PhosDhorus
Planf
roduction of elemental phosphorus by the electric
Pfurnace method, long a relatively static technology, is now subject to a number of new trends in commercial conditions, equipment design, and power costs. This article will discuss these trends with particular regard to the effects on future plant design. T h e electric furnace method is now used exclusively worldwide for phosphorus production and is expected to remain the dominant technology for some 10-15 years at least. Increasing size of electric furnaces is a major trend. The growth in maximum size, or capacity, of furnaces for phosphorus manufacture is shown in Figure 1. This plot shows three growth periods: a small increase prior to 1900 in the early development of the industry, a period between 1930 and 1940 terminated by World War 11, and the postwar period. These were interrupted by two periods of fixed furnace size, 1900-1930 and the World War I1 period. The curve of Figure 1 concerns U. S. phosphorus furnaces only. A recent European furnace of about 70 MW built about 1966 coincides with U. S . performance shown in Figure 1. The extent to which furnace size will increase in the future is a matter of some speculation; technical factors affecting maximum size are discussed below. Demand for phosphorus has been growing at an average rate of about 770 per annum in the U. S. with a corresponding growth worldwide. Future demand could be substantially different depending on market trends for several major phosphorus derivatives. The detergent phosphates, a major class of derivatives, can enjoy increased markets due to trends toward enzyme incorporation or, on the other hand, reduced usage due to replacement by other detergent “builders.” Promotion of algae growth in streams by residual detergent 8
INDUSTRIAL AND ENGINEERING CHEMISTRY
phosphates is a current problem, but the cost and performance relative to substitute products and the possibility of adequate municipal effluent treatment offer hope that current phosphate detergent markets will not be strongly affected. Another elemental phosphorus derivative, phosphoric acid, could see greatly increased use in the future in fertilizer markets. The current trend toward higher analysis fertilizers is a case where phosphoric acid produced from elemental phosphorus could have cost-performance characteristics more favorable than those of wet process phosphoric acid, which now serves the large majority of fertilizer markets. Other current trends which, if continued, will support fertilizer end uses of phosphorus are: a decreasing ratio of cost of electric power/cost of sulfur and increasing economies of scale of elemental phosphorus production. As in the case of ammonia production technology in recent years, new large-scale, strategically situated phosphorus plants could result in the shutdown of older, smaller, and less efficient units. Electric Furnace Technology
A block flow diagram for the overall system is shown in Figure 2. In addition to phosphate ore, the electric furnace must be charged with sources of carbon (for ore reduction) and of silica (for proper composition and hence flow properties of the slag by-product). Metallurgical grade coke is the usual carbon source. Silica is normally provided by addition of pebble silica although it may largely be contained in the phosphate ore raw material. The overall reaction may be approximated : 2 Cas(POd2
+ 10 C + 6 Si02 Pd
+ 10 CO + G CaSiOa
(1)
H. S. BRYANT N.G. HOLLOWAY A.D. SILBER
Technical factors affecting size of electric furnaces are discussed
T h e phosphorus vapor is condensed, the CO-rich gases are used as fuel or flared and the calcium silicate slag is removed in the molten state from the furnace, usually by intermittent tapping. The phosphorus is purified by removal of entrained solids either by electrostatic precipitator treatment of the vapor or by filtration of condensed product or a combination of the two. Other constituents of the phosphate ore, depending on source, can be iron oxides, calcium fluorides, vanadium oxides, various aluminum compounds, hydrocarbons and other organics. The iron forms by-product ferrophosphorus which separates in the furnace as a molten layer below the molten slag layer. Both ferrophosphorus and slag are removed from the furnace by tapping separately. Aluminum concentrates in the slag, whereas vanadium and other heavy metals are usually reduced and appear in the ferrophosphorus byproduct. Organics and fluorides form compounds which leave in the vapor state; most of the fluorine, however, leaves in the slag. The slag is largely calcium silicate. The phosphate ore, as mined, must be treated in some manner before being fed to the electric furnace. Considerable advantages, particularly on the larger furnaces, accrue from agglomerating the ore and calcining the agglomerates prior to feeding into the electric furnace. These advantages are discussed below.
1900
'IO
'20
'40
'30
'50
'60
'70
YEAR
Figure 7.
Growth in phosphorus furnace size
Maximum size single furnace in operation in U. S. in given year
Selection of Phosphate Ore
Domestic phosphate ore sources exploited in the past are those in Southern Idaho, South Central Tennessee, and West Central Florida. These are still the most important but several new areas are being opened up. VOL. 6 2
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APRIL 1970
9
Mining has commenced in Coastal North Carolina and in Northern Florida for fertilizer usage, and exploration is currently active in Georgia, South Carolina, and some other areas. I t is evident from Figure 1 2 that raw materials are one of the major cost components in elemental phosphorus manufacture. Some 40% to 60y0 of total raw materials cost (15-20% of total manufacturing cost) can be ascribed to phosphate ore in the U. S. The desire to minimize total manufacturing costs, therefore, can in the future, lead the phosphorus industry away from the familiar grades of beneficiated phosphate rock products which are well established in both the domestic and foreign fertilizer industries. This is especially true of elemental phosphorus plants located in the Florida or western United States phosphate fields. In the case of Florida phosphate, the beneficiation process could easily be oriented toward the production of reasonably highconcentration materials which would not be suitable as feed for wet acid or other fertilizer plants due to excessive silica content, inappropriate particle size distribution, or impurities which cause processing difficulties. The value at which such nonmarketable phosphatic materials could be costed into an elemental phosphorus
plant could be the subject of considerable economic analysis. The three major factors are : The operating cost per ton for processing and delivery to the battery limits phosphorus plant The cost requirement and profit potential to upgrade further the material to a marketable phosphate mineral product Whether the material is an unavoidable and normally unused by-product of the phosphate rock beneficiation process-eg., tailing sand from flotation beneficiation (6) Increased production of elemental phosphorus in Florida, for example, would appear to be favored by several current and future trends : Low-cost sources of phosphorus plant feedstock are available from current mining and beneficiation operations 'The high silica content of certain of these potential raw materials could supply the bulk of the silica requirement for the electric furnace phosphorus process if provision were made in the process design for using silica-included rock as feed
Figure 2. Elemental phosphorus manufacture block Juw diagram 10
INDUSTRIAL AND ENGINEERING C H E M I S T R Y
Figure 3. Spherical pellet production f o r furnace feed
'The electric furnace process, when combined with future low-cost power, represents an economically viable method for utilizing central Florida's large reserves of low-grade phosphate rock Selection of Feed Preparation Method
As in most other areas of chemical manufacture, minimum phosphorus production costs are obtained by operating large plants. 'The maximum capacity of a submerged arc-type phosphorus furnace can usually be extended and manufacturing costs reduced if the phosphate ore feedstock is first prepared by sizing and heat treatment before being introduced into the furnace. There is a definite trend away from the use of phosphate feed without any pretreatment before electric furnace processing. Careful sizing of the furnace solids feed, or burden, promotes the even distribution of gas flow within the furnace and thereby results in more efficient heat transfer and lower total energy costs. T h e pretreatment of the phosphate feed a t temperatures in the range 1750 to 2400°F results in a t least a partial liberation of certain chemically bound rock constituents such as water of hydration, organics, carbon dioxide, and fluorine. The electric furnace energy requirement in the form of higher cost electric energy is thereby partly replaced by the use of lower cost fuel combustion in calcining or heat treatment. T h e result is a net decrease in total energy costs per unit of product. T h e heat treatment or pyro processing of furnace feed is also important to impart strength and hardness to the furnace burden particle. These characteristics minimize feed decrepitation a t high temperatures which liberates fines and causes erratic temperature profiles within the furnace. The sizing of phosphate rock feed, usually by some form of agglomeration, followed by a heat hardening and prereduction step improves overall phosphorus plant economics also by increasing the maximum permissible size and throughput of a single furnace. While the following discussion points out certain technical and economic differences between the various agglomeration processes of pelletizing, briqueting, flake
compacting, and nodulizing, the basic objective of all these processes and the associated heat treating processes is the same---vir., the preparation of a satisfactory feed to an electric furnace. The various agglomeration and heat treatment methods are reviewed below : Agglomeration processes. The agglomeration of phosphate ore for use as electric furnace feed requires the formation of strong physical bonds between discrete particles so as to form larger particles. The enlarged particles might be in the form of spheres (pellets), briquets, flakes, or other shapes. Furnace performance is probably rnore dependent upon the strength, shock resistance, and volatiles content of the agglomerate than upon shape. The ideal prepared feed would consist of agglomerates which are equal in density a n d mechanical integrity to the original discrete ore particle and which evidence no weight loss when heated to the melting point. Also, porosity of the charge (a function of agglomerate ,shape) is important to allow countercurrent flow between the gas and the incoming charge with a low pressure drop but good heat transfer. T h e most commonly attempted ore agglomeration techniques are :
PELLETIZING. Spherical pellets approximately 3/8 in. to 3/4 in. in diameter can be produced providing that a specific particle size distribution of ore is first obtained and moisture level is controlled within specific limits. A suitable system for producing pellets is shown in Figure 3. The pellets are usually formed on a rotating disk although a drum or cone can also be used. T h e disk is preferred because of its ability to pelletize nearly dry, finely ground phosphate ore a t low recycle rates. The major factor affecting adhesion of ore particles to form a pellet is the total surface area of the feed--i.e., particle size. For effective pelletizing, this surface area, often called the Blaine Number, should be in the range of 2000-4000 cm2/gm. The moisture content of a newly formed pellet is about 13yoand it must possess sufficient green strength and shock resistance for further processing. T h e procedure used to pelletize phosphate ore is in the following sequence : VOL. 6 2
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(a) The ore is first dried to a moisture level of about 1% or less. This is necessary to prevent the buildup of deposits on grinding media. (b) The dried ore is subjected to ball mill grinding to develop the required Blaine Number for pelletizing. The size of the ball mill used and the power input to the mill will be strongly influenced by the silica content of the ore. Closed circuit grinding is preferred to prevent the desired Blaine Number from being developed by overgrinding one component of the ore. (c) The dry pellet feed, which has been reduced to a size distribution of perhaps 60 to 80% passing 200 mesh, is fed onto the rotating disks where water sprays cause seed pellets to form. These are built up in size by continued rotation and material addition. A stronger pellet is sometimes achieved by the use of binders such as natural clays or slimes made from furnace precipitator dust and “phossy water”. (d) The green pellets collected as product from the rotating disk can be screened with undersize recycled as seed or they may be fed directly to a heat treating step and screened after conversion to a calcined product. T h e moisture content and cohesion of the pellet when removed from the disk are critical in achieving product quality in the subsequent drying and heat hardening steps. Advantages. Operating equipment and procedures have been established for many years. Good performance has been demonstrated on large furnaces. I t provides maximum flexibility for chemical and physical variations in feed. Where drying and grinding are not required, capital investment for pelletizing is moderate. Recycle rates are lower than for most other methods. Disadvantages. The pelletizing of phosphate requires the availability of a dry and finely ground feed. Use of such a feed, in most instances, adds to overall investment and operating cost. The formation of the pellet does not lend itself to automatic process control; frequent operator adjustments are necessary. Large-scale operations require multiple units. Overall processing rates are usually lower for the spherical pellet compared with other agglomerates. This results from the fact that the pellets are more dense and less porous than the other shapes; outward diffusion rates of escaping product gases are thus lower. If product gases are evolved at rates higher than the diffusion rate, shattering of the pellet can occur. Thus heat treating rates must be limited in practice and heat treating equipment sizes must be larger. Maximum pellet size is limited to about 3/4-in. 12
INDUSTRIAL A N D ENGINEERING CHEMISTRY
diameter but this is of questionable significance in furnace operation. BRIQUETIKG.,4 closed-circuit system is shown schematically in Figure 4. Agglomeration by briqueting results from the application of force by t ~ 7 0rolls rotating counter to each other. The raw material phosphate ore is carried into the narrowest gap between the rolls either by gravity feeding or by the use of a screw type force feeding device. The actual shaping of the briquet is accomplished by means of molds or pillow-shaped cavities in the rolls. The pressure exerted on each briquet is dependent upon the amount of material in the molds as well as the width and diameter of the briqueting rolls. The angle of nip between the rolls largely determines the amount of material in the molds. The most important criteria for designing a briqueting mill are the bulk density of the original ore and the desired density of the product briquet. The maximum ratio of product to feed density attainable in a single stage of briqueting normally will not exceed 2.5. The ideal density of briqueted phosphate ore for use as furnace feed is likely to be the same as the particle density of the ore before agglomeration. Temperature is an important variable in briqueting phosphate ore. Some ores can be easily briqueted at ambient temperatures especially if they have some clay content to act as a binder. The particle size distribution or grain characteristics of the ore can also influence the ease of briqueting. Hot briqueting can be the preferred technique if the ore is abrasive due to silica content or when the green strength that can be developed by cold briqueting is inadequate. In either case, the design of a briqueting system is highly empirical.
Figure 4. Briqueting of ore f o r furnace feed Flaking process uses similar equipment except that smooth rolls produce sheet of material which is broken i n j a k e breaker and requires both ouersize andjnes screening
Figure 5. Nodulizing kiln for furnace feed
Advantages. Uniformly shaped particles are obtained which have been demonstrated as being acceptable in an electric furnace. Drying and grinding requirements are minimal. T h e recycle rates compare with those for pelletizing. The briqueting process is particularly suitable for ores containing natural binders-i.e., Western and Tennessee ores. While the process is not amenable to a high degree of automation, operator attention is minimal. Disadvantages. Roll wear can be a major operating cost. While the extent of this varies with briquet size, ore characteristics, or temperatures, it can be of the order of 50 cents per ton of furnace feed. The production available from a single machine is limited. A multiplicity of units is necessary for large modern plants. This process is similar to FLAKING OR COMPACTION. briqueting except that smooth surfaced rolls are used to compact the ore into flakes. Because compaction is essentially a densifying or voids reduction process, a finely divided material (
