(658d) Calcination and Applications of Sludge from Softening Water Process

Cachimayo industries is a chemical plant located in Cusco, Peru. Its main product is nitrate ammonia that is used as an explosive in mining operations. Within this plant there is a primary water treatment plant, specifically a softening reactor. In this reactor, hardness of water (calcium and magnesium ions) are removed by precipitation using limewater (calcium hydroxide) and ferric chloride as a coagulant. The reactor has a capacity of 125 cubic meters, and lime sludge is generated at large quantities. The purpose of this research was to find a conversion of this lime sludge to lime, and its application in the softening water process. In addition, the adsorption of NOx gases was studied using this regenerated material as an adsorbent.

Firstly, the sludge was sampled from the sludge disposal area, and lime water preparation tank. Those materials were dried at 800, 950, and 1100 °C during 30, 60, 90, and 120 minutes. Temperature, and time are critical variables at the calcination process. Those conditions were selected in order to evaluate either the calcination at lower temperatures, and large residential times or higher temperatures, and small residential times. The performance of calcination was tested by concentration of lime (% CaO). Using sludge from sludge disposal area for calcination experiments, each temperature was fixed to a value, and time was changed; for example, set 1: 950 °C and 30 min, set 2: 950 °C and 60 min, set 3: 950 °C and 90 min, and set 4: 950 °C and 120 min. For those sets, the best response of concentration of lime was the set 4: 62 % of CaO that was recovered; according to the experimental results at 950 °C, more residential time of calcination was needed in order to achieve more concentration of CaO. The second one, set 5: 800 °C and 30 min, set 6: 800 °C and 60 min, set 7: 800 °C and 120 min, and set 8: 800 °C and 180 min. For those second sets, the best response of lime concentration was the set 7 with 56.8 % of CaO that was recovered; according to the experimental results at 800 °C, at more residential time of calcination, less concentration of CaO was achieved. It was due to recarbonation of the recovered lime, and more quantities of generated carbon dioxide at the calcination process. The last sets of calcination, set 9: 1100 °C and 30 min, set 10: 1100 °C and 60 min, set 11: 1100 °C and 90 min, and set 12: 1100 °C and 120 min. For those last sets, the best response of lime concentration was the set 10 with 56.8 % of CaO that was recovered. In this case, like the second one, there was a recarbonation process of CaO and reversibility of calcination reaction at more time. Therefore, the best response was achieved at 950 °C and 120 min with 62 % of CaO. The next sample that was studied was sludge from a lime water preparation tank. In this case, the calcination parameters were fixed at 950 °C and 120 min where 78.1 % of CaO was achieved. The result of the last test was better than the previous ones, it is probably due to more concentration of calcium carbonate in sludge from this lime water preparation tank than sludge from the softening water reactor. Lime that was recovered from sludges was tested in the softening water process, and compared with commercial lime that is applied in plants.

Secondly, the tests of softening water process were conducted at the same operational conditions of plant: That kind of lime that is applied in plant is a commercial lime with a 75 % of CaO, and its concentration is 20 g/L that get into to the softening reactor with 30.4 g/L of ferric chloride as a coagulant, and the velocity of agitation is 95 RPM. Water that gets into the plant was characterized with the following values: 25.05 °dH (degree of general hardness or German degree), and pH 8.2. The samples were lime converted from sludge disposal area (sample 1), lime converted from lime water preparation tank (sample 2), and commercial lime (sample 3) that is applied in the plant. The results were, sample 1: 16.75 °dH, sample 2: 16.9 °dH, and sample 3: 16.8 °dH. The hardness was removed at the same values using regenerated lime (samples 1 and 2), and commercial lime (sample 3). Those results showed a good performance from regenerated lime, and a competitive removal of global hardness in comparison with commercial lime. However, there was an increase of pH during the softening water process. Sample 1: pH 10.4, and sample 2: pH 11.3 in comparison with commercial lime, sample 3: pH 9.4. It was due to the presence of dissolved cations that regenerated lime provided to water. Within the process, after primary water treatment plant, the next stage is secondary water treatment plant that is basically anion, and cation mixed-bed ion exchange in order to produce ultrapure water. Probably, the increase of pH could accelerate the regeneration of ions at the secondary water treatment plant. However, it needs more study and tests.

Finally, the regenerated lime was tested as an adsorbent of NOx gases at the nitric acid plant in Cachimayo Industries. Those remanent gases are expelled to the atmosphere after passing absorption columns. For experimental tests, a line of those gases was conducted to a packed column with regenerated lime from the primary water treatment plant. The adsorption process of NOx gases was tested at different times. The flowrate was regulated using a rotameter, and the concentration of NOx gases was measured using UV-vis spectrophotometry. Sets 1, 2, 3 and 4 were tested at 3, 6, 20, and 40 minutes respectively. For set 1, the initial concentration of NOx gases was 693 mg/m³, and after passing the column the final concentration of NOx gases was 104.8 mg/m³ with 84.9 % of adsorption. For set 2, the initial concentration of NOx gases was 693 mg/m³, and after passing the column the final concentration of NOx gases was 213.6 mg/m³ with 69.2 % of adsorption. For set 3, the initial concentration of NOx gases was 883.2 mg/m³, and after passing the column the final concentration of NOx gases was 287.9 mg/m³ with 67.4 % of adsorption. And, for set 4, the initial concentration of NOx gases was 883.2 mg/m³, and after passing the column the final concentration of NOx gases was 294.1 mg/m³ with 66.7 % of adsorption. According to the results, at the beginning, the adsorption was very quick, but after some minutes, the equilibrium was achieved. In addition, a pseudo kinetic model of second order was fitted to the adsorption process, and it showed a good performance of the adsorbent until 720 hours of operation. However, more studies are needed in order to implement that kind of adsorbent in industrial plants.

To conclude, the generated sludge in the primary water treatment plant in Cachimayo industries could achieve 175 kg per day, approximately 63 tons per year. Therefore, it was necessary to propose a recovery system of this sludge; hence, this research was focused in the calcination of this residue, its evaluation in the softening water process, and its application as an adsorbent of NOx gases. During the calcination process, the best response was achieved at 950 °C and 120 min with 62 % of CaO from sludge disposal area, and 78.1 % of CaO from lime water preparation tank. Then those regenerated lime were evaluated in the softening water process, where they showed a good performance in comparison with commercial lime. The removal of hardness were 16.75 °dH for lime from sludge disposal area, 16.9 °dH for lime from lime water preparation tank, and 16.8 °dH for commercial lime. Finally, the regenerated lime was tested as an adsorbent from NOx gases in a nitric acid plant. For an initial concentration of NOx gases of 693 mg/m³, the packed bed column with regenerated lime could adsorb 84.9 % of NOx gases in 3 minutes, then it achieved the equilibrium around 66 % of adsorption in the next minutes. Those results showed that regenerated lime from the primary water treatment plant could be applied in the softening water treatment and as an adsorbent of NOx gases in a nitric acid plant.