Improving the specifications of Syrian raw phosphate by thermal treatment

Abdullah Watti; Mohamed Alnjjar; Abdulrzak Hammal

doi:10.1016/j.arabjc.2011.07.009

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Original Article

9 (

1_suppl

); S637-S642

doi:

10.1016/j.arabjc.2011.07.009

Improving the specifications of Syrian raw phosphate by thermal treatment

Abdullah Watti, Mohamed Alnjjar, Abdulrzak Hammal^⁎

Chemistry Department, Faculty of Science, University of Aleppo, Aleppo, Syria

⁎Corresponding author. Tel.: +963 940957320. abdlrzak1986@hotmail.com (Abdulrzak Hammal)

Received: 2011-4-9, Accepted: 2011-7-11, Published: 2011-09-01

Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

1 General company for phosphate and mines in Syria.

Abstract

Because of availability of raw phosphate in Syria and Syria’s advanced position in the production of raw phosphate, it is necessary to search for methods of treatment which allow to use the low-quality raw phosphate, and ensure good position in Syria, especially that the good raw phosphate continues to decrease. As well as the industry of phosphate fertilizers by acid treatment is associated with many difficulties which needs large production lines and advanced technologies.

In this research we treated the Syrian raw phosphate by thermal way in order to: (1) Enriching of studied Syrian raw phosphate that contains proportions of 28.60% of phosphorus pentoxide P₂O₅, 6.12% of carbonate, which we got after treatment at 850 °C for 30 min on a phosphate containing proportion of 33.95% of phosphorus pentoxide P₂O₅, small amount of carbonate 0.75% and almost free of organic materials. (2) Preparing phosphate fertilizer by thermal treatment in the presence of sodium carbonate, where it was found that the best conditions are adding 40% of sodium carbonate by weight of phosphate ore; temperature 1100 °C; time 120 min.

Keywords

Raw phosphate

Thermal treatment

Calcinations

Chemical reactivity

Fertilizers

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1 Introduction

Raw phosphate is the main ingredient for many important industries such as the fertilizers industry which consume about (90%) of global production, animal feed, as well as the manufacture of several phosphorus compounds.

The annual consumption of raw phosphate is close to (150) million tons, and demand for raw phosphate is increasing day after day especially by the countries that are interested in agricultural production to defend their food security. This increase is associated with a decrease in the sources of good raw phosphate. Phosphate is considered marketable if it contains at least 30% of phosphorus pentoxide P₂O₅. Consequently the sources of phosphate in future will depend on the poor raw phosphate (where the proportion of Phosphorus pentoxide P₂O₅ in it is less than 30%), which cannot be used directly in the industry and it needs different processing or enriching according to the type of raw and accompanying impurities (Abouzeid, 2008).

Syria is one of the countries developed in the production of phosphate, which has the eighth ranke in the world, and produces about 2.4% of world production (Jasinski, 2009), where it invests raw phosphate from two main mines Khnevis and Eastern. Annual production has reached to about 3.85 million tons, 600–700 thousand tons of which is invested locally in fertilizers plant in Homs for the manufacturing of phosphoric acid and triple super phosphate. The rest of the quantity is exported to a number of countries.¹

1.1

1.1 Types of raw phosphate

There are more than 200 types of raw phosphate in nature. These types differ a lot from each other in chemical and physical specifications.

Generally, raw phosphate occurs in nature in these forms:

Fluor-apatite Ca₁₀(PO4)₆F₂.
Hydroxy-apatite Ca₁₀(PO₄)₆(OH)₂.
Carbonate-hydroxy-apatites Ca₁₀(PO₄,CO₃)₆(OH)₂.
Francolite Ca₁₀₋_x₋_yNa_xMg_y(PO₄)₆₋_z(CO₃)_zF₀.₄_zF₂.
Dahllite 3Ca₃ (PO₄)₂·CaCO₃.
Collophane 3Ca₃(PO₄)₂·nCa(CO₃,F₂,O)·xH₂O.
Crandallite CaAl₃(PO₄)₂(OH)₅·(H₂O).
Variscite AlPO₄·2H₂O.
Strengite FePO₄·2H₂O (Straaten, 2002).

1.2

1.2 Evaluation of raw phosphate

The grade of raw phosphate depends on the total phosphor content, type of impurities and amount of impurities. Total phosphor is expressed as:

Phosphorus pentoxide P₂O₅.
Calcium phosphate Ca₃(PO₄)₂ (BPL), where:

% P_{2} O_{5} \times 2.1853 = % BPL (BPL \times 0.4576 = % P_{2} O_{5})

Phosphate is usually enriched or concentrated until the total phosphor concentration becomes more than 65% BPL or 30% P₂O₅ (Abouzeid, 2008; Zafar et al., 1996).

1.3

1.3 Chemical reactivity of raw phosphate (solubility)

The most important methods which are used in measuring the solubility of raw phosphate are

1.3.1

1.3.1 Neutral ammonium citrate (NAC)

A 1-g sample of PR is extracted with 100 ml of NAC solution at 65 °C for 1 h, then the solution is filtered and the amount of phosphorus in it is determined. This method is used mainly in the United States.

1.3.2

1.3.2 Citric acid 2% (volumetric) (CA)

A 1-g sample of PR is extracted with 100 ml of 2% CA solution at room temperature for 1 h, then the solution is filtered and the amount of phosphorus in it is determined. This method is used mainly in Malaysia, Brazil and New Zealand.

1.3.3

1.3.3 Formic acid 2% (volumetric) (FA)

A 1-g sample of PR is extracted with 100 ml of 2% FA solution at room temperature for 1 h, then the solution is filtered and the amount of phosphorus in it is determined. This method is used in countries of the European Union (Chien, 1993).

1.4

1.4 Enriching of raw phosphate

There are several techniques for upgrading phosphate ores. The choice of one or more of these techniques depends on the type of ore as well as the associated impurities. Among these techniques thermal treatment.

Traditionally, thermal treatment of raw phosphate is defined as heating up the raw to a certain temperature to obtain a product with specific properties (Abouzeid, 2008).

There are three types of thermal treatment of raw phosphate: calcination, sintering and fusion.

Calcination is a process that breaks down existing carbonates and drives off CO₂, sintering is agglomeration of small particles to form larger ones without reaching to the melting point, fusion is heating raw above the melting point. The objective of the first type of thermal treatment (calcination) is to enrich the raw phosphate, but the objective of the other types of thermal treatment (sintering and smelting) is to prepare fertilizers (Straaten, 2002).

1.5

1.5 Thermal phosphate

Thermal phosphate is preparation by several methods among them:

Sintering the apatite phosphate with sodium carbonate and silica in the presence of water vapor at temperature range between 1100–1200 °C. The resultant sodium-silicophosphates then quenched with water and ground to a fine powder. This fertilizer called Rhenania phosphate.
Mixing of raw phosphate with magnesium ores such as olivine (3MgO·FeO) 2SiO₂ or serpentine 3MgO·2SiO₂·2H₂O, then the mix is fused at about 1500–1600 °C.
This fertilizer called fused magnesium phosphate (Straaten, 2002; Pozin, 1986).
Calcination of raw aluminum phosphate such as crandallite CaAl₃(PO₄)₂(OH)₅·(H₂O) and wavellite Al₃(PO₄)₂(OH)₃·5H₂O at a temperature of about 550 °C where the crystalline non-soluble phosphate transferred to non-crystalline soluble phosphate (Straaten, 2002).

2 Importance of research

The importance of this research evident through:

Improving the specification of the Syrian raw phosphate and increasing the productivity by treatment of the unmarketable ores.
Production of phosphate fertilizer by simple technological processes.
The fertilizer can be prepared from a low-quality raw phosphate which is not suitable for use in acid treatment.

3 Experimental section

3.1

3.1 Apparatus and reagents

3.1.1

3.1.1 Apparatus

Grain gradation device manufactured by Retsch model (AS.200).
Electric oven 1200 °C manufactured by Nabertherm.
Differential thermal analysis device (DTA) manufactured by Linseis.
Spectrophotometer manufactured by Jasco model V-530.
Flame spectrophotometer manufactured by BWB.
Determination of moisture device manufactured by Sartorius model (MA150).
Nanocolor – 300D device to determine the concentration of fluoride.
Sensitive balance manufactured by Denver.
Laboratory mill with balls.
Electric vibrator.
Electric dryer.

3.1.2

3.1.2 Reagents and chemicals

Nitric acid, hydrochloric acid, citric acid, sulfuric acid, hydrofluoric acid, iron nitrate, silver nitrate, sodium hydroxide, ammonia, ammonium molybdate, ammonium metavanadate, sodium carbonate, potassium carbonate, sodium tetra borate, ethylene diamine tetra acetic acid (EDTA), calcon, eriochrome black, all of which are with high purity, produced by MERCK, distilled water.

3.2

3.2 Results and discussion

3.2.1

3.2.1 Granule gradation and chemical analysis

We took the raw phosphate from the eastern region (mine B), then we ground the sample by laboratory mill with balls. The grain size became less than 500 μm. We did not grind the sample finer because of economic consideration. We determined the granule gradation of the studied sample by using five standard sieves their holes are: 100, 200, 300, 400, and 500 μm, then we calculated the remaining weight on each sieve (Table 1).

Table 1 Granule gradation of studied phosphate.

Percentage (%)	Grain size (μm)
0	500<
4.16	500–400
10.39	400–300
20.96	300–200
39.51	200–100
24.44	100

We determined P₂O₅ spectrophotometrically by measuring the color density of yellow complex phospho-vanado-molybdate at a wavelength of 430 nm, (CaO, MgO, and Al₂O₃) by calibration with a standard solution of EDTA, (Na₂O and K₂O) by flame spectrophotometer, total CO₂ by heating at temperature 1000 °C, organic materials by heating at a temperature of 700 °C, SiO₂ by weight analysis, Fe₂O₃ spectrophotometrically by measuring the color density of the red complex of iron ions with thiocyanate ions at a wavelength of 420 nm, Chloride and Fluoride spectrophotometrically, moisture by heating of sample at 105 °C (Table 2).

Table 2 Chemical analysis of studied phosphate.

Compound	Percentage (%)	Compound	Percentage
H₂O	5.01	Na₂O	0.44
P₂O₅	28.6	K₂O	0.19
CaO	46.07	Total CO₂	6.12
Al₂O₃	0.52	Organic materials	0.78
Fe₂O₃	0.25	F	3.61
MgO	0.57
SiO₂	3.44

After that we determined P₂O₅ percentage in raw phosphate and in each granule size, the results were as in Table 3.

Table 3 P₂O₅% in studied phosphate and in different granule sizes.

P₂O₅ (%)	Grain size (μm)
28.6	Raw phosphate
28.51	500–400
29.41	400–300
30.26	300–200
30.84	200–100
25.43	100

We note from Table 3 that the highest proportion of P₂O₅ exists in the granules sizes ranging from 100 to 300 μm, and the fine granule sizes (less than 100 μm) contain the lowest proportion of P₂O₅, because most of the clay materials and sands are present in this granule size, so we cannot enrich this sample by crushing and screening, because of only 60.47% of the sample contains greater than 30% of P₂O₅ (marketable), consequently the rest will be neglected.

3.2.2

3.2.2 Effect of temperature on the solubility

We treated the raw phosphate at different temperatures according to the condition:

Quantity of sample 5 g.
Tim of treatment 60 min.
Fast cooling.

We determined the solubility by a solution of citric acid 2% and the results are shown in Table 4.

Table 4 Solubility of the treated phosphate at different temperatures.

P₂O₅ (%) dissolved in a solution of citric acid 2%	Temperature of treatment (°C)
9.32	25
8.93	300
7.86	400
7.02	500
6.68	600
6.18	700
4.11	800
3.85	900
3.55	1000

We note a decrease in the solubility with an increase of the temperature treatment, where the proportion of the dissolved P₂O₅ in the sample without thermal treatment is equal to 9.32%, and begin to decrease gradually with increasing the temperature of treatment, then it decreased sharply when treated at 800 °C, this is consistent with the previous researches where it gets re-crystallized and forms products with low solubility.

3.2.3

3.2.3 Study curve of differential thermal analysis (DTA)

We Studied the DTA curve by taking 72 mg of sample, then heating the sample from 20 °C to 1000 °C with constant heating rate of 10 degrees per minute (Fig. 1).

We note from the curve the presence of three peaks:

Endothermic peak between 49.6 and 101 °C resulting from evaporation of water moisture.
Endothermic peak between 129.2 and 141.8 °C resulting from removal of hydroxide radicals (bound water).
Endothermic peak between 739.9 and 768.1 °C resulting from dismantling of carbonate.

We concluded that the treatment must be performed at a temperature greater than 768 °C to remove the carbonate.

3.2.4

3.2.4 Thermal treatment without additives (enriching)

We treated the studied phosphate at three temperatures: 770 °C, 800 °C and 850 °C for 15, 30, 45, and 60 min, where we noted the weight to be constant at 30 min. Therefore, treatment of the sample for 30 min is sufficient.

We performed the thermal treatment by electric furnace. Where we put the samples in platinum crucibles, then raised the temperature gradually from room temperature until reaching the required level, then we fixed the temperature for 30 min. Then we washed the hot samples with distilled water to reduce chloride content, calcium oxide, and magnesium oxide (which are formed as resulting of decomposition of carbonate), then we dried and preserved the samples.

We analyzed the treated phosphate and put the results in Table 5.

Table 5 Chemical analysis of treated studied phosphate.

Treatment temperature (°C)			Compound (%)
850	800	770	Compound (%)
33.95	33.31	32.62	P₂O₅
49.42	49.31	49.13	CaO
0.11	0.13	0.14	MgO
3.49	3.49	3.48	SiO₂
0.55	0.55	0.54	Al₂O₃
0.26	0.25	0.25	Fe₂O₃
0.17	0.18	0.21	Na₂O
0.06	0.07	0.1	K₂O
0.75	0.88	1.35	Total CO₂
0.03	0.03	0.04	Organic materials
3.90	3.86	3.78	F

We note from Table 5 that when we heated at 770 °C, percentage of P₂O₅ increased to 32.62% and the percentage of total CO₂ decreased to 1.35%, while heating at 800 °C increased the percentage of P₂O₅ to 33.31% and decrease the percentage of total CO₂ to 0.88%. Also percentage of P₂O₅ increased to 33.95% and the percentage of total CO₂ decreased to 0.75% when the treatment was performed at 850 °C. Also organic materials were almost removed. Therefore, the thermal treatment at 850 °C for 30 min is sufficient and above this temperature makes the treatment ineffective.

3.2.5

3.2.5 Thermal treatment in presence of sodium carbonate

We prepared mixtures containing increasing proportions of sodium carbonate, then we studied the effect of temperature and time on thermal treatment

Sodium carbonate (g)	Raw phosphate (g)	Mixture
10	100	A
20	100	B
30	100	C
40	100	D

3.2.5.1

3.2.5.1 Effect of temperature on thermal treatment

Thermal treatment was performed according to the following conditions:

Quantity of treated sample 5 g.
Time of treatment 60 min.
Temperature of treatment 900, 1000, 1100 °C.

The results are listed in Tables 6–9 .

Table 6 Results of the mixture (A).

Temperature of treatment (°C)	900	1000	1100
Phosphorus pentoxide %, dissolved in a solution of citric acid 2%	7.41	8.33	9.01
Total Phosphorus pentoxide %	30.94	31.86	32.06
Phosphorus pentoxide %, converted to the dissolved form	23.94	26.56	28.10

Table 7 Results of the mixture (B).

Temperature of treatment (°C)	900	1000	1100
Phosphorus pentoxide %, dissolved in a solution of citric acid 2%	12.88	15.31	16.01
Total Phosphorus pentoxide %	28.14	29.66	29.96
Phosphorus pentoxide %, converted to the dissolved form	45.77	51.68	53.43

Table 8 Results of the mixture (C).

Temperature of treatment (°C)	900	1000	1100
Phosphorus pentoxide %, dissolved in a solution of citric acid 2%	19.78	22.11	23.21
Total Phosphorus pentoxide %	25.4	27.05	27.81
Phosphorus pentoxide %, converted to the dissolved form	77.87	81.73	83.45

Table 9 Results of the mixture (D).

Temperature of treatment (°C)	900	1000	1100
Phosphorus pentoxide %, dissolved in a solution of citric acid 2%	22.4	23.53	25.86
Total Phosphorus pentoxide %	24.06	25.17	26.95
Phosphorus pentoxide %, converted to the dissolved form	93.1	93.48	95.95

We note from the previous Tables 6–9 that increasing the solubility of all the mixtures with increasing the temperature of treatment from 900 °C to 1100 °C, therefore, the thermal treatment at 1100 °C is the best.

3.2.5.2

3.2.5.2 Effect of time on thermal treatment

We studied the effect of time (60, 90, and 120 min) on the mixtures (C) and (D) (the best mixtures) and the results are listed in Tables 10 and 11.

Table 10 Results of the mixture (C).

Temperature of treatment (°C)	900			1000			1100
Time of treatment (min)	60	90	120	60	90	120	60	90	120
Phosphorus pentoxide %, dissolved in a solution of citric acid 2%	19.78	22.4	23.02	22.11	24.64	24.8	23.21	25.38	25.91
Total Phosphorus pentoxide %	25.4	27.66	27.78	27.05	28.02	28.16	27.81	28.75	28.96
Phosphorus pentoxide %, converted to the dissolved form	77.87	80.98	82.59	81.73	87.93	88.06	83.45	88.27	89.46

Table 11 Results of the mixture (D).

Temperature of treatment (°C)	900			1000			1100
Time of treatment, min	60	90	120	60	90	120	60	90	120
Phosphorus pentoxide %, dissolved in a solution of citric acid 2%	22.4	23.01	23.49	23.53	24.55	24.66	25.86	26.14	26.86
Total Phosphorus pentoxide %	24.06	24.32	24.44	25.17	25.52	25.63	26.95	27.15	27.69
Phosphorus pentoxide %, converted to the dissolved form	93.1	94.61	96.11	93.48	96.19	96.25	95.95	96.27	97

We note increase in the solubility with increase in the time of thermal treatment from 60 to 120 min, therefore, the thermal treatment for 120 min is the best.

4 Conclusions

4.1

4.1 Thermal treatment without additions

The studied phosphate cannot be enriched by crushing and screening because this method causes a big loss of phosphate in the coarse parts and in the very fine parts.
The solubility of the studied Syrian phosphate decreases with the increase in temperature of the treatment gradually until the temperature is 800 °C then it decreases sharply above this temperature.
Treatment at 850 °C for 30 min is sufficient and above this temperature the treatment becomes ineffective.
We obtained raw phosphate containing a very low percentage of carbonate and almost free of organic material, where the impurities cause increase of consumption of sulfuric acid and many technical problems during the manufacturing of phosphoric acid by wet way.

4.2

4.2 Thermal treatment in presence of sodium carbonate

Phosphate fertilizer can be prepared by thermal treatment of the studied raw phosphate in the presence of sodium carbonate at 30% or 40% of the weight of the raw phosphate.
Thermal treatment at 1100 °C for 120 min gave the highest solubility in all of the mixtures, but raising the temperature above 1100 °C and increasing the time of treatment above 120 min are ineffective economically.
Adding sodium carbonate at a percentage higher than 40% of the weight of the raw phosphate is ineffective practically.

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