CA1335752C - Agglomerated phosphate furnace charge - Google Patents

Abstract

Calcined phosphate fines, recovered from an electric phosphorus furnace, are converted into a supplementary furnace charge by mixing the fines with carbon reduc-tant, the requisite amount of silica fluxing agent and phosphoric acid binder and the mixture formed into bri-quettes which are cured by heating at sufficient temperatures to effect reaction between the acid and fines to give hardened briquettes of phosphate salts. The supplemental charge is higher in P2O5 than the main charge and thus provides increased phosphorus output.

Description

~ ~ 335752 AGGLOMERATED PHOSPHATE FURNACE CHARGE
This invention relates to an agglomerated phosphate material suitable for charging in an electric-type phos-phorus furnace. More particularly, the invention is concerned with a briquetted phosphatic material as a supplemental furnace feed for increased phosphorus production.
In the electrothermal manufacture of elemental phos-phorus, feed streams of phosphatic material such as calcined phosphate ore, a carbonaceous reductant such as coke and optionally a fluxing agent such as silica are charged into an electric furnace. The charge materials undergo resistive heating which results in the formation of a molten reaction mass. Reduction of the phosphate ore to phosphorus produces a gaseous mixture of phos-phorus vapor, carbon monoxide and particulates. After freeing of particulates by electrostatic precipitation, the gaseous stream is water quenched and the condensed phosphorus recovered and stored under water. The fur-nace is tapped periodically to remove molten slag and liquid ferrophosphorus.
In a typical method of preparing the phosphatic feed material, raw phosphate ore is first formed into aggre-gates or agglomerates of the requisite size by compact-ing comminuted phosphate ore with a binder to form shaped articles such as pellets or briquettes, usually the latter. These are then calcined to increase their crush strength and thereby minimize breakage. The procedure is much used in the processing of phosphate shales such as are found in the Western areas of the United States. These shales usually contain clay which undergoes sintering during calcination thereby acting as a binder for the phosphate particles to give a high strength agglomerate. The technique is not, however, .. , . . - ~ . . . . -. . . . . . .

` --2- ~ 335752 applicable to clay deficient shales or to other phos-phates lacking binding properties. Agglomeration of these materials requires additional clay or other bind-er.
Although the manufacture of phosphorus by reduction of phosphate ore in an electric furnace is an establish-ed industry, it is not entirely free of operational problems. For example, the briquettes of calcined phos-phate shale are subject to a certain amount of abrasion during handling and while in transport to the furnace.
As a consequence, a certain amount of calcined ore particulates are generated. Over extended periods of operation, these fines, commonly referred to as nodule fines, accumulate in considerable amounts.
Fines build up can be ameliorated to some extent by blending a stream of recycle nodule fines with fresh shale ore. However, this approach tends to be self-defeating owing to the increased susceptibility to abrasion of shale agglomerates containing recovered nodule fines. Nodule fines, unlike raw phosphate shale, cannot be compacted into strong shapes. Calcining destroys the binding properties of the shale. Hence the decrease in strength of phosphate aggregates or bri-quettes cont~;n;ng nodule fines. Clearly, recycling is not the answer to nodule fines build up.
Another problem that frequently confronts phosphorus producers involves matter of supply and demand and comes about in the following manner. From time to time there is a marked increase in the consumption of phosphorus.
The timing and extent of these increases are not always predictable although they tend to parallel periods of heightened business and economic activity. In some instances, the demand for phosphorus may exceed the output of a given plant resulting in loss of potential sales. Of course, additional furnace capacity could be ; . ... ... . . . . . . . . . . . . . ..

_3_ 1 3 3 5 7 ~ 2 installed but then the plant would be underutilized during intervals of normal or baseline operation. What is needed is a means of dealing with the wide swings in phosphorus markets while keeping within the general bounds of existing plant design and capacity.
Now, therefore, it is a principal object of the present invention to provide a process of reducing the buildup of calcined phosphate fines while simultaneously increasing the yield of phosphorus from an electric-type phosphorus furnace in which the charge material is cal-cined phosphate agglomerates, carbon and the requisite silica flux. Other general objects and advantages of the invention will become apparent in the more detailed description as subsequently set forth herein.
Broadly, the objects and advantages of the invention are realized by the steps of:
(1) mixing calcined phosphate fines, phosphoric acid, carbon and silica to form a phosphatic feed material suitable for winning phosphorus in an electric-type phosphorus furnace, said charge material containing proportions of phosphoric acid and phosphate fines such that on reaction with the phosphate fines the phosphoric acid is converted to phosphate salts, the carbon being in at least a stoichiometric amount required to reduce the phosphorus values in the charge material to elemental values thereof and the quantity of silica being sufficient to provide the required amount of flux;
(2) forming agglomerates of said phosphatic feed material;

(3) forming cured agglomerates by heating the green agglomerates to effect reaction between the phosphate fines and phosphoric acid whereby the phosphoric acid is converted into phosphate salts resulting in increased hardness and strength of the agglomerates, and (4) introducing the cured agglomerates into the _4_ t 3 3 5 7 5 2 phosphorus furnace as a supplemental charge stream.
The homogeneous phosphatic feed material of step (1) is produced by bringing the components together in a known blending device such as a pug mill or the like until a homogeneous mixture is obtained. Desirably, the mixing device is made of acid-resistant material. The amounts of components in the phosphatic feed material should satisfy the requisite stoichiometry as explained further herein while at the same time providing a con-sistency that is suitable for converting into agglom-erates.
The agglomerates in step (2) are produced by forming shaped structures or configurations of the phosphatic feed material of step (1). Exemplary shaping techniques include nodulizing, pelletizing or briquetting. Bri-quetting is most commonly employed and is carried out in commercially available equipment such as a roll briquet-ting press.
The green (uncured) agglomerates are much weaker than their uncured counterparts and consequently require more careful handling to prevent undue crumbling. Green strength can, however, be improved by employing high phosphoric acid binder levels and/or by precompacting the blended components prior to agglomeration. As carried out herein, precompaction comprises the follow-ing sequence of steps:
(1) compacting a mixture of the calcined phosphate fines, carbon, silica and acid binder;
(2) granulating the compacted mixture;
(3) sieving the granulated material through a half inch screen, and (4) recompacting the screen material.
The precompacting can be performed using a roll briquetting press of the standard type. Improved strength derives from the fact that the effective .
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_5_ 1 3 3 5 7 S 2 pressure exerted on the material in the second compact-ing step is significantly greater than in the first step.
The freshly formed or green agglomerates are cured in order to increase their crush strength and render them more resistant to abrasion. In general, curing consists in heating the green agglomerates at tempera-tures ranging from about 100C to 500C. Heating periods can range from about 0.3 hours at the higher temperature to about 2.0 hours at the lower temperature.
Preferred curing conditions are from about 150C to 200C for one to two hours.
During the curing step, the calcined phosphate fines and phosphoric acid react to produce complex acid phos-phate salts, examples of which are thought to include CaHP04 and Ca(H2P04)2. It is believed that such salt formation is responsible for the increase in strength and abrasion resistance of the cured agglomerates.
Curing is also advantageous in that it promotes expulsion of water, both free and hydrated, as well as other volatile materials which would be detrimental if released in the furnace.
As previously pointed out, during curing the phos-phate fines and phosphoric acid react to give complex phosphate salts. Such reaction is necessary since any free phosphoric acid in the agglomerates would be expelled as P2O5 in the furnace and thus unavailable for conversion into elemental phosphorus.
The phosphoric acid binder used in practicing the invention need not be pure or highly refined. In fact, a low cost technical grade of phosphoric acid, such as commercially available green wet process acid (WPA) is entirely satisfactory and even preferred. Acids con-t~;n;ng by weight from about 30% up to about 100% H3P04 constitute suitable binder materials. The amount of ~ 1 335752 phosphoric acid binder in the phosphatic feed material can be expressed as follows:

% (by weight) Binder = (gms as X% H3PO~) flOO) gms total solids + gms as X% H3P04 Green acid assay at about 70% (X=70% in the formula) is preferred. Binder levels ranging from about 7~ to about 15% are satisfactory.
The percentage of carbon in the phosphatic feed material should be sufficient to reduce the phosphate values to phosphorus as well as other reducible sub-stances present in the calcined phosphate fines to their elemental forms. The stoichiometry for reducing phos-phate values, expressed as P205, can be depicted as follows:
(1) 2P205 + lOC----> P4 + lOCO
Suitable sources of carbon reductant are various cokes derived from the pyrolysis of petroleum and coal.
About 9% to 10% phosphoric acid binder is usually req~ired to give cured agglomerates of sufficient strength and hardness. If it is assumed, for instance, that a 9.6% level of 70% H3P04 is used and that the car-bon, as coal derived coke such as coke breeze, and the calcined phosphate fines contain 77% fixed carbon and 25% P20s, respectively, the charge material required to satisfy equation (1) supra would consist of 14.7% of coke and 85.3% of nodule fines. From an economic stand-point, however, it is generally desirable to include an excess of one or the other solid components. For example, use of excess calcined phosphate fines would ensure that the more expensive coke component undergoes essentially complete reaction.
Silica is commonly added to the charge materials as a fluxing agent which facilitates smelting of the phos-phatic agglomerates. The flux forming reaction is .. . . . .

~7~ 1 3 3 5 7 5 2 generally expressed in simplified form, as follows:
(2) 2Ca3(PO4)2 + lOC + 6sio2 ---> P4 + lOCO + 6CaSiO3 Representative of the phosphate fines used in producing the furnace charge material of the invention are the nodule fines recovered from a commercial phos-phorus plant in which a charge of calcined phosphate ore agglomerates, coke and silica are smelted in an electric phosphorus furnace. Typically, such nodule fines assay at about 32% CaO and about 24% sio2, corresponding to a CaO/SiO2 mole ratio of about 1.4. The charge material of the invention has yielded strong composite cured agglomerates containing about 6.5 parts added silica per 100 parts of nodule fines. This gives a CaO/SiO2 mole ratio of about 1.1, close to the stoichiometry required by equation (2).
The particle size of nodule fines recovered from the calciner unit of a commercial electric phosphorus plant consists mainly of ~4.75mm (U.S.A. No. 4 Standard Sieve) material. Another source of calcined phosphate fines is burden dust recovered from the plant electrostatic precipitators. Burden dust consists of <150Jum material and ordinarily contains a few percent of coke dust.
Strong composite agglomerates, highly suitable as a charge stream, containing calcined phosphate fines mixed with up to 55% burden dust, have been produced by the process of the invention.
Size distribution of representative nodule fines and commercially available particulate carbons suitable as carbon reductants are listed in Table I.
The cured agglomerates of the invention prepared with nodule fines are introduced into a phosphorus fur-nace as a supplemental furnace charge. The combination - of supplemental and main furnace charges gives a yield of phosphorus that is greater than that of the main charge alone. Whereas the P205 assay of a conventional , j, ", . : .. . . .

furnace charge is about 25% that of the invention is about 27~. The use of such an enriched phosphatic charge material translates into increased phosphorus production on the order of several million pounds per year for a typical commercial phosphorus plant.
Thus, the invention solves the problem of nodule fines buildup while providing means of increasing phos-phorus output without having to install added plant capacity.
The invention is illustrated in further detail by the following test procedures and examples in which compositions are on a weight basis unless stated other-wise.
General Preparations Agqlomerate PreParation - Large cylindrical pellets (ca 2.8 cm. diameter x 2.8 cm. height) were prepared for the mech~nical strength evaluations which consisted of Abradability and Tumble Tests (see below). Small (1.27 diameter x 1.1 cm. height) pellets were used in the Furnace Reactivity Tests.
The large pellets were prepared in a Carver Press from 35.0 gram portions of well mixed (Hobart Blender) blends containing nodule fines, coke fines, 70% H3P04 and, optionally, burden dust and/or silica. In most cases small quantities of water were also added to facilitate the blending and pelletizing steps. The small pellets were prepared from 2.0 gram portions of the well mixed blends in a hand operated Pellet Press (Parr Instrument Co.). The solids were used "as received" for all of the m~r-h~n;cal strength tests and for most of the reactivity evaluations. In some of the latter, the solids were ground in a mortar and pestle to obtain <25~um (U.S.A. No. 60 Sieve) material for small pellet preparation. Typical size distributions for the "as received" materials are given in Table I.

.. . . .

Curing Ske~ - All of the pellets in Table I were routinely cured in a laboratory oven at 200C + 15C for one hour. In one case, the cured pellets were additionally "fired" by heating them at 1000C in a laboratory tube furnace under nitrogen for 0.5 hour.
Test Procedures Abradability Test - Four large pellets were weighed and placed on a U.S.A Standard Series No. 6 Sieve (3.35mm) equipped with a metal cover and receiving pan.
This assembly was shaken in a Portable Sieve Shaker (Tyler Model RX24) for 20 minutes. The total quantity (both >3.35mm and <3.35mm) of material abraded from the pellets was determined by weighing, and calculated as percent abraded based on the original sample weight.
Tumble Test - This test was performed on green (uncured) and cured pellets. Four large green pellets were weighed and rotated at 43 rpm for exactly one minute in a 21.6 cm. diameter steel drum containing four one inch high steel baffles. The 0.6 cm. material re~in;ng after this treatment was obtained by sieving the sample in a 0.6 cm. screen. This material was weighed and the percent 0.6 cm. was calculated based on the original sample weight. Four large cured pellets were similarly treated for 10 minutes and the percent plus 0.6 cm. material was calculated in the same manner.
Furnace ReactivitY Tests - Three or four small pellets were weighed in a ceramic boat. The boat was placed in a 5.1 cm. ID mullite tube equipped with end plates containing 0.6 cm. stainless steel tubes for entry and exiting of sweep gases. The tube was positioned in a Lindberg 1500C horizontal tube furnace in such a way that the ceramic boat and contents were in the center (hot zone) of the furnace. The mullite tube was blanketed with dry nitrogen. The temperature was preset to the desired value and the power turned on.

~ -10- 1335752 The onset of the P4-forming reaction was noted by the appearance of a small flame at the exit end of the apparatus. The reaction was allowed to take place at the set temperature for three hours (see Examples 5 and 6) after which the power was turned off. The contents of the mullite tube were allowed to cool to room temperature under nitrogen. The boat plus contents were then reweighed and the percent weight loss accompanying the reaction was calculated. The total percent P205 in the cured pellets and that remaining ln the reacted agglomerates was determined by a reliable titrimetric analytical procedure (see Anal. Chem., 20 p.
1052, 1948). The percent conversion in each case was calculated from the loss in total P2O5 in the agglomerates by means of the following formula:
(original spl,wt)(l- %wt.loss)(% P205 remaining) % Conversion = 100 - 100 (original spl,wt)(original % P2O5) The above formula represents:
% Conversion - g. P2O5 in original - g. P205 in final g- P205 in original x 100 Example 1 Efect of Binder in Abradability Test Large pellets were prepared at 27.58 KPa (4000 psi) from a mixture containing 100.0 grams of nodule fines, 17.Z grams of petroleum coke fines, 12.4 grams of 70% H3PO4 (9.6% binder level) and 8.8 grams of water. The cured pellets abraded to the extent of only 3.1% after 20 minutes. For comparison purposes, similarly prepared and cured pellets containing no binder abraded completely after only three minutes demonstrating that the 9.6%
binder level used in the example markedly decreased the tendency of composite agglomerates to undergo abrasion under adverse handling conditions.

~. ~; ~
~ ~ . ~ .

~xamPle 2 Effect of Binder with Silica Present Pellets were prepared as per Example 1 except that 6.4 grams of silica per 100 grams of nodule fines was incorporated into the blend. Cured pellets containing a 9.1% H3P04 (70%) binder level abraded to the extent of only 3.5% after 20 minutes while those containing no binder abraded essentially completely after three minutes. These results demonstrate that phosphoric acid is also an effective binder for agglomerates containing approximately reactant proportions of silica and lime (CaO/SiO2 mole ratio = 1.1) according to Equation 2.
Example 3 ~ffect of "Firing" at 1000C
Large pellets were prepared at 41.37 kPa (6000 psi) pressure in the Carver Press from a mixture containing the following proportions of ingredients by weight:
100.0 parts of nodule fines, 14.6 parts of metallurgical coke fines, 8.7 parts of green H3PO4 (70%) and 10.0 parts of water. The binder level, 70% H3P04, was 7.1%.
The pellets, after curing at 200C for one hour, abraded to the extent of 2.5% in the Abradability Test. Several cured pellets were additionally "fired" at 1000C for 30 minutes under a reducing atmosphere (ca 95% N2 + 5%
CH4). These pellets, after cooling, were evaluated in the Abradability Test in which they underwent only 7.3%
abrasion after 20 minutes.
Prior Art Comparison In another experiment, pellets were prepared using molasses as a binder. The mixture contained 140 parts of nodule fines, 60 parts of burden dust, 24.4 parts of metallurgical coke breeze, 30.6 parts of molasses (12%
binder level) and 19.8 parts of water. After having been cured and additionally fired at 1000C, these pellets underwent 83.7~ abrasion in only ten minutes .
-indicating that molasses, a typical carbohydrate, is an ineffective binder for agglomerates consisting mostly of calcined phosphate. This example demonstrates that phosphoric acid is a uniquely effective binder for composite agglomerates in that these agglomerates are not significantly weakened at high temperatures unlike those prepared with carbohydrate binders.
Exam~le 4 Tumble Test The Tumble Test is somewhat more severe than the Abradability Test described in the previous examples.
It was implemented to further determine whether composite agglomerates prepared with H3PO4 binder are likely to disintegrate in transit to the furnaces. A
mixture of nodule fines and burden dust containing 30%
by weight of the latter was combined with coke in pro-portions such that the coke comprised 10.9% of the solid blend. This mixture was combined with three levels of phosphoric acid and additional water in separate experi-ments. Large pellets were formed at 27.58 kPa (4000 psi) in the Carver Press. Tumble Test results on both green and cured agglomerates are shown in Table II.
These results demonstrate that mechanical strength is increased with increasing levels of the binder in both cases.
ExamPle 5 Furnace ReactivitY of ComPosite Pellets Small pellets containing approximately stoichio-metric proportions (Equations 1 and 2) of total P2O5 and coke carbon were prepared from the following blend:
Nodule Fines 72.2%
Coke Fines 12.4%
Green Acid (70%) 9.0%
Free Water 6.4%

-13- l 335752 The pellets were cured at 200C for one hour then heated in a tube furnace for three hours at 1300C.
Coke fines were omitted from the pellets in one experi-ment. Formation of elemental P4 was indicated by the appearance o~ a flame at the exit end of the tube in the experiments in which coke fines were present. When coke was omitted the conversion value as defined previously was negligible at 1.6% (see Table III) indicating that little or no free P2O5 was volatilized at 1300C. When "as received" coke and nodule fines were used, moderate (31% to 58%) conversions were obtained. However, by grinding these materials to <25~m (0.0098 inch) it was possible to boost the conversions to 81% to 84% under otherwise identical reaction conditions (Table III). It was also observed that the pellets did not melt in any of the 1300C experiments cited below.
ExamPle 6 Effect of Temperature In this example, the phosphorus forming reaction was 2Q allowed to proceed at higher temperatures than Example 5. Large pellets were prepared in the Carver Press at 27.58 kPa (4000 pounds/in2) from a mixture containing <18~um particles and green phosphric acid. The follow-ing percentages of these ingredients were used to give composite pellets containing a 7.9% molar excess of carbon over that required to react with the nodule plus binder P2Os content. The Sio2/CaO molar ratio was 0.97.
Nodule Fines 74.7%
Coke (98.7% fixed carbon) 10.2%
Silica (100%) 5.1%
Green Acid (66.7% H3PO4) 10.0%
The pellets were cured at 200C. One large (20.9 gm) pellet was heated in a molybdenum crucible, contain-ed in a vertical high temperature furnace equipped with a controller-programmer. The temperature-time relation-ships for the reactant pellet (obtained by optical pyro-meter readings through a sight port in the reactor head) were as follows:
Time Hours Reaction TemP. C
2.0 1070 3.0 1200 4.0 1340 5.0 1490 5.3 1530 (maximum) 6.0 1460 8.0 1190 10.0 850 (estimated) The P4-forming reaction started at about 1080C and a melt was observed at 1375C. The slag remaining after the reaction contained only 0.09% P20s, corresponding to 99.7% conversion of the total P20s in the mixture to P4.
This example illustrates that very high conversions of the P2Os in the composite feed are possible at maximum temperatures, in the neighborhood of 1500C.
Ex~m~le 7 Effect of Particle Sizinq and Silica This example demonstrates that neither fine (<18~m) mesh particle sizing nor added silica is required to achieve high conversion of P2O5 under high temperature (1500-1550C) reaction conditions. In this case, pellets were prepared as described in the previous example, from <3.35mm solids and other ingredients, to give an 8.0% molar excess of carbon and approximately a 0.70 SiO2/CaO molar ratio.
Nodule Fines 72.9%
Coke (83.3% fixed carbon)12.5%
Green Acid (66.8~ H3P04)10.7%
Water 3-9%
The pellets were cured at 200C and heated to a maximum temperature of 1530C at a slightly faster rate than in the previous example. The P4-forming reaction started at about 1120C and a melt was observed at about 1250C. A weight loss of 37.3% occurred during the reaction and the slag P205 content was only 0.4%, corresponding to 98.9~ conversion of P205 to P4.

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TABLE I
PARTICLE SIZE DISTRIBUTIONS OF SOLIDS
Cumulative Weiqht % on Each Sieve U.S.A. Standard Nodule Petroleum Series Sieve Fines Coke Coke*
No. 8 16.3 57.0 0.7 No. 18 48.2 95.8 14.0 No. 30 59.9 99.6 27.6 No. 50 71.8 99.8 51.3 No. 100 82.8 99.8 72.0 * Low temperature coke; see U.S. Patents 3,140,241 and 3,140,242.

TABLE II
TUMBLE TEST RESULTS
Percent Plus 0.6 Cm. Remaininq % Binder Green Pellets Cured Pellets (70% H3PO4) (1 min.) ~20 min.) 8.0 34.9 35.9 10.0 51.3 81.6 12.0 76.3 89.7 .
TABLE III
FURNACE EXPERIMENTS AT 1300C (3 HOURS) Solids Experi- Coke Particle % Wt. % Con-5 ment No. TyPe Size Loss version 1 None As Added Received 4.7 1.6 2 *Coke As Received 27.3 58.4 3 Coke As (repeat) Received 26.8 54.7 4 Petro-leum As Coke Received 15.3 31.4 Coke -60 mesh 39.7 81.2 6 Petro-leum Coke -60 mesh 39.7 82.9 7 Metal-lurgical Coke -60 mesh 40.0 83.5 *See Table I

TABLE IV
EFFECT OF TEMPERATURE ON CONVERSION
OF P2Os TO p~a) Reaction %
Temp. CConversion 1300 58.4 1300 54.7 1500 97.7 a) Reaction Time: 3 hours .

Claims (8)

The embodiments of the invention in which an exclu-sive property or privilege is claimed are defined as follows:

1. In the electrothermal manufacture of phospho-rus in which a phosphatic feed charge of calcined phos-phate shale agglomerates is fed into an electric fur-nace, the improvement of utilizing the calcined phos-phate fines produced from abrasion of the said phosphate shale agglomerates characterized by the steps of:
1) mixing the calcined phosphate fines, phospho-ric acid, carbon and silica to form a charge material, said charge material containing pro-portions of phosphoric acid and phosphate fines such that on reaction with the phosphate fines, the phosphoric acid is converted to phosphate salts, the carbon being in at least a stoichiometric amount required to reduce the phosphorus values in the charge material to elemental values thereof and the quantity of silica being sufficient to provide the re-quired amount of flux;
2) forming green agglomerates of said charge ma-terial;
3) forming cured agglomerates by heating said green agglomerates to effect reaction between the phosphate fines and phosphoric acid where-by the phosphoric acid is converted into phos-phate salts resulting in increased hardness and strength of the agglomerates; and 4) introducing the cured agglomerates into the phosphorus furnace as a supplemental charge stream.

2. The process of claim 1 characterized in that the agglomerates are pillow briquettes.

3. The process of claim 1 characterized in that the carbon is selected from the group consisting of par-ticles of petroleum coke and coal derived coke.

4. The process of claim 3 characterized in that the carbon is coal derived coke.

5. The process of claim 1 characterized in that the phosphoric acid is 70% wet process acid.

6. The process of claim 5 characterized in that the percent of phosphoric acid is from 7% to 15%.

7. The process of claim 5 characterized in that the percent of phosphoric acid is from 9% to 10%.

8. The process of claim 1 characterized in that the molar ratio of CaO/SiO2 is from 1.1 to 1.4.