Classifying – separating – and assorting solids – Fluid suspension – Liquid
Reexamination Certificate
2000-10-30
2002-01-29
Lithgow, Thomas M. (Department: 1724)
Classifying, separating, and assorting solids
Fluid suspension
Liquid
C252S061000
Reexamination Certificate
active
06341697
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to separation of phosphate minerals from other minerals, particular from phosphate containing ores.
BACKGROUND OF THE INVENTION
World fertilizer production continues to be a crucial factor for the efficient growth of crops to feed the peoples of the earth. Phosphate is an essential ingredient of fertilizer, and the world production of phosphate rock is more than 147.1 million mt per annum as indicated in Table 1. It should be noted that the U.S. is the largest producer of phosphate rock with most of its 40-plus million mt per annum coming from the vast sedimentary deposits in central Florida. However as discussed at recent Engineering Foundation Conferences on Phosphate, December 1993 and 1998, Palm Coast, Fla., many technological problems must be solved if we are to continue to produce phosphate rock at our current rate of consumption. Unless these technological problems are solved, phosphate rock may be in short supply. The critical nature of this situation is clarified by Isaac Asimov, “We may be able to substitute nuclear power for coal power; and plastics for wood; and yeast for meat, and friendship for isolation-but for phosphorus there is neither substitute nor replacement.”
One of the most important processing technologies which accounts for this significant production of phosphate is the process of froth flotation which is used exclusively around the world. The flotation process is based on appropriate surface chemistry control in order to selectively generate hydrophobic surfaces on certain mineral particles while maintaining the surfaces of other mineral particles in a hydrophilic state. The phosphate industry has relied on the flotation process since the 1950's and will continue to do so for decades to come.
TABLE 1
Phosphate Production in 1997
Country
Millions of Tons
USA
42.3
USSR/CIS
12.0
Morocco
22.3
China
31.3
Tunisia
7.2
Other (Israel, Jordan, Egypt,
32.0
Saudi Arabia, Brazil, India,
Australia, S. Africa)
TOTAL
147.1
A significant problem which now faces the phosphate industry worldwide is the selective separation of phosphate minerals from carbonate minerals, particularly dolomite, by froth flotation. The Florida phosphate industry is no exception to this problem, and future production from the phosphate deposits of central Florida will require the development of new flotation technology for improved separation efficiency. Typical specifications of the phosphate rock concentrate for the production of fertilizer are:
<1% MgO
>30% P
2
O
5
<4% SiO
2
Traditionally these specifications have been relatively easy to meet because the run-of-mine phosphate rock has been mostly siliceous rock rather than calcereous or dolomitic rock. The current state-of-the-art has been discussed in the literature and at the Engineering Foundation Conferences on Phosphate, December 1993 and 1998, Palm Coast, Fla. Now, it is evident that the siliceous resources will soon be exhausted, and only the difficult-to-separate, carbonate-bearing rock will remain as our country's major phosphate resource. Efforts have been made for some time to treat such carbonate-bearing rock, particularly the dolomitic phosphate reserves of Florida.
Froth flotation for the separation of phosphate minerals from other gangue minerals has been practiced by fatty acid flotation with pine oil as frother since as early as 1928. Many flotation strategies for the processing and concentration of phosphate ores have been developed since then. The conventional phosphate flotation process for sedimentary deposits of central Florida is the “double float” process, viz. anionic flotation of phosphate minerals at alkaline pH, followed by cationic “reverse” flotation of silica from the initial phosphate concentrate at neutral or acidic conditions. The Florida phosphate industry, with few exceptions, still uses this standard method.
Summary of Phosphate Flotation Processes
Flotation is the most widely used method for the treatment of phosphate rock. The flowsheet design depends on the type of ore (endogenic or sedimentary deposit) and the nature of impurities (silica or carbonate) to be removed. Phosphate flotation strategies can be classified as follows:
Direct Flotation of Phosphate
The phosphate minerals are floated directly using carboxylate (fatty acids and the corresponding soaps) collectors, often co-added with hydrocarbon supplements (such as kerosene, fuel oil, etc), and appropriate reagents for gangue depression. This process is very successful for endogenic siliceous phosphate deposits. In this process the advantages are a relatively simple flowsheet, and low cost.
Reverse Flotation of Carbonate Minerals
Dolonite and other carbonate minerals are floated using carboxylate collectors under slightly acid conditions with phosphoric acid added for the depression of phosphate minerals. If the feed contains a significant amount of silica a final concentrate cannot be obtained with this strategy alone.
Double Float Flotation Processes
There are two flotation processes which fall in this category. One is the Direct-Reverse flotation process. In this process the first stage is designed to float as much phosphate mineral as is possible using carboxylate collector. In this stage some of the silica and/or carbonate gangue is rejected. The second stage is referred to as reverse flotation. In this stage only as much silica or carbonate mineral is floated from the initial phosphate concentrate as is required to meet the final desired product specifications. For example this “double float” process is widely used in central Florida phosphate industry, viz. the anionic flotation of phosphates at pH8-9.5, followed by cationic flotation of quartz from the acid-scrubbed rougher concentrate at pH 6-7.5.
Another process that falls in this category is the Reverse-Direct flotation process. In the first stage the carbonate or silica gangue mineral is floated and then the phosphate flotation is carried out. When the feed contains two types of gangue mineral (silica and carbonate) this double stage strategy may not be efficient.
Phosphate flotation efficiency needs to be improved in several ways. Technology needs to be developed to eliminate the double flotation processes, to improve the flotation efficiency for both coarse and fine phosphate, and to solve the dolomite problem. A very important factor in flotation technology is the use of appropriate reagents. Collectors and other reagents need to be developed to improve coarse particle flotation and to achieve selectivity with respect to carbonate minerals, particularly dolomite. The development of highly selective collectors, which are specific to the surface structure of a particular mineral, is essential for the exploitation of relatively more difficult-to-process ore deposits, particularly for the separation of semi-soluble minerals having a common cation. The difficulty in the separation of phosphate from dolomite is probably due to the fact that both minerals have the same cation Ca
2+
, and similarly sized anions, PO
4
3−
and CO
3
2−
.
The major dolomite problem associated with the future reserves in central Florida is found with the pebble fraction (~6% MgO). The development of a satisfactory processing strategy will probably involve grinding and classification followed by flotation or some other method to separate the dolomite from the phosphate. A number of flotation technologies have been under development for the Florida carbonate-bearing phosphate rock, and most under current study involve the anionic flotation of carbonate minerals from an acid suspension. These include,
USBM—pH 6.0 depression of phosphate with hydrofluosilic acid.
TVA Diphosphonic Acid—pH 6.5 phosphate depression with ethylidene hydroxydiphosphonic acid.
Aluininum Sulfate/Tartrate—pH 7.5 to 8.2 phosphate depression with Al
2
(SO
4
)
3
/Na tartate.
Sulfuric Acid—pH 5.0 to 5.5 phosphate depression simply by sulfuric acid, fast conditioning and flotation time in order to maintain pH. Even better flotation separation efficiencies
Li Minhua
Miller Jan D.
Wang Xuming
Lithgow Thomas M.
Sonntag, Esq. James L.
University of Utah Research Foundation
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