Analysis

It would be an inefficient use of time if every consideration for a waste source feedstock was simulated in order to determine if the choice is economically feasible or not. Luckily, there are screening methods that provide a rough but quick analysis as to whether or not a process may be profitable. For these methods, the only necessary information is the purchase price of feedstock, the selling price of products, and the stoichiometric relationships between the materials. One such method is the Metric for Inspecting Sales and Reactants (MISR) which is a ratio of product sales to reactant purchases (El-Halwagi, 2017). Any process that has an MISR greater than 1 has a chance of being profitable, while any value lower than 1 indicates the process is not economically feasible. The MISR equation is defined below as:

It should be noted that an MISR value greater than 1 does not guarantee a profitable process, as a full detailed analysis is necessary in order to determine profitability. However, a value greater than 1 does justify further investigation into the process. This method helps quickly weed out any processes that have no chance of viability rather than wasting effort on unnecessary analysis.

Waste evaluation

In order to perform an MISR analysis on previously mentioned waste materials, the purchasing prices and product selling prices must be realistically set first. This data was gathered through literature and will be used for this analysis. The more difficult aspect of the MISR evaluation is determining the stoichiometric relationship between the waste and the carbonate products due to the variation in composition. For the purposes of this analysis, it will be assumed that the wastes consist of 30 wt% calcium oxide, as this falls within the range of each of the considered waste sources. The CO, feed is obtained from the shale gas process, and brine is considered to be present within the existing infrastructure and, therefore, assumed to also be available as feed.

The representative reaction scheme for creating calcium carbonate from calcium oxide and carbon dioxide is as follows:

If we assume the reaction goes to completion, a stoichiometric conversion of one mole of calcium oxide produces one mole of calcium carbonate. By incorporating molecular weights, we can describe the reaction in terms of mass as:

For the purposes of this analysis, it will be assumed that the wastes consist of 30 wt% calcium oxide, as this falls within the range of each of the considered waste sources. In order to get the desired amount of calcium oxide from the waste to satisfy the conversion scheme above, the following must be true:

Finally, the reaction scheme can now incorporate this theoretical waste as:

The primary target product for this study will be precipitated calcium carbonate (PCC), a high purity material used in ceramics, fillers, and other chemical applications. From the reaction scheme above, a stoichiometric ratio of 1.87 kg waste/kg PCC is established, assuming all reactant material is converted. While ultrahigli purity calcium carbonate can reach market prices of USD 10,000 per metric ton, a more realistic quality (~ 98%) will be considered here with a typical market price around USD 400 per metric ton (Katsuyama et al., 2005). The CO, feed is obtained fr om the shale gas process, and brine is considered to be present within the existing infrastructure and. therefore, assumed to also be available as feed. Table 2 shows the results for the MISR calculations.

As indicated by the MISR values in Table 2, the waste carbonation reaction appears to be potentially profitable. Of course, this evaluation does not consider utility costs or other potential reactants, but this is saved for further analysis.

Table 2. MISR evaluations of waste carbonation.

Waste material

Approximate price (USD/metric ton)

MISR

Fly Ash

15

14.3

Cement Kiln Dust

17

12.6

Furnace Slag

13

16.5

The two-step approach

Two main strategies exist for mineralizing waste particles: The single-step approach and the two-step approach. A single step approach involves mixing waste particles in an aqueous solution where CO, is then also bubbled through in the same reactor. The two-step approach separates these processes, where the calcium ions are first leached into solution and the remaining solid waste is filtered out so CO, can be mixed to form pure calcium carbonate without other solids affecting purity. While the single-step approach is much more direct and easier to implement, the two-step approach will produce the purified product of interest and is more economically favorable in this case. The two-step approach also allows further process control and tuning. Leaching calcium ions out of solid particle waste is favored at lower pH ranges, while carbonation only occurs in more basic conditions. These competing reaction conditions make it difficult to be efficient in the single-step approach without the use of complicated pH swing teclmiques. All simulated processes in this research will implement the two-step approach in order to increase efficiency of producing high purity PCC.

Reaction modelling

Other than the weight percent of calcium content, the maj or difference between the three types of industrial waste being considered in this project is the form in which the calcium is stored. These waste particles contain various types of silicates, ores, metals, and hydrates which influence the effects and composition of the leaching solution. In order to account for these differences, the proper chemical systems of each waste type need to be considered in order to describe the processes. Before these systems are described, there are a few simplifications that can be made. Fu st, it has been noted that with these types of industrial wastes, calcium, hydroxide, and sulfate ions are the main leachable components, followed by potassium and mmor levels of sodium, aluminum, and magnesium. The sum of the three main leachable ions typically account for 90-95% of the electroneutrality condition. With this information, we can drastically reduce the amount of chemical equilibriums that need to be incorporated in the simulation and focus on the major components. While it has been stated that the dissociation of calcium sulfide (CaS) is also present, this can be neglected at lower mass fractions (< 0.1 wt%). Other than these leachable components, the rest of the waste can be assumed as an inert solid which will be filtered out following the leaching stage.

Leaching waste cement

Waste cement is typically composed of SiO,, CaO, Al,Or MgO. and Fe,0,. From this, we can infer that the free lime (CaO) component will be the main calcium source. When free lime is mixed within an aqueous solution, calcium hydroxide (Ca(OH),) is formed. This is a somewhat soluble precipitate which will dissociate a hydroxide group fir st, followed by the remaining hydr oxide and calcium ions in a second equilibrium step. The equilibrium descriptions are stated below.

While the first step can be assumed to react to completion, the following two dissociations require equilibrium data to determine the resulting concentrations. Both equilibrium constants were found through literature and were input into Aspen to help model the system.

Leaching fly ash

Fly ash can contain a great variety of materials in varying amounts; however, the main components are typically SiO,, CaO, Al,Or MgO. K,0, CaS04 and Fe.O ,. While similar to the components found in waste cement, one key difference is the presence of calcium sulfate, which represents another viable calcium source. In the presence of water, calcium sulfate forms a hydrated complex known as gypsum. This solid hydrate is slightly soluble and dissociates to form calcium and sulfate ions along with the complexed water. While the free lime component can be assumed to follow the same equilibrium conditions as the waste cement, this gypsum component is described by the equations below.

The parameters for these equilibrium states are present in Aspen and were utilized in the simulation.

Leaching steelmaking slag

Steelmaking slag contains a complex mixture of silicates, srebrodolskite, and calcium magnesium- wustite type phases. While free lime is present in small amounts, it is typically bound within the wustite phases and carmot react in the leaching process. In this case, the main leachable calcium content comes in the form of dicalcium silicate (Ca,Si04). For this project, the dissociation equation will be assumed as stated below.

The calcium in this phase is more difficult to extract than the calcium present in the previous waste types. While acids can improve extraction efficiency, they impose greater environmental concerns as well as a higher operating cost. An interesting solution that will be implemented in this project is the use of ammonium salts, as described in the next section.

Ammonium salts

As mentioned in the section on the two-step approach, the leaching process favors acidic conditions while the carbonation stage requires basic conditions. Using acid to improve extraction efficiencies would cause difficulties and the possible necessity of additional basic material to make the carbonation process possible. One suggestion is the addition of ammonium salts, like NH,C1, which, while less effective at leaching than acids, still manage to improve efficiencies without drastically impacting the pH level. In addition to this, the ammonium salts are regenerated after the carbonation step, which helps to improve process economics. The following reaction equations show an example of how the salts interact in the leaching and carbonation stages.

Different ammonium salts har e different impacts on both the extraction efficiencies as well as the carbonation efficiencies of varying waste types. A high extraction efficiency doesn't correlate to a good carbonation efficiency. Additionally, the efficiency of some salts may vary greatly with concentration while others aren't impacted as heavily. For example, in the study of ammonium salt effects on waste cement leaching, NH4NO, varied from an efficiency of 68.8% at 1 M to an efficiency of 60.1% at 0.5 M, while the salt CH.COONH. varied from 69% at 1 M to 23.8% at 0.5 M.

5 4

Carbonation of leached calcium

While the extraction mechanisms differ between the waste types, the carbonation process is virtually the same between them all since it is only the leached calcium ions taking part in the reaction and none of the other ions. Because of this, the reactor can be modelled using the same set of equilibrium equations across all three waste types. In this reactor, CO, is bubbled through the solution to first produce bicarbonate, which in turn reacts to form the carbonate species. This carbonate ion reacts with the leached calcium ions to precipitate as the target calcium carbonate. The simplified set of equations is shown below.

As an example, literature has shown that with the process of leaching and carbonation of ash waste, a precipitated calcium carbonate product of ~ 99% purity is obtained.

Defined project ejficiencies

While it is possible to simulate these systems in Aspen utilizing kinetic and equilibrium data, the results are not always accurate. Instead, this project will utilize present experimental data from literature to define reaction efficiencies. While this may limit the operating conditions to those defined in the literature, the resulting computational model should produce a more realistic simulation. With everything considered so far, the following three waste source scenarios will be explored, along with the determined efficiencies:

  • 1. Fly ash (no ammonium salts)-extraction efficiency will be determined by Gibbs reactor; Carbouation efficiency is approximately 87%.
  • 2. Waste Cement with NH4NO, salt-extraction efficiency of 60% at salt concentration of 0.5 M; Carbouation efficiency is approximately 74%.
  • 3. Steehnaking Slag with NH4C1 salt-extraction efficiency of 35% at salt concentration of 2.0 M; Carbouation efficiency is approximately 84%.

All data was produced at ambient condition (25 °C, 1 bar) and will be reflected as such in simulation parameters.

 
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