ELUTION BEHAVIOR OF DEPHOSPHORAZATION SLAG TO VARIOUS ACIDS AND EFFECT OF ADDITION OF AQUEOUS ALKALI TO ACID ELUATE

We recently developed a method for efficiently recovering phosphoric acid from dephosphorization slag. The most important processes in this recovery technique involve elution of the dephosphorization slag into an acid solution (acid-elution process) and the addition of an alkali to the acid eluate to form a precipitate (alkali-precipitation process). Ultimately, we settled the use of an aqueous nitric acid solution for the acid-elution step, while an aqueous ammonia solution proved to be the optimal choice to accomplish alkali precipitation. Herein, we describe the procedures used to compare the utility of nitric acid with that of hydrochloric acid, sulfuric acid, and citric acid in the acid-elution process, and describe our comparison of an aqueous sodium hydroxide with that of an aqueous ammonia in the alkali-precipitation process. Finally, we summarize our findings on the acids and alkalis that proved to be suitable for this technology

Recovery of Phosphoric Acid and Calcium Phosphate from Dephosphorization Slag

We previously reported that by adding aqueous ammonia to the nitric acid extract of dephosphorization slag, a solid with enhanced concentrations of calcium and phosphorus, could be recovered. The present study shows that a considerable amount of manganese and iron also remains, however, which creates difficulties in directly reusing the recovered solid. The recovered material was again dissolved in nitric acid and the resultant filtrate was passed through a cation exchange resin that mostly removed various cations from the yield of an aqueous phosphoric acid solution. The recovery of phosphoric acid was confirmed via 31P NMR. Furthermore, when calcium nitrate was added to this aqueous solution, calcium hydroxyapatite, which was converted to calcium phosphate after the calcination at 1073 K. Phosphoric acid, calcium hydroxyapatite, and calcium phosphate are raw materials that are used to produce various industrial products containing phosphorus, and the suggested process greatly improves the technology for recovering phosphorus-containing materials that are mostly used as fertilizer

KEY FACTORS FOR THE SEPARATION OF SILICON AND IRON DURING PHOSPHORUS RECOVERY FROM SLAG DISCHARGED FROM THE DOUBLE-SLAG REFINING PROCESS

In the present study, we developed a technology for concentrating and recovering phosphorus from slag-like phosphorus-containing unused resources and applied it to slag discharged during the latest steelmaking process, that is, double-slag refining process (DRP). The technology we developed consists of the following four processes: Process (1) is the initial acid elution; Process (2) involves alkali precipitation; Process (3) is the second acid elution; and, Process (4) utilizes ion-exchange. In Process (1), the addition of DPR slag to 0.5 M of a nitric acid solution for 24 min resulted in sufficient phosphorus dissolution. In Process (2), ammonia was added to the dissolved solution, and phosphorus was precipitated with high efficiency. The timing of the addition of ammonia significantly influenced the removal of silicon and iron, which would have been inconvenient to accomplish in subsequent processes. In Process (3), the precipitation obtained in Process (2) was re-dissolved in a nitric acid solution. The dissolution of phosphorus together with other elements progressed sufficiently, and we confirmed that silicon could be completely separated as silica by using high-concentration nitric acid at this stage. The fact that silicon could be removed during Process (3) was an important finding, since silicon could not have been separated in the Process (4). In Process (4), by passing the phosphorus-containing solution obtained in Process (3) through an ion exchange resin, elements other than phosphorus and silicon could be removed, which confirms that the range of applications for this technology could be expanded

PHOSPHORUS RECOVERY FROM SEWAGE-SLUDGE MOLTEN SLAG USING A COMBINATION OF ACID-DISSOLUTION, ALKALI-PRECIPITATION, AND ION-EXCHANGE

We recently reported an efficient procedure for recovering phosphoric acid from dephosphorization slag. This recovery procedure consists of a combination of the following four processes: (1) A first dissolution process of slag in a nitric acid solution; (2) a precipitation process then adds ammonia to the obtained eluate; (3) a second dissolution process dissolves the precipitation from the nitric acid eluate; and, (4) the final process involves ion exchange in which the obtained eluate is passed through an ion exchange resin. In the present study, this recovery procedure was applied to concentrate and recover phosphorus from sewage-sludge molten slag, which is an unused resource that should be considered a new resource for phosphorus. As a result, our procedure for recovery from dephosphorization slag was viable following two revisions. Initially, the time for the first dissolution process was extended from 0.2 h to 1 h, but 0.2 h proved to be the optimum time for dephosphorization slag. Next, we discovered it was better to perform the filtration one day after adding the ammonia instead of immediately after adding it. The other two processes could be treated under substantially the same conditions as in the case of dephosphorization slag, and high-purity phosphorus was obtained

Measurement of Interfacial Tension between Liquid Pb and a Molten LiCl-NaCl-KCl Mixture via a Floating Drop Method

The floating drop method has the potential for high-accuracy measurement of liquid–liquid interfacial tension. It was applied here to measure the interfacial tension between liquid Pb and a molten mixture of 8.9% LiCl, 50% NaCl, and KCl (mass%). The heavier Pb droplet floated on the lighter molten salt and enabled the acquisition of data at 673 K, 723 K, and 773 K under vacuum. The results agreed with previous reports. The temperature (T) dependence of the interfacial tension s was given by s = 459–1.27 T mN/m