SysCAD + ChemApp for Copper Smelting: A Step-by-Step Guide to Building an Integrated Pyrometallurgical Flowsheet - Part II
Tanai Marin-Alvarado (SysCAD) and Shivani Gonde (GTT-Technologies)
June, 2026
Categories: SysCAD, ChemApp for SysCAD, Process Simulation
Figure 1: Copper primary production from chalcopyrite - SysCAD flowsheet using ChemApp for SysCAD TCE Add-on (Copper Smelter with Slag Recirculation)
A recent post by Tanai Marin-Alvarado and Shivani Gonde (GTT Technologies, June 2026) demonstrated how a simple integrated copper smelting flowsheet can be modelled using SysCAD and the ChemApp for SysCAD TCE (Thermodynamic Calculation Engines) Add-On. ChemApp for SysCAD performs rigorous Gibbs free energy minimisation calculations within a fully integrated graphical flowsheet, where stream connections, phase splitting, and solver convergence are handled automatically by SysCAD, making the workflow more accessible to plant engineers and metallurgists who work primarily in a simulation environment rather than a scripting one.
This post is the second in a series that builds progressively in complexity. The earlier post (Part I) introduced a simplified steady-state flowsheet: Smelter_4 → Converter_12 → Converter_19 (Part-I). In this post (Part II), we will build on this foundation by introducing a more realistic flowsheet that incorporates slag recirculation and multiple slag blow stages.
Building up from the first simple example, the flowsheet has been modified by adding a second slag blow stage and, more importantly, recirculation of slag from the first two slag blow stages to the furnace, and the slag or mush from the copper blow stage back to the first slag blow stage, simulating in a steady-state solver the fact that any residue or crust left over after casting blister copper will remain in the converter for the next campaign.
From an industrial point of view, it is extremely important to recover copper losses from all converter stages to maximise metallurgical recovery. This can be achieved by adding slag cleaning stages or by recycling slag from more oxidative processes back to the furnace. The latter is the approach demonstrated in this example, where the only slag stream that is discarded is the furnace slag. This adds computational complexity as solving the model to steady-state now becomes an iterative process, which in the case of SysCAD is handled automatically, facilitating significantly the solution of systems involving recirculating streams and complex thermodynamic calculations.
For this example, several targets or setpoints were specified, including copper grade in furnace matte (65%), iron end-point in matte for first and second slag blows (5% and 1.5%, respectively) and final Sulfur end-point in Blister Copper (0.5%). In addition, flux addition to furnace and converters was controlled to target typical slag quality in industry by monitoring the Fe/SiO2 mass ratio in the furnace (1.5 kg/kg) and converters slag blows (1.8 kg/kg), reflecting a typical silica-saturated operating zone for the furnace and a magnetite-saturated zone for slag blow converters.
To demonstrate the importance of slag recycling, key results such as copper recovery, throughput and reagent consumption per unit of copper produced were compared using slag recirculation versus full slag discard, while keeping all the set-points and targets mentioned above constant between the two cases.
The modified pyrometallurgical copper extraction process from chalcopyrite concentrate in this example consists of three main unit operations, as shown in Figure 1:
- Furnace – converts copper concentrate to a copper-rich matte phase, in this case using 65% Copper grade as target for Furnace Matte and slag close to silica saturation (1.5 Fe/SiO2 mass ratio).
- Converter Slag Blow 1 (slag-forming) – oxidizes the matte to remove iron as slag with 5% iron target in the matte and 1.8 Fe/SiO2 kg/kg in the slag as targets
- Converter Slag Blow 2 (slag-forming) – further oxidizes matte to remove iron as slag with 1.5% iron target in the matte and 1.8 Fe/SiO2 kg/kg in the slag as targets
- Converter Copper Blow (copper-making) – produces blister copper with a 0.5 wt% S in Blister as target
Each major unit operation (Equilibrium Calculations) is implemented in SysCAD as a ChemApp (CA) TCE Reactor and operates at 1250°C, with oxygen-enriched air at 95, 23 and 24 O2 vol% for Furnace, slag blows and copper Blow, respectively.
Although the real copper converting is a batch process, where multiple converters perform the various stages of converting sequentially — some in stand-by, others slag blowing and others performing the final Copper Blow — this example models the process from a Steady-State point of view, abstracting the batch nature of converting by representing the various stages as process units connected one after another. This modelling approach allows long-term stable results to be calculated while missing some of the characteristics of batch processing. A more detailed, true dynamic modelling will be shown and described in future examples in this series.
The matte produced from the furnace is mainly directed as feed for the first slag blow, and a smaller portion is fed to the second slag blow which also receives the partially converted matte from the first blow. This mimics the fact that at the beginning of the second slag blow, fresh furnace matte is used to top-up the converter after slag skimming.
More importantly, this example highlights the use of recycling streams by directing the slag produced from the two slag blows back to the Furnace, and the residue or slag from the final Copper Blow back to the first slag blow. The distribution, mixing and splitting of streams is handled in SysCAD using a Tie unit as described in Part I of this series.
The same thermodynamic database, KWA_Cu_Smelting_CST, used in Part I is used again in this project across all ChemApp TCE reactors. It contains Gibbs energy data and interaction parameters for all relevant species and mixture phases in the Cu–Fe–S–O–Si–H–N system. This database file was generated using SysCAD's private thermodynamic solution data and is included with the example project files.
Setting up this project is relatively simple as it uses the same configuration file (.cfg) and thermodynamic database file (KWA_Cu_Smelting_CST.cst) as Part I of the series. Therefore the only step required is to create a new SysCAD Project in the same Project Group Folder as before and then create the flow diagram shown in Figure 1. The ready-built example project files are provided - see the Accessing the Example Project Files section below.
The key setup steps have already been performed in the distributed example. However, creating a new project reusing the previous configuration from Part I requires the setup steps as follows:
1. Create a new steady-state SysCAD project and select the previous configuration file created in Part I, which already includes the ChemApp TCE units and species database required for this example. The project will be created in the same group folder as the previous example as a new (.spf) SysCAD project folder.
2. Add a TCE ChemApp Chem Model Configuration (TMC) unit to the flowsheet. Load the KWA_Cu_Smelting_CST.cst database file via the TMC unit and run the species auto-mapping to link ChemApp species to SysCAD species.
3. Build the Flowsheet by inserting ChemApp reactors, feeders, sinks, ties, connecting links and setting up input and equilibrium conditions as in Figure 1.
4. Set input values to define feeder composition and flow, set temperatures for feeders and reactors and define equilibrium conditions to ChemApp unit models.
5. Solve and Analyse Results by running the simulation, during which all the ChemApp call equilibrium results and stream flow and recycle results are automatically propagated through the model and ready to review and analyse.
This project contains an additional slag blow step as compared to the previous example. In general, two or three converters performing slag blow and one in copper blow are common configurations in industry. Usually the initial slag blows have a higher iron end-point before moving into Copper Blow. This is reflected in this example by adding a second slag blow step and setting 5% and 1.5% iron end-points to the first and second slag blow steps, respectively.
Slag from the two slag blows must be redirected to the furnace as recirculation streams; in this case these are connected to a Tie unit and from there to the furnace as an additional input. Other inputs to the furnace are of course dry concentrate feed, O2-enriched air and flux.
Matte produced from the furnace is mainly directed to the first slag blow converter, whereas a small portion (25% by mass) is sent to the second slag blow converter to simulate the top-up feed typically received by the second slag blow after skimming slag. All of the furnace slag is discarded as final slag in this example and the off-gas is captured by a common off-gas Tie and Sink.
The furnace and all converter ChemApp reactor unit models use fixed temperature operation mode set to 1250°C and 1 atm in this example.
The converting process follows the same concept explained in the previous blog post. The difference in this case is the addition of a second slag blow and the recirculation of copper slag or residue back to the first slag blow step.
All converters receive matte or white metal generated in the preceding step, in addition to O2-enriched air and flux. For simplicity, all are set to fixed temperature operation mode and 1 atm (absolute) of pressure. The off-gas is connected to a common Tie unit model together with the furnace off-gas, simulating the gas train that would typically be treated in an acid plant.
The following table summarises the input feed conditions and settings used for the furnace (Fce) and Peirce-Smith converters (PS) as in the provided example project files.
| Units | Concentrate | Fce Flux | Fce Air | PS1 Flux | PS1 Air | PS2 Flux | PS2 Air | PS3 Flux | PS3 Air | |
|---|---|---|---|---|---|---|---|---|---|---|
| Mass Flow | t/h | 10.94 | 0.60 | 4.02 | 0.10 | 0.58 | 0.10 | 0.60 | 0.03 | 2.29 |
| CuFeS2(s) | wt% | 61.33 | ||||||||
| FeS2(s) | wt% | 24.28 | ||||||||
| SiO2(s) | wt% | 12.38 | 100 | 100 | 100 | 100 | ||||
| H2O(l) | wt% | 2.00 | ||||||||
| O2(g) | vol% | 95.0 | 23.0 | 23.0 | 24.0 | |||||
| N2(g) | vol% | 5.0 | 77.0 | 77.0 | 76.0 |
Note: all input feed streams are set at 25 deg C and 1 atm (absolute) pressure
One advantage of using SysCAD to simulate processes that contain multiple and complex recirculating streams is that SysCAD handles recycles automatically. The user only needs to connect the streams as necessary and SysCAD will detect if recirculations exist, perform a flow network analysis and automatically converge the solution using an iterative Tear Solver to reach steady state.
In this example, we have several recirculating streams: the two slag streams from the first two converter steps are recycled back to the furnace and the residue from the Copper Blow converter is recirculated back to the first slag blow. This has two main implications, from the metallurgical point of view and from the computational perspective. These two topics are discussed in detail below.
To evaluate the importance of recirculating slag, it is necessary to compare to a case where slag is not recirculated and is discarded while keeping all the metallurgical targets constant, that is, furnace matte grade, converter end-points, slag quality, etc.
The example provided can easily be modified to represent the case where no slag is recycled to either the furnace or other converters, and is thus fully discarded at each step, by simply rearranging how the slag streams are connected. The modified flow diagram used as comparison is shown below.
Figure 2: SysCAD flowsheet for Copper Smelter without Slag Recirculation
The table below summarises the key operating conditions and copper outputs as reported in the SysCAD model:
| Units | No Recirculation | With Recirculation | |
|---|---|---|---|
| Cu Recovery | wt% | 97.6 | 98.7 |
| Cu in Blister | wt% | 99.4 | 99.4 |
| O2 to Cu | kg/kg | 2.10 | 2.07 |
| Flux to Cu | kg/kg | 0.35 | 0.36 |
The results highlight the clear metallurgical benefit of slag recycling. Overall copper recovery improves from 97.6% to 98.7% — a difference of 1.1 percentage points that, at industrial throughputs, represents a significant reduction in copper losses to discarded slag. The converter slags from the slag blow stages carry entrained matte droplets and dissolved copper oxide that would otherwise be lost; routing these streams back to the furnace allows that copper to re-partition into the matte and ultimately report to blister copper.
The marginal reduction in specific oxygen consumption (2.07 vs. 2.10 kg O2 per kg Cu produced) reflects the fact that the recycled converter slags already contain partially oxidised iron species. Less fresh oxygen is therefore required to reach the same metallurgical end-points compared to treating only fresh concentrate feed. The flux requirement is essentially unchanged (0.36 vs. 0.35 kg/kg), indicating that the additional silica needed to maintain the target Fe/SiO2 ratio in the furnace when processing recycled converter slag is minimal.
Importantly, the purity of the blister copper product is identical in both cases (99.4 wt% Cu), confirming that slag recirculation does not compromise product quality as the main controlling factor for blister quality is the %S end-point set at the copper blow in both cases. The gains are therefore in yield and reagent efficiency, making slag recycling an attractive strategy in industrial operations where copper price and smelter throughput drive economic performance. An alternative to slag recycling to furnace, that is also widely implemented in industry is to implement a separate slag cleaning proces, that route could also be modelled in SysCAD. The reader is encourage to modify the project to use slag cleaning instead of recyrculation and potentially do a techno-economic comparison of both scenarios.
The introduction of recirculating streams increases the computational complexity of the model. Without recirculation, SysCAD evaluates the flowsheet in a straightforward sequential pass and reaches a steady-state solution in 5 iterations (by default, this is the minimum required number of subsequent convergence iterations) with a total of 24 ChemApp calls. With recirculation, SysCAD first performs a flow network analysis to identify the recirculation loops and determine an efficient evaluation order — as illustrated in the Evaluation Order Diagram (Figure 3). It then applies a Tear Solver, which iteratively updates estimates for the tear streams (the recirculating streams) until mass and energy balances converge simultaneously across all loops.
Figure 3: SysCAD Evaluation Order Diagram - single iteration step
This iterative process requires 18 solver iterations and 76 ChemApp calls in total. However, because SysCAD automatically parallelizes independent unit operations within each iteration — units that have no upstream dependency on each other are evaluated concurrently - the actual ChemApp calculation time per iteration drops from 0.169 s (no recirculation) to 0.125 s. The net effect is that while the recirculating case requires approximately 3.5× more iterations to converge, each iteration completes faster due to parallel evaluation, partially offsetting the additional computational cost.
| Metric | Unit | No Recirculation | With Recirculation |
|---|---|---|---|
| Number of Calls to ChemApp | # | 24 | 76 |
| Total SysCAD iterations* | # | 5 | 18 |
| CA calls per iteration | # | 4.80 | 4.22 |
| CA Calc time per iteration | s | 0.169 | 0.125 |
*a minimum of 5 consecutive converged iterations is the default setting in SysCAD before considering the model fully converged.
The convergence behaviour of key process variables across solver iterations is shown in Figure 4. Starting from a reset state, the variables associated with the recirculating streams, such as slag flow rates and copper content, oscillate over the first several iterations before settling to stable steady-state values, which is the expected behaviour for a Tear Solver approaching convergence.
Figure 4: Convergence during SysCAD Solve from Reset State - key variables vs iterations
The SysCAD project files for this example are available through two channels. First, the project is distributed as a built-in example with the SysCAD installation - users can find it in the examples directory after installing or updating SysCAD. Second, the files are hosted in the GTT Technologies ChemApp Examples repository on GitHub, where they can be browsed, downloaded, or cloned directly (ChemApp for sysCAD).
The repository also contains examples for other tools and workflows in the ChemApp ecosystem, making it a useful reference point as you work through subsequent parts of this series.
License requirements: To open and reproduce this example, the following licenses are required:
- SysCAD Steady State Solver, with Energy Balance, Integrated Libraries and TCE Add-on
- ChemApp for SysCAD license from GTT Technologies
Subsequents parts will build on the foundations established in this series up-to-date, so if you want to take your Thermodynamic model to the next level using SysCAD and its Thermodynamic Calculation Engines (TCE) as part of a feature-rich process simulation platform, this is the right place to learn. The future parts will be available on the sysCAD website with summary on GTT's website. Stay tuned!
For more information on SysCAD and the TCE Add-on with ChemApp for SysCAD, visit www.syscad.net and gtt-technologies.de.