation pH.on the available phosphorus content of the precipltated product.. ..-

Figure 3 .
Figure 3.Effect of concentration of H2S04 on the extent of,conversion., ,

Figure 4 .
Figure 4. Variation of total, available and water soluble phosphorus contents with the extent of H2S04-acidulation.

Figure 5 .
Figure 5.Effect of curing period on the available eo.te.nt of thc H2S04-:

Table 2 . Results of HC1 acidulation followed by precipitation.
% in the filtrateNote: Base used is aqueous sodium hydroxide .

Table 3 . Variation o f available P205 content with the extent of H2S04 acidulation.
Note: Original rock phosphate contained 35.3% wt.total P205 ; curing period = 4 weeks

Table 4 .
H2S04 acidulation : Effect of curing time on the availble P205 content