Other Thermal Acid Processes Including Improved Hard Process
Other Thermal Acid Processes – Blast Furnace Process
- The Blast Furnace Process operated commercially in the US for twelve years beginning in the 1920’s. The process produced
phosphoric acid from phosphate rock, sand and coal at temperature below that which a fluid melt is obtained.
- The potential cost advantage of the blast furnace compared to the electric furnace was well understood, although greater gas volumes lead to larger equipment and greater capital cost. Favorable hydroelectricity rates awarded to the electric furnaces undermined the advantage of the Blast Furnace and they were closed and dismantled.
- The Blast Furnace Process had very poor heat integration (5x theoretical carbon is required), but the process worked, proving the thermal process concept without the need for in situ resistance heating of the process solids with electricity.
Other Thermal Acid Processes – Kiln Process giving rise to IHP
- Portions of the Kiln Process have similarities with the feed preparation for a phosphorous furnace and the hydrator section of a furnace acid plant.
- Phosphate rock, silica and coke are dried, milled, formed into pellets and fired in a rotary kiln.
- The superior heat integration characteristics of a rotary kiln are fundamental to the kiln phosphoric acid process.
- As kiln feedstock is heated to the reaction temperature, carbon is oxidized to carbon monoxide, and phosphorus is reduced to its elemental form P2 which evolve to occupy voids in the kiln solids, forming a gas layer that blankets the kiln bed and reduces mass transfer from the gas phase.
- Because mass transfer from gas to solids is poor, there is little oxidation of carbon in the feedstock by the oxygen and carbon dioxide in the kiln freeboard.
- As phosphorus and carbon monoxide evolves from the solids, they are convectively transferred to the freeboard and oxidized to phosphorus oxide and carbon dioxide.
- The oxidation of phosphorus and carbon monoxide in the kiln freeboard provides the heat to produce these reactants from the solid feedstock.
- The Kiln Process was investigated in the 1960’s by FMC, in the 1970’s by Olin, and in the 1980’s by Occidental Research.
- The FMC process achieved a good degree of heat integration, a headache for the blast furnaces in the 1920’s and 1930’s, but did not conquer the problem of the charge melting in the kiln.
The Occidental Research process, also known as the Hard Process, after Robert Hard, one of the inventors, achieved stability in a 0.84 m diameter by 9.1 m long pilot rotary kin in the early 1980’s, the breakthrough being that additional silica addressed the melting problem. The kiln processed 1,000 lbs/hr (limited by design, not the process), had consistent yields of 75-87%, made phosphoric acid in a hydrator and operated up to 1500 C without melting.
- Problems with the Hard Process were a hot spot in the kiln that limited performance, use of calcined coke instead of green coke, use of coke at 2x theoretical to reduce phosphate, high silica in the kiln feed that resulted in low P2O5, and a reducing atmosphere that required an after burner. Consequently, engineering and cost estimates showed the Hard Process was not competitive with the Wet Acid Process.
- JDC has subsequently improved the Hard Process. JDC’s Improved Hard Process has a better understanding of the kiln operating temperature range, the specification for the petroleum coke, optimum size distribution of the feed materials and feed ratios, reaction kinetics and the characteristics of the co-product aggregate.